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L_0393 | the respiratory system | T_2219 | After the blood in the capillaries in the lungs picks up oxygen, it leaves the lungs and travels to the heart. The heart pumps the oxygen-rich blood into arteries, which carry it throughout the body. The blood passes eventually into capillaries that supply body cells. | text | null |
L_0393 | the respiratory system | T_2220 | The cells of the body have a lower concentration of oxygen that does blood in the capillaries that supply body cells. Therefore, oxygen diffuses from the blood into the cells. Carbon dioxide, which cells produce in cellular respiration, is more concentrated in the cells. Therefore, carbon dioxide diffuses out of the cells and into the blood. The carbon dioxide travels in capillaries to veins and then to the heart. The heart pumps the blood to the lungs, where the carbon dioxide diffuses into the alveoli. It passes out of the body during exhalation. This brings the process of respiration full circle. | text | null |
L_0393 | the respiratory system | T_2221 | No doubt youve had the common cold. When you did, you probably had respiratory system symptoms. For example, you may have had a stuffy nose that made it hard to breathe. While you may feel miserable when you have a cold, it is generally a relatively mild disease. Many other respiratory system diseases are more serious. | text | null |
L_0393 | the respiratory system | T_2222 | Common diseases of the respiratory system include asthma, pneumonia, and emphysema. All of them are diseases of the lungs. You can see some of the changes in the lungs that occur in each of these diseases in Figure 19.4. Asthma is a disease in which bronchioles in the lungs periodically swell and fill with mucus. Symptoms of asthma may include difficulty breathing, wheezing, coughing, and chest tightness. An asthma attack may be triggered by allergies, strenuous exercise, stress, or another respiratory illness such as a cold. Pneumonia is a disease in which some of the alveoli of the lungs fill with fluid so they can no longer exchange gas. Symptoms of pneumonia typically include coughing, chest pain, difficulty breathing, and fatigue. Pneumonia may be caused by an infection or an injury to the lungs. Emphysema is a disease in which the walls of the alveoli break down so less gas can be exchanged by the lungs. The main symptom of emphysema is shortness of breath. The damage to the alveoli is usually caused by smoking and is permanent. | text | null |
L_0393 | the respiratory system | T_2223 | The main way to keep your respiratory system healthy is to avoid smoking or breathing in the smoke of others. Smoking causes, or makes you more susceptible to, many respiratory diseases, including asthma, bronchitis, em- physema, and lung cancer. Other steps you can take to keep your respiratory system healthy are listed below. Eat well, get enough sleep, and be active every day. These healthy lifestyle choices will help keep your immune system healthy so it can fight off respiratory infections and other diseases. Wash your hands often. This will reduce your risk of picking up viruses or bacteria that could make you sick with colds or other respiratory infections. Avoid contact with other people when they are sick and stay home when you are sick. These steps will help reduce the spread of infectious diseases. | text | null |
L_0394 | the excretory system | T_2224 | Excretion is any process in which excess water or wastes are removed from the body. Excretion is the job of the excretory system. Besides the kidneys, other organs of excretion include the large intestine, liver, skin and lungs. The large intestine eliminates food wastes that remain after digestion takes place. The liver removes excess amino acids and toxins from the blood. Sweat glands in the skin excrete excess water and salts in sweat. The lungs exhale carbon dioxide and also excess water as water vapor. Each of the above organs of excretion is also part of another body system. For example, the large intestine and liver are part of the digestive system, and the lungs are part of the respiratory system. The kidneys are the main organs of excretion. They are part of the urinary system. | text | null |
L_0394 | the excretory system | T_2225 | The urinary system is shown in Figure 19.6. It includes two kidneys, two ureters, the urinary bladder, and the urethra. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body as urine. For a visual presentation on the urinary system and how it works, watch this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0394 | the excretory system | T_2226 | The kidneys are a pair of bean-shaped organs at each side of the body just above the waist. You can see a diagram of a kidney in Figure 19.7. The function of the kidneys is to filter blood and form urine. Tiny structures in the kidneys, called nephrons, perform this function. Each kidney contains more than a million nephrons. | text | null |
L_0394 | the excretory system | T_2226 | The kidneys are a pair of bean-shaped organs at each side of the body just above the waist. You can see a diagram of a kidney in Figure 19.7. The function of the kidneys is to filter blood and form urine. Tiny structures in the kidneys, called nephrons, perform this function. Each kidney contains more than a million nephrons. | text | null |
L_0394 | the excretory system | T_2227 | Blood with wastes enters each kidney through an artery, which branches into many capillaries. After passing through capillaries and being filtered, the clean blood leaves the kidney through a vein. The part of each nephron called the glomerulus is where blood in the capillaries is filtered. Excess water and wastes are filtered out of the blood. The tubule of the nephron collects these substances. Some of the water is reabsorbed. The remaining fluid is urine. | text | null |
L_0394 | the excretory system | T_2228 | From the kidneys, urine enters the ureters. These are two muscular tubes that carry urine to the urinary bladder. Contractions of the muscles of the ureters move the urine along by peristalsis. The urinary bladder is a sac-like organ that stores urine. When the bladder is about half full, a sphincter relaxes to let urine flow out of the bladder and into the urethra. The urethra is a muscular tube that carries urine out of the body through another sphincter. The process of urine leaving the body is called urination. The second sphincter and the process of urination are normally under conscious control. | text | null |
L_0394 | the excretory system | T_2229 | The kidneys help the body maintain homeostasis in several ways. They filter all the blood in the body many times each day and produce urine. They control the amount of water and dissolved substances in the blood by excreting more or less of them in urine. The kidneys also secrete hormones that help maintain homeostasis. For example, they produce a hormone that stimulates bone marrow to produce red blood cells when more are needed. They also secrete a hormone that regulates blood pressure and keeps it in a normal range. | text | null |
L_0394 | the excretory system | T_2230 | You need only one kidney to live a normal, healthy life. A single kidney can do all the work of filtering the blood and maintaining homeostasis. However, at least one kidney must function properly to maintain life. Diseases that threaten the health and functioning of the kidneys include kidney stones, infections, and diabetes. You can learn more about kidney diseases in this video: . MEDIA Click image to the left or use the URL below. URL: Kidney stones are mineral crystals that form in urine inside a kidney, as shown in Figure 19.8. The stones may be extremely painful. If a kidney stone blocks a ureter, it must be removed so urine can leave the kidney and be excreted. Bacterial infections of urinary organs, especially the urinary bladder, are common. They are called urinary tract infections. Generally, they can be cured with antibiotic drugs. However, if they arent treated, they can lead to more serious infections and damage to the kidneys. Untreated diabetes may damage capillaries in the kidneys so the nephrons can no longer filter blood. This is called kidney failure. The only cure for kidney failure is to receive a healthy transplanted kidney from a donor. Until that happens, a patient with kidney failure can be kept alive by artificially filtering the blood through a machine. This is called hemodialysis. You can see how it works in Figure 19.9. | text | null |
L_0396 | chemistry of living things | T_2237 | All known matter can be divided into a little more than 100 different substances called elements. | text | null |
L_0396 | chemistry of living things | T_2238 | An element is pure substance that cannot be broken down into other substances. Each element has a particular set of properties that, taken together, distinguish it from all other elements. Table 2.1 lists the major elements in the human body. As you can see, you consist mainly of the elements oxygen, carbon, and hydrogen. Element Oxygen Carbon Hydrogen Nitrogen Calcium Phosphorus Potassium Sulfur Percent of Body Mass 65 18 10 3 1.5 1.0 0.35 0.25 In your body, most elements are combined with other elements to form chemical compounds. A compound is a unique type of matter in which two or more elements are combined chemically in a certain ratio. For example, much of the oxygen and hydrogen in your body are combined in the chemical compound water, or H2O. | text | null |
L_0396 | chemistry of living things | T_2239 | The smallest particle of an element that still has the properties of that element is an atom. Atoms are extremely tiny. They can be observed only with an electron microscope. They are commonly represented by models, like the one Figure 2.6. An atom has a central nucleus that is positive in charge. The nucleus is surrounded by negatively charged particles called electrons. The smallest particle of a compound that still has the properties of that compound is a molecule. A molecule consists of two or more atoms. For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen. Thats why the chemical formula for water is H2 O. You can see a simple model of a water molecule in Figure 2.7. | text | null |
L_0396 | chemistry of living things | T_2239 | The smallest particle of an element that still has the properties of that element is an atom. Atoms are extremely tiny. They can be observed only with an electron microscope. They are commonly represented by models, like the one Figure 2.6. An atom has a central nucleus that is positive in charge. The nucleus is surrounded by negatively charged particles called electrons. The smallest particle of a compound that still has the properties of that compound is a molecule. A molecule consists of two or more atoms. For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen. Thats why the chemical formula for water is H2 O. You can see a simple model of a water molecule in Figure 2.7. | text | null |
L_0396 | chemistry of living things | T_2240 | Besides water, most of the compounds in living things are biochemical compounds. A biochemical compound is a carbon-based compound that is found in living organisms. Carbon is an element that has a tremendous ability to form large compounds. Each atom of carbon can form four chemical bonds with other atoms. A chemical bond is the sharing of electrons between atoms. Bonds hold the atoms together in chemical compounds. A carbon atom can form bonds with other carbon atoms or with atoms of other elements. | text | null |
L_0396 | chemistry of living things | T_2241 | Biochemical compounds make up the cells and tissues of living things. They are also involved in all life processes. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. Even so, all biochemical compounds can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in Table 2.1. Class Elements Examples Functions Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starch glycogen cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids fats oils phospholipids DNA RNA Functions provide energy to cells stores energy in plants stores energy in animals makes up the cell walls of plants speed up biochemical re- actions regulate life processes store energy in animals store energy in plants make up cell membranes stores genetic information in cells helps cells make proteins | text | null |
L_0396 | chemistry of living things | T_2242 | You can see from Table 2.1 that all biochemical compounds contain hydrogen and oxygen as well as carbon. They may also contain nitrogen, phosphorus, and/or sulfur. Almost all biochemical compounds are polymers. Polymers are large molecules that consist of many smaller, repeating molecules, called monomers. Most biochemical molecules are macromolecules. The prefix macro- means large, and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. The largest known biochemical molecule contains more than 34,000 monomers! | text | null |
L_0396 | chemistry of living things | T_2243 | Carbohydrates are biochemical compounds that include sugar, starch, glycogen, and cellulose. Sugars are simple carbohydrates with relatively small molecules. Glucose is the smallest of all the sugar molecules with its chemical formula of C6 H12 O6 . This means that a molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose can obtain it by consuming plants or organisms that consume plants. Starches are complex carbohydrates. They are polymers of glucose. Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers can get starches by eating plants. Common sources of starches in the human diet are pictured in the Figure 2.8. Our digestive system breaks down starches to sugar, which our cells use for energy. Like other animals, we store any extra glucose as the complex carbohydrate called glycogen. Glycogen is also a polymer of glucose. Cellulose is another complex carbohydrate found in plants that is a polymer of glucose. Cellulose molecules bundle together to form long, tough fibers. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. | text | null |
L_0396 | chemistry of living things | T_2244 | Proteins are biochemical compounds that consist of one or more chains of small molecules called amino acids. Amino acids are the monomers of proteins. There are only about 20 different amino acids. The sequence of amino acids in chains and the number of chains in a protein determine the proteins shape. Shapes may be very complex. You can learn more about the shapes of proteins at this link: MEDIA Click image to the left or use the URL below. URL: The shape of a protein determines its function. Proteins have many different functions. For example, proteins: make up muscle tissues. speed up chemical reactions in cells. regulate life processes. help defend against infections. 2.2. Chemistry of Living Things transport materials around the body in the blood. blood How hemoglobin transports oxygen in the | text | null |
L_0396 | chemistry of living things | T_2245 | Lipids are biochemical compounds that living things use to store energy and make cell membranes. Types of lipids include fats, oils, and phospholipids. Fats are solid lipids that animals use to store energy. Examples of fats include butter and the fat in meat. Oils are liquid lipids that plants use to store energy. Examples of oils include olive oil and corn oil. Phospholipids contain the element phosphorus. They make up the cell membranes of living things. Lipids are made of long chains consisting almost solely of carbon and hydrogen. These long chains are called fatty acids. Fatty acids may be saturated or unsaturated. The Figure 2.10 shows an example of each type of fatty acid. | text | null |
L_0396 | chemistry of living things | T_2246 | Nucleic acids are biochemical compounds that include RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). Nucleic acids consist of chains of small molecules called nucleotides. Nucleotides are the monomers of nucleic 40 acids. A nucleotide is shown in Figure 2.11. Each nucleotide consists of: 1. a phosphate group, which contains phosphorus and oxygen. 2. a sugar, which is deoxyribose in DNA and ribose in RNA. 3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same. RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form bonds with each other. The bonded bases are called base pairs. Bonds form only between adenine and thymine, and between guanine and cytosine. They hold together the two chains of DNA and give it its characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 2.12. Sugars and phosphate groups form the backbone of each chain of DNA. Determining the structure of DNA was a huge scientific breakthrough. You can read the interesting story of its discovery and why it was so important at this link: DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in DNAs nucleotide chains. RNA copies and interprets the genetic code in DNA. RNA is also involved in the synthesis of proteins based on the code. You can watch these events unfolding at this link: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0396 | chemistry of living things | T_2246 | Nucleic acids are biochemical compounds that include RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). Nucleic acids consist of chains of small molecules called nucleotides. Nucleotides are the monomers of nucleic 40 acids. A nucleotide is shown in Figure 2.11. Each nucleotide consists of: 1. a phosphate group, which contains phosphorus and oxygen. 2. a sugar, which is deoxyribose in DNA and ribose in RNA. 3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same. RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form bonds with each other. The bonded bases are called base pairs. Bonds form only between adenine and thymine, and between guanine and cytosine. They hold together the two chains of DNA and give it its characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 2.12. Sugars and phosphate groups form the backbone of each chain of DNA. Determining the structure of DNA was a huge scientific breakthrough. You can read the interesting story of its discovery and why it was so important at this link: DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in DNAs nucleotide chains. RNA copies and interprets the genetic code in DNA. RNA is also involved in the synthesis of proteins based on the code. You can watch these events unfolding at this link: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0396 | chemistry of living things | T_2247 | The student athlete in Figure 2.13 is practically flying down the track! Running takes a lot of energy. But you dont have to run a race to use energy. All living things need energy all the time just to stay alive. The energy is produced in chemical reactions. A chemical reaction is a process in which some substances, called reactants, change chemically into different substances, called products. Reactants and products may be elements or compounds. Chemical reactions that take place inside living things are called biochemical reactions. Living things depend on biochemical reactions for more than just energy. Every function and structure of a living organism depends on thousands of biochemical reactions taking place in each cell. | text | null |
L_0396 | chemistry of living things | T_2248 | The sum of all of an organisms biochemical reactions is called metabolism. Biochemical reactions of metabolism can be divided into two general categories: catabolic reactions and anabolic reactions. You can watch an animation showing how the two categories of reactions are related at this link: Anabolic reactions involve forming bonds. Smaller molecules combine to form larger ones. These reactions require energy. For example, it takes energy to build starches from sugars. Catabolic reactions involve breaking bonds. Larger molecules break down to form smaller ones. These reactions release energy. For example, energy is released when starches break down to sugars. | text | null |
L_0396 | chemistry of living things | T_2249 | Each of the trillions of cells in your body is continuously performing thousands of anabolic and catabolic reactions. Thats an amazing number of biochemical reactionsfar more than the number of chemical reactions that might take place in a science lab or chemical plant. So many biochemical reactions can take place simultaneously in our cells because biochemical reactions occur very quickly. Thats because of enzymes. Enzymes are proteins that increase the rate of biochemical reactions. Enzymes arent changed or used up in the reactions, so they can be used to speed up the same reaction over and over again. Enzymes are highly specific for certain chemical reactions, so they are very effective. A reaction that would take years to occur without its enzyme might occur in a split second with the enzyme. | text | null |
L_0396 | chemistry of living things | T_2250 | Some of the most important biochemical reactions are the reactions involved in photosynthesis and cellular respira- tion. Photosynthesis is the process in which producers capture light energy from the sun and use it to make glucose. This involves anabolic reactions. Cellular respiration is the process in which energy is released from glucose and stored in smaller amounts in other molecules that cells can use for energy. This involves catabolic reactions. Photosynthesis and cellular respiration together provide energy to almost all living cells. Figure 2.14 shows how photosynthesis and cellular respiration are related. You can read more about both processes in the chapter Cell Functions. | text | null |
L_0398 | the nervous system | T_2257 | Controlling muscles and maintaining balance are just two of the functions of the human nervous system. What else does the nervous system do? It senses the surrounding environment with sense organs that include the eyes and ears. It senses the bodys own internal environment, including its temperature. It controls internal body systems to make sure the body maintains homeostasis. It prepares the body to fight or flee in the case of an emergency. It allows thinking, learning, memory, and language. Remember Hakeem the skater from the first page of the chapter? When Hakeem started to fall off the railing, his nervous system sensed that he was losing his balance. It responded by sending messages to his muscles. Some muscles contracted while other relaxed. As a result, Hakeem gained his balance again. How did his nervous system accomplish all of this in just a split second? You need to know how the nervous system transmits messages to answer that question. | text | null |
L_0398 | the nervous system | T_2258 | The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron. The messages carried by neurons are called nerve impulses. A nerve impulse can travel very quickly because it is an electrical signal. Think about flipping on a light switch when you enter a room. When you flip the switch, electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light. Nonetheless, the light still comes on as soon as you flip the switch. Nerve impulses travel just as quickly through the network of nerves inside the body. | text | null |
L_0398 | the nervous system | T_2259 | The structure of a neuron suits it for its function of transmitting nerve impulses. You can see what a neuron looks like in Figure 20.2. It has a special shape that lets it pass electrical signals to and from other cells. A neuron has three main parts: cell body, dendrites, and axon. 1. The cell body contains the nucleus and other organelles. 2. Dendrites receive nerve impulses from other cells. A single neuron may have thousands of dendrites. 3. The axon passes on the nerve impulses to other cells. It branches at the end into multiple nerve endings so it can transmit impulses to many other cells. | text | null |
L_0398 | the nervous system | T_2260 | There are three basic types of neurons: sensory neurons, motor neurons, and interneurons. All three types must work together to receive and respond to information. 1. Sensory neurons transmit nerve impulses from sense organs and internal organs to the brain via the spinal cord. In other words, they carry information about the inside and outside environment to the brain. 2. Motor neurons transmit nerve impulses from the brain via the spinal cord to internal organs, glands, and muscles. In other words, they carry information from the brain to the body, telling the body how to respond. 3. Interneurons carry nerve impulses back and forth between sensory and motor neurons. | text | null |
L_0398 | the nervous system | T_2261 | The nerve endings of an axon dont actually touch the dendrites of other neurons. The messages must cross a tiny gap between the two neurons, called the synapse. Chemicals called neurotransmitters carry the message across this gap. When a nerve impulse arrives at the end of an axon, neurotransmitters are released. They travel across the synaptic gap to a dendrite of another neuron. The neurotransmitters bind to the membrane of the dendrite, triggering a nerve impulse in the next neuron. You can see how this works in Figure 20.3 and in this animation: The transmission of nerve impulses between neurons is like the passing of a baton between runners in a relay race. After the first runner races, she passes the baton to the second runner. Then the second runner takes over. Instead of a baton, a neuron passes neurotransmitters to the next neuron. | text | null |
L_0398 | the nervous system | T_2262 | The nervous system has two main parts, called the central nervous system and the peripheral nervous system. The peripheral nervous system is described later in this lesson. The central nervous system is shown in Figure 20.4. It includes the brain and spinal cord. | text | null |
L_0398 | the nervous system | T_2263 | The human brain is an amazing organ. It is the most complex organ in the human body. By adulthood, the brain weighs about 3 pounds and consists of billions of neurons. All those cells need a lot of energy. In fact, the adult brain uses almost a quarter of the total energy used by the body! The brain serves as the control center of the nervous system and the body as a whole. It lets us understand what we see, hear, or sense in other ways. It allows us to learn, think, remember, and use language. It controls all the organs and muscles in our body. | text | null |
L_0398 | the nervous system | T_2264 | The brain consists of three major parts, called the cerebrum, cerebellum, and brain stem. You can see these three parts of the brain in Figure 20.5. You can use this interactive animation to explore these parts of the brain: http://s 1. The cerebrum is the largest part of the brain. It controls conscious functions, such as thinking, sensing, speaking, and voluntary muscle movements. Whether you are chatting with a friend or playing a video game, you are using your cerebrum. 2. The cerebellum is the next largest part of the brain. It controls body position, coordination, and balance. Hakeems cerebellum kicked in when he started to lose his balance on the railing in the opening photo. It allowed him to regain his balance. 3. The brain stem (also called the medulla) is the smallest part of the brain. It controls involuntary body functions such as breathing, heartbeat, and digestion. It also carries nerve impulses back and forth between the rest of the brain and the spinal cord. | text | null |
L_0398 | the nervous system | T_2265 | The cerebrum is divided down the middle from the front to the back of the head. The two halves of the cerebrum are called the right and left hemispheres. The two hemispheres are very similar but not identical. They are connected to each other by a thick bundle of axons deep within the brain. These axons allow the two hemispheres to communicate with each other. Did you know that the right hemisphere of the cerebrum controls the left side of the body, and vice versa? This can happen because of the connections between the two hemispheres. Each hemisphere is further divided into four parts, called lobes, as you can see in Figure 20.6. Each lobe has different functions. One function of each lobe is listed in the figure. | text | null |
L_0398 | the nervous system | T_2266 | The spinal cord is a long, tube-shaped bundle of neurons. It runs from the brain stem to the lower back. The main job of the spinal cord is to carry nerve impulses back and forth between the body and brain. The spinal cord is like a two-way road. Messages about the body, both inside and out, pass through the spinal cord to the brain. Messages from the brain pass in the other direction through the spinal cord to tell the body what to do. | text | null |
L_0398 | the nervous system | T_2267 | All the other nervous tissues in the body are part of the peripheral nervous system. If you look again at Figure 20.1, you can see the major nerves of the peripheral nervous system. They include nerves that run through virtually every part of the body, both inside and out, except for the brain and spinal cord. The peripheral nervous system has two main divisions: the sensory division and the motor division. The divisions carry messages in opposite directions. Figure 20.7 shows these divisions of the peripheral nervous system. | text | null |
L_0398 | the nervous system | T_2268 | The sensory division of the peripheral nervous system carries messages from sense organs and internal organs to the central nervous system. For example, it carries messages about images from the eyes to the brain. Once the messages reach the brain, the brain interprets the information. | text | null |
L_0398 | the nervous system | T_2269 | The motor division of the peripheral nervous system carries messages from the central nervous system to muscles, internal organs, and glands throughout the body. The brain sends commands to these tissues, telling them how to respond. As you can see in Figure 20.7, the motor division is divided into additional parts. The autonomic part of the motor division controls involuntary responses. It sends messages to organs and glands. These messages control the body both during emergencies (sympathetic division) and during none- mergencies (parasympathetic division). The somatic part of the motor division controls voluntary responses. It sends messages to the skeletal muscles for movements that are under conscious control. | text | null |
L_0398 | the nervous system | T_2270 | Nervous system problems include diseases and injuries. Most nervous system diseases cant be prevented. But you can take steps to decrease your risk of nervous system injuries. | text | null |
L_0398 | the nervous system | T_2271 | Bacteria and viruses can infect the brain or spinal cord. An infection of the brain is called encephalitis. An infection of the membranes that cover the brain and spinal cord is called meningitis. A vaccine is available to prevent meningitis caused by viruses (see Figure 20.8). Encephalitis and meningitis arent very common, but they can be extremely serious. They may cause swelling of the brain, which can be fatal. Thats why its important to know the symptoms of these diseases. Both encephalitis and meningitis typically cause a severe headache and a fever. Meningitis also causes a stiff neck. Both require emergency medical treatment. | text | null |
L_0398 | the nervous system | T_2272 | Epilepsy is a disease in which seizures occur. A seizure is a period of lost consciousness that may include violent muscle contractions. It is caused by abnormal electrical activity in the brain. Epilepsy may result from an infection, injury, or tumor. In many cases, however, the cause cant be identified. There is no known cure for epilepsy, but the seizures often can be prevented with medicine. Sometimes children with epilepsy outgrow it by adulthood. | text | null |
L_0398 | the nervous system | T_2273 | A stroke occurs when a blood clot blocks blood flow to part of the brain. Brain cells die quickly when their oxygen supply is cut off. Therefore, a stroke may cause permanent loss of normal mental functions. Many stroke patients suffer some degree of paralysis, or loss of the ability to feel or move certain parts of the body. If medical treatment is given very soon after a stroke occurs, some of the damage may be reversed. Strokes occur mainly in older adults. | text | null |
L_0398 | the nervous system | T_2274 | Alzheimers disease is another disease that occurs mainly in older adults. In Alzheimers disease, a person gradually loses most normal mental functions. The patient typically suffers from increasing memory loss, confusion, and mood swings. The cause of Alzheimers isnt known for certain, but it appears to be associated with certain abnormal changes in the brain. There is no known cure for this devastating disease, but medicines may be able to slow its progression. | text | null |
L_0398 | the nervous system | T_2275 | The brain and spinal cord are protected within bones of the skeletal system, but injuries to these organs still occur. With mild injuries, there may be no lasting effects. With severe injuries, there may be permanent disability or even death. Brain and spinal cord injuries most commonly occur because of car crashes or athletic activities. Fortunately, many injuries can be prevented by wearing seat belts and safety helmets (see Figure 20.9). Avoiding unnecessary risks, such as doing stunts on a bike or diving into shallow water, can also reduce the chances of brain and spinal cord injuries. The most common type of brain injury is a concussion. This is a bruise on the surface of the brain. It may cause temporary symptoms such as headache and confusion. Most concussions heal on their own in a few days or weeks. However, repeated concussions can lead to permanent changes in the brain. More serious brain injuries also often cause permanent brain damage. Spinal cord injuries may cause paralysis. Some people recover from spinal cord injuries. However, many people remain paralyzed for life. This happens when the spinal cord can no longer transmit nerve impulses between the body and brain. | text | null |
L_0398 | the nervous system | T_2276 | A drug is any chemical substance that affects the body or brain. Some drugs are medicines. Although these drugs are helpful when used properly, they can be misused like any other drug. Drugs that arent medicines include both legal and illegal drugs. Both can do harm. | text | null |
L_0398 | the nervous system | T_2277 | Many drugs affect the brain and influence how a person feels, thinks, or acts. Such drugs are called psychoactive drugs. They include legal drugs such as caffeine and alcohol, as well as illegal drugs such as cocaine and heroin. They also include certain medicines, such as antidepressant drugs and medical marijuana. Some psychoactive drugs, such as caffeine, stimulate the central nervous system. They may make the user feel more alert. Some psychoactive drugs, such as alcohol, depress the central nervous system. They may make the user feel more relaxed. Still other psychoactive drugs, such as marijuana, are hallucinogenic drugs. They may make the user have altered sensations, perceptions, or thoughts. | text | null |
L_0398 | the nervous system | T_2278 | Psychoactive drugs may bring about changes in mood that users find desirable. These drugs may be abused. Drug abuse is use of a drug without the advice of a medical professional and for reasons not originally intended. Continued use of a psychoactive drug may lead to drug addiction. This occurs when a drug user is unable to stop using the drug. Over time, a drug user may need more of the drug to get the desired effect. This can lead to drug overdose and death. | text | null |
L_0399 | the senses | T_2279 | The ability to see is called vision. It depends on both the eyes and the brain. The eyes sense light and form images. The brain interprets the images formed by the eyes and tells us what we are seeing. For a fascinating account of how the brain helps us see, watch this short video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0399 | the senses | T_2280 | Did you ever use 3-D glasses to watch a movie, like the teens in Figure 20.11? If you did, then you know that the glasses make images on the flat screen seem more realistic by giving them depth. The images seem to jump right out of the screen toward you. Unlike many other animals, human beings and other primates normally see the world around them in three dimen- sions. Thats because we have two eyes that face the same direction but are a few inches apart. Both eyes focus on the same object at the same time but from slightly different angles. The brain uses the different images from the two eyes to determine the distance to the object. Human beings and other primates also have the ability to see in color. We have special cells inside our eyes that can distinguish different wavelengths of visible light. Visible light is light in the range of wavelengths that the human eye can sense. The exact wavelength of visible light determines its color. | text | null |
L_0399 | the senses | T_2281 | The function of the eye is to focus light and form images. We see some objects, such as stars and light bulbs, because they give off their own light. However, we see most objects because they reflect light from another source such as the sun. We form images of the objects when some of the reflected light enters our eyes. Look at the parts of the eye in Figure 20.12. Follow the path of light through the eye as you read about it below. 1. Light from an object passes first through the cornea. This is a clear, protective covering on the outside of the eye. 2. Then light passes through the pupil, an opening in the center of the eye. The pupil, which looks black, is surrounded by the colored part of the eye, called the iris. 3. Light entering through the pupil next passes through the lens. The lens is a clear, curved structure, like the lens of a magnifying glass. Along with the cornea, the lens focuses the light on the back of the eye. 4. The back of the eye is covered by a thin layer called the retina. This is where the image of the object normally forms. The retina consists of special light-sensing cells called rods and cones. Rods sense dim light. Cones sense different colors of light. 5. Nerve impulses from rods and cones travel to the optic nerve. It carries the nerve impulses to the brain. | text | null |
L_0399 | the senses | T_2282 | You probably know people who need eyeglasses or contact lenses to see clearly. Maybe you need them yourself. Lenses are used to correct vision problems. Two of the most common vision problems in young people are myopia and hyperopia. You can compare myopia and hyperopia in Figure 20.13. To learn about astigmatism, another common vision problem, watch this very short video: . MEDIA Click image to the left or use the URL below. URL: Myopia is commonly called nearsightedness. People with myopia can see nearby objects clearly, but distant objects appear blurry. Myopia occurs when images focus in front of the retina because the eyeball is too long. This vision problem can be corrected with concave lenses, which curve inward. The lenses focus images correctly on the retina. Hyperopia is commonly called farsightedness. People with hyperopia can see distant objects clearly, but nearby objects appear blurry. Hyperopia occurs when images focus in back of the retina because the eyeball is too short. This vision problem can be corrected with convex lenses, which curve outward. The lenses focus images correctly on the retina. | text | null |
L_0399 | the senses | T_2283 | Vision is just one of several human senses. Other human senses include hearing, touch, taste, and smell. Imagine shopping at the fruit market in Figure 20.14. It would stimulate all of these senses. You would hear the noisy bustle of the market. You could feel the smooth skin of the fruit. If you tried a sample, you could smell the fruity aroma and taste its sweet flavor. | text | null |
L_0399 | the senses | T_2284 | What do listening to music and riding a bike have in common? Both activities depend on the ears. The ears are organs that sense sound. They also sense the position of the body and help maintain balance. Hearing is the ability to sense sound. Sound travels through the air in waves. Suppose a car horn blows in the distance. Sound waves spread through the air from the horn. Some of the sound waves enter your ears and cause vibrations. The vibrations trigger nerve impulses that travel to the brain through the auditory nerve. You can learn how this happens in Figure 20.15. The brain then interprets the impulses and tells you what you are hearing. To find out how the brain determines where a sound is coming from, watch this amusing video: MEDIA Click image to the left or use the URL below. URL: The parts of the ears involved in balance are the semicircular canals. These are the curved structures above the cochlea in the inner ear in Figure 20.15. Like the cochlea, the semicircular canals contain liquid and are lined with tiny hair cells. As the head changes position, the liquid moves. This causes the hair cells to bend. The bending of the hair cells triggers nerve impulses that travel to the cerebellum in the brain. The cerebellum uses the information to maintain balance. | text | null |
L_0399 | the senses | T_2285 | Touch is the ability to sense pain, pressure, or temperature. Nerve cells that sense touch are found mainly in the skin. The skin on the palms, soles, face, and lips has the most neurons. Neurons that sense pain are also found inside the body inside the body in the tongue, joints, muscles, and other organs. Suppose you wanted to test the temperature of bath water before getting into the tub. You might stick one toe in the water. Neurons in the skin on your toe would sense the temperature of the water and send a message about it to the brain through the spinal cord. The brain would process the information. It might decide that the water is too hot and send a message to your muscles to pull your toe out of the water. | text | null |
L_0399 | the senses | T_2286 | The sense of taste is controlled by sensory neurons on the tongue. They are grouped in bundles called taste buds. You can see taste buds on the tongue in Figure 20.16. Taste neurons sense chemicals in food. They can detect five different tastes: sweet, salty, sour, bitter, and umami, which is a meaty taste. When taste neurons sense chemicals, they send messages to the brain about them. The brain then decides what you are tasting. The sense of smell also involves sensory neurons that sense chemicals. These neurons are found in the nose, and they sense chemicals in the air. Unlike taste neurons, smell neurons can detect thousands of different odors. Your sense of smell plays a big role in your sense of taste. You can use your sense of taste alone to learn that a food is sweet. However, you have to use your sense of smell as well to learn that the food tastes like apple pie. | text | null |
L_0400 | the endocrine system | T_2287 | The endocrine system is a system of glands that release chemical messenger molecules into the blood stream. The messenger molecules are called hormones. Hormones act slowly compared with the rapid transmission of electrical impulses of the nervous system. Endocrine hormones must travel through the bloodstream to the cells they control, and this takes time. On the other hand, because endocrine hormones are released into the bloodstream, they travel to cells everywhere in the body. For a good visual introduction to the endocrine system, watch this short video: http MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0400 | the endocrine system | T_2288 | An endocrine gland is a gland that secretes hormones into the bloodstream for transport around the body (instead of secreting hormones locally, like sweat glands in the skin). Major glands of the endocrine system are shown in Figure 20.17. The glands are the same in males and females except for the ovaries and testes. | text | null |
L_0400 | the endocrine system | T_2289 | The hypothalamus is actually part of the brain, but it also secretes hormones. Some of its hormones go directly to the pituitary gland in the endocrine system. These hypothalamus hormones tell the pituitary to either secrete or stop secreting its hormones. In this way, the hypothalamus provides a link between the nervous and endocrine systems. The hypothalamus also produces hormones that directly regulate body processes. For example, it produces antid- iuretic hormone. This hormone travels to the kidneys and stimulates them to conserve water by producing more concentrated urine. | text | null |
L_0400 | the endocrine system | T_2290 | The pea-sized pituitary gland is just below the hypothalamus and attached directly to it. The pituitary receives hormones from the hypothalamus. It also secretes its own hormones. Most pituitary hormones control other endocrine glands. Thats why the pituitary gland is called the master gland of the endocrine system. Table Pituitary Hormone Adrenocorticotropic (ACTH) hormone Target Glands/Cells adrenal glands Thyroid-stimulating (TSH) Growth hormone (GH) hormone thyroid gland Follicle-stimulating (FSH) hormone body cells ovaries or testes Luteinizing hormone (LH) ovaries or testes Prolactin (PRL) mammary glands Effects(s) Stimulates the cortex (outer layer) of the adrenal glands to secrete their hormones Stimulates the thyroid gland to se- crete its hormones Stimulates body cells to make pro- teins and grow Stimulates the ovaries to develop mature eggs; stimulates the testes to produce sperm Stimulates the ovaries or testes to secrete sex hormones; stimulates the ovaries to release eggs Stimulates the mammary glands to produce milk | text | null |
L_0400 | the endocrine system | T_2291 | There are several other endocrine glands. Find them in Figure 20.17 as you read about them below. The thyroid gland is a relatively large gland in the neck. Hormones secreted by the thyroid gland include thyroxin. Thyroxin increases the rate of metabolism in cells throughout the body. The pancreas is a large gland located near the stomach. Hormones secreted by the pancreas include insulin. Insulin helps cells absorb glucose from the blood. It also stimulates the liver to take up and store excess glucose. The two adrenal glands are glands located just above the kidneys. Each adrenal gland has an outer layer (cortex) and inner layer (medulla) that secrete different hormones. The hormone adrenaline is secreted by the inner layer. It prepares the body to respond to emergencies. For example, it increases the amount of oxygen and glucose going to the muscles. The gonads are glands that secrete sex hormones. Male gonads are called testes. They secrete the male sex hormone testosterone. The female gonads are called ovaries. They secrete the female sex hormone estrogen. Sex hormones stimulate the changes of puberty. They also control the production of sperm or eggs by the gonads. | text | null |
L_0400 | the endocrine system | T_2292 | Endocrine hormones travel throughout the body in the blood. However, each endocrine hormone affects only certain cells, called target cells. | text | null |
L_0400 | the endocrine system | T_2293 | A target cell is the type of cell on which a given endocrine hormone has an effect. A target cell is affected by a given hormone because it has proteins on its surface to which the hormone can bind. When the hormone binds to target cell proteins, it causes changes inside the cell. For example, binding of the hormone might cause the release of enzymes inside the cell. The enzymes then influence cell processes. | text | null |
L_0400 | the endocrine system | T_2294 | Endocrine hormones control many cell activities, so they are very important for homeostasis. But what controls the hormones? Most endocrine hormones are controlled by feedback loops. In a feedback loop, the hormone produced by a gland feeds back to control its own production by the gland. A feedback loop can be negative or positive. Most endocrine hormones are controlled by negative feedback loops. .Negative feedback occurs when rising levels of a hormone feed back to decrease secretion of the hormone or when falling levels of the hormone feed back to increase its secretion. You can see an example of a negative feedback loop in Figure 20.18. It shows how levels of thyroid hormones regulate the thyroid gland. This loop involves the hypothalamus and pituitary gland as well as the thyroid gland. Low levels of thyroid hormones in the blood cause the release of hormones by the hypothalamus and pituitary gland. These hormones stimulate the thyroid gland to secrete more hormones. The opposite happens with high levels of thyroid hormones in the blood. The hypothalamus and pituitary gland stop releasing hormones that stimulate the thyroid. | text | null |
L_0400 | the endocrine system | T_2295 | Diseases of the endocrine system are fairly common. An endocrine disease usually involves the secretion of too much or not enough hormone by an endocrine gland. This may happen because the gland develops an abnormal lump of cells called a tumor. For example, a tumor of the pituitary gland can cause secretion of too much growth hormone. If this occurs in a child, it may result in very rapid growth and unusual tallness by adulthood. This is called gigantism. Type 1 diabetes is another endocrine system disease. In this disease, the bodys own immune system attacks insulin- secreting cells of the pancreas. As a result, not enough insulin is secreted to maintain normal levels of glucose in the blood. Patients with type 1 diabetes must regularly check the level of glucose in their blood. When it gets too high, they must give themselves an injection of insulin to bring it under control. You can learn more about glucose, insulin, and type 1 diabetes by watching this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0401 | infectious diseases | T_2296 | An infectious disease is a disease that is caused by a pathogen. A pathogen is an organism or virus that causes disease in another living thing. Pathogens are commonly called germs. Watch this dramatic video for an historic perspective on infectious diseases and their causes: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0401 | infectious diseases | T_2297 | There are several types of pathogens that cause diseases in human beings. They include bacteria, viruses, fungi, and protozoa. The different types are described in Table 21.1. The table also lists several diseases caused by each type of pathogen. Many infectious diseases caused by these pathogens can be cured with medicines. For example, antibiotic drugs can cure most diseases caused by bacteria. | text | null |
L_0401 | infectious diseases | T_2298 | Different pathogens spread in different ways. Some are easy to catch. Others are much less contagious. Some pathogens spread through food or water. When harmful bacteria contaminate food, they cause foodborne illness, commonly called food poisoning. An example of a pathogen that spreads through water is the protozoan named Giardia lamblia, described in Table 21.1. It causes a disease called giardiasis. Some pathogens spread through sexual contact. In the U.S., the pathogen most commonly spread this way is HPV, or human papillomavirus. It may cause genital warts and certain types of cancer. A vaccine can prevent the spread of this pathogen. Many pathogens spread by droplets in the air. Droplets are released when a person coughs or sneezes, as you can see in Figure 21.2. The droplets may be loaded with pathogens. Other people may get sick if they breathe in the pathogens on the droplets. Viruses that cause colds and flu can spread this way. Other pathogens spread when they are deposited on objects or surfaces. The fungus that causes athletes food spreads this way. For example, you might pick up the fungus from the floor of a public shower. You can also pick up viruses for colds and flu from doorknobs and other commonly touched surfaces. Still other pathogens are spread by vectors. A vector is an organism that carries pathogens from one person or animal to another. Most vectors are insects such as ticks or mosquitoes. They pick up pathogens when they bite an infected animal and then transmit the pathogens to the next animal they bite. Ticks spread the bacteria that cause Lyme disease. Mosquitoes spread the protozoa that cause malaria. | text | null |
L_0401 | infectious diseases | T_2299 | What can you do to avoid infectious diseases? Eating well and getting plenty of sleep are a good start. These habits will help keep your immune system healthy. With a healthy immune system, you will be able to fight off many pathogens. Vaccines are available for some infectious diseases. For example, there are vaccines to prevent measles, mumps, whooping cough, and chicken pox. These vaccines are recommended for infants and young children. You can also take the following steps to avoid picking up pathogens or spreading them to others. Watch this video for additional information on preventing the spread of infectious diseases: MEDIA Click image to the left or use the URL below. URL: Wash your hands often with soap and water. Spend at least 20 seconds scrubbing with soap. See Figure 21.3 for effective hand washing tips. Avoid touching your eyes, nose, or mouth with unwashed hands. Avoid close contact with people who are sick. This includes kissing, hugging, shaking hands, and sharing cups or eating utensils. Cover your coughs and sneezes with a tissue or shirt sleeve, not your hands. Disinfect frequently touched surfaces, such as keyboards and doorknobs, especially if someone is sick. Stay home when you are sick. The best way to prevent diseases spread by vectors is to avoid contact with the vectors. For example, you can wear long sleeves and long pants to avoid tick and mosquito bites. Using insect repellent can also reduce your risk of insect bites. | text | null |
L_0402 | noninfectious diseases | T_2300 | Cancer is a disease in which cells divide out of control. Normally, the body has ways to prevent cells from dividing out of control. However, in the case of cancer, these ways fail. The rapidly dividing cells may form a mass of abnormal tissue called a tumor. This is illustrated in Figure 21.4. Watch this video for an animated introduction to cancer: . MEDIA Click image to the left or use the URL below. URL: As a tumor increases in size, it may harm normal tissues around it. Sometimes cancer cells break away from a tumor. If they enter the bloodstream, they are carried throughout the body. Then the cells may start growing in other tissues. This is usually how cancer spreads from one part of the body to another. Once this happens, cancer is very hard to stop. | text | null |
L_0402 | noninfectious diseases | T_2301 | Most cancers are caused by mutations. Mutations are random errors in genes. Mutations that lead to cancer usually occur in genes that control the cell cycle. Because of the mutations, abnormal cells are allowed to divide. Some mutations that lead to cancer may be inherited. However, most of the mutations are caused by environmental factors. Anything in the environment that can cause cancer is called a carcinogen. Common carcinogens include certain chemicals and some types of radiation. Many different chemicals can cause cancer. For example, tobacco contains dozens of chemicals, including nicotine, that have been shown to cause cancer. Figure 21.5 shows some of these chemicals. Smoking tobacco or using smokeless tobacco increases the risk of cancer of the lung, mouth, throat, and urinary bladder. Types of radiation that cause cancer include ultraviolet (UV) radiation and radon. UV radiation is part of sunlight. It is the leading cause of skin cancer. Radon is a naturally occurring radioactive gas that escapes from underground rocks. It may seep into the basements of buildings. It can cause lung cancer. | text | null |
L_0402 | noninfectious diseases | T_2302 | Cancer occurs most often in adults, especially adults over the age of 50. The most common types of cancer in adults differ between males and females. The most common type of cancer in adult males is cancer of the prostate gland. The prostate gland is part of the male reproductive system. About one third of all cancers in men are prostate cancers. The most common type of cancer in adult females is cancer of the breast. About one third of all cancers in women are breast cancers. In both men and women, the second most common type of cancer is lung cancer. Most cases of lung cancer develop in people who smoke. Childhood cancer is rare. The main type of cancer in children is leukemia. It makes up about one third of all childhood cancers. It occurs when the body makes abnormal white blood cells. | text | null |
L_0402 | noninfectious diseases | T_2303 | Many cases of cancer can be cured if the cancer is diagnosed and treated early. Treatment often involves removing a tumor with surgery. This may be followed by other types of treatments. These treatments may include drugs and radiation, both of which target and kill cancer cells. Its important to know the warning signs of cancer so it can be diagnosed as early as possible. Having warning signs doesnt mean that you have cancer, but you should check with a doctor to be sure. Warning signs of cancer include: a change in bowel or bladder habits. a sore that doesnt heal. unusual bleeding or discharge. a lump in the breast or elsewhere. frequent, long-term indigestion. difficulty swallowing. obvious changes in a wart or mole. persistent cough or hoarseness. | text | null |
L_0402 | noninfectious diseases | T_2304 | Making healthy lifestyle choices can help prevent some types of cancer. For example, you can reduce your risk of lung cancer by not smoking. You can reduce your risk of skin cancer by using sunscreen (see Figure 21.6). | text | null |
L_0402 | noninfectious diseases | T_2305 | Diabetes is another type of noninfectious disease. Diabetes occurs when the pancreas doesnt make enough insulin or else the bodys cells are resistant to the effects of insulin. Insulin is a hormone that helps cells absorb glucose from the blood. When there is too little insulin or cells do not respond to it, the blood contains too much glucose. High glucose levels in the blood can damage blood vessels and other cells in the body. The kidneys work harder to filter the extra glucose from the blood and excrete it in urine. This leads to frequent urination, which in turn causes excessive thirst. Watch this short video for an animated introduction to diabetes, its causes, and its consequences: MEDIA Click image to the left or use the URL below. URL: There are two main types of diabetes: type 1 diabetes and type 2 diabetes. The two types of diabetes have different causes. | text | null |
L_0402 | noninfectious diseases | T_2306 | Type 1 diabetes is caused by the immune system attacking and destroying normal cells of the pancreas. As a result, the cells can no longer produce insulin. Why the immune system acts this way is not known for certain. Its possible that a virus may trigger the attack. This type of diabetes usually develops in childhood or adolescence. At present, there is no known way to prevent the development of type 1 diabetes. However, it is a treatable disease. Treatment of type 1 diabetes includes: taking several insulin injections every day or using an insulin pump (see Figure 21.7). monitoring blood glucose levels several times a day. eating a healthy diet that spreads out carbohydrate intake throughout the day. regular physical activity, which helps the body use insulin more efficiently. regular medical checkups. | text | null |
L_0402 | noninfectious diseases | T_2307 | Type 2 diabetes is much more common than type 1 diabetes. Type 2 diabetes occurs when body cells no longer respond normally to insulin. The pancreas still makes insulin, but the cells of the body cant use it. Being overweight and having high blood pressure increase the chances of developing type 2 diabetes. This type of diabetes usually develops in adulthood. However, it is becoming more common in teens and children because more young people are overweight now than ever before. You can greatly reduce your risk of developing type 2 diabetes by maintaining a healthy body weight. Some cases of type 2 diabetes can be cured with weight loss. However, most people with the disease need to take medicine to control their blood glucose. Regular exercise and balanced eating also help. Like people with type 1 diabetes, people with type 2 diabetes must frequently check their blood glucose. | text | null |
L_0402 | noninfectious diseases | T_2308 | The immune system is the body system that normally fights infections and defends against other causes of disease. When the immune system is working well, it usually keeps you from getting sick. But like any other body system, the immune system can have problems and develop diseases. Two types of immune system diseases are autoimmune diseases and allergies. | text | null |
L_0402 | noninfectious diseases | T_2309 | An autoimmune disease is a disease in which the immune system attacks the bodys own cells. Why this happens is not known for certain, but a combination of genetic and environmental factors are likely to be responsible. Type 1 diabetes is an example of an autoimmune disease. In this case, the immune system attacks cells of the pancreas. Two other examples are multiple sclerosis and rheumatoid arthritis. In multiple sclerosis, the immune system attacks nerve cells. This causes weakness and pain that gradually get worse over time. In rheumatoid arthritis, the immune system attacks joints. This causes joint damage and pain. These diseases cant be prevented and have no known cure. However, they can be treated with medicines that weaken the immune systems attack on normal cells. | text | null |
L_0402 | noninfectious diseases | T_2310 | An allergy is a disorder in which the immune system responds to a harmless substance as though it was a pathogen. Any substance that causes an allergy is called an allergen. The most common allergens are pollen, dust mites, mold, animal dander, insect stings, latex, and certain foods and medications. To see in greater detail how allergies occur, watch this animated video: . MEDIA Click image to the left or use the URL below. URL: Did you ever hear of hay fever? Its not really a fever, and it may have nothing to do with hay. Its actually an allergy to plant pollens. People with this type of allergy generally have seasonal allergies that come back year after year. Symptoms commonly include watery eyes and nasal congestion. Ragweed, shown blooming in Figure 21.8, causes more pollen allergies than any other plant. Allergy symptoms can range from mild to severe. Mild symptoms might include itchy eyes, sneezing, and a runny nose. Severe symptoms can cause difficulty breathing, which may be life threatening. Keep in mind that it is the immune system and not the allergen that causes the allergy symptoms. Allergy symptoms can be treated with medications such as antihistamines. Severe allergic reactions may require an injection of the hormone epinephrine. These treatments lessen or counter the immune systems response. Often, allergy symptoms can be prevented. One way is to avoid exposure to the allergens that cause your symptoms. If you are allergic to pollen, for example, you can reduce your exposure by staying inside when pollen levels are highest. Some people receive allergy shots to help prevent allergic reactions. The shots contain tiny amounts of allergens. After many months or years of shots, the immune system gets used to the allergens and no longer reacts to them. | text | null |
L_0405 | male reproductive system | T_2327 | The male reproductive system has two main functions: producing sperm and releasing testosterone. Sperm are male gametes, or reproductive cells. Sperm form when certain cells in the male reproductive system divide by meiosis to form haploid cells. Being haploid means they have half the number of chromosomes of other cells in the body. An adult male may produce millions of sperm each day! Testosterone is the major sex hormone in males. Testosterone has two primary roles: 1. During adolescence, testosterone causes most of the changes associated with puberty. It causes the reproduc- tive organs to mature. It also causes other adult male traits to develop. For example, it causes the voice to deepen and facial hair to start growing. 2. During adulthood, testosterone is needed for the production of sperm. | text | null |
L_0405 | male reproductive system | T_2328 | The male reproductive organs include the penis, testes, epididymis, vas deferens, and prostate gland. These organs are shown in Figure 22.1. The figure also shows some other parts of the male reproductive system. Find each organ in the drawing as you read about it below. For a cartoon about the male reproductive system, watch this video: http MEDIA Click image to the left or use the URL below. URL: The penis is an external, cylinder-shaped organ that contains the urethra. The urethra is the tube that carries urine out of the body. It also carries sperm out of the body. The two testes (testis, singular) are oval organs that produce sperm and secrete testosterone. They are located inside a sac called the scrotum that hangs down outside the body. The scrotum also contains the epididymis. | text | null |
L_0405 | male reproductive system | T_2329 | Sperm are tiny cells. In fact, they are the smallest of all human cells. They have a structure that suits them well to perform their function. | text | null |
L_0405 | male reproductive system | T_2330 | As you can see in Figure 22.2, a sperm has three main parts: the head, connecting piece (or midpiece), and tail. 1. The head of the sperm contains the nucleus. The nucleus holds the chromosomes. In humans, the nucleus of a sperm cell contains 23 chromosomes. The acrosome on the head contains enzymes that help the sperm penetrate an egg. 2. The connecting piece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy to move. 3. The tail of the sperm moves like a propeller. It spins around and around and pushes the sperm forward. Sperm can travel about 30 inches per hour. | text | null |
L_0405 | male reproductive system | T_2331 | It takes up to two months for mature sperm to form. The process occurs in several steps: 1. Special cells in the testes go through mitosis to make identical copies of themselves. 2. The copies of the original cells divide by meiosis. This results in haploid cells called spermatids. These cells lack tails and cannot yet swim. 3. Spermatids move from the testes to the epididymis, where they slowly mature. For example, they grow a tail and lose some of the cytoplasm from the head. 4. Once sperm are mature, they can swim. The mature sperm remain in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with secretions from these glands, forming semen. Semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as half a billion sperm! | text | null |
L_0406 | female reproductive system | T_2332 | Two functions of the female reproductive system are similar to the functions of the male reproductive system: producing gametes and secreting a major sex hormone. In the case of females, however, the gametes are eggs, and they are produced by the ovaries. The hormone is estrogen, which is the main sex hormone in females. Estrogen has two major roles: During adolescence, estrogen causes the changes of puberty. It causes the reproductive organs to mature. It also causes other female traits to develop. For example, it causes the breasts to grow and the hips to widen. During adulthood, estrogen is needed for a woman to release eggs from the ovaries. The female reproductive system has another important function, which is not found in males. It supports a baby as it develops before birth. It also gives birth to the baby at the end of pregnancy. | text | null |
L_0406 | female reproductive system | T_2333 | The female reproductive organs include the ovaries, fallopian tubes, uterus, and vagina. These organs are shown in Figure 22.3, along with some other structures of the female reproductive system. Find each organ in the drawing as you read about it below. For a cartoon about the female reproductive system, watch this video: http://education-por The two ovaries are small, oval organs on either side of the abdomen. Each ovary contains thousands of eggs. However, the eggs do not develop fully until a female has gone through puberty. Then, about once a month, an egg is released by one of the ovaries. The ovaries also secrete estrogen. The two fallopian tubes are thin tubes that are connected to the uterus and extend almost to the ovaries. The upper end of each fallopian tube has fingers (called fimbriae) that sweep an egg into the fallopian tube when it is released by the ovary. The egg then passes through the fallopian tube to the uterus. If an egg is fertilized, this occurs in the fallopian tube. The uterus is a hollow organ with muscular walls. The uterus is where a baby develops until birth. The walls of the uterus stretch to accommodate the growing fetus. The muscles in the walls contract to push the baby out during birth. The uterus is connected to the vagina by a small opening called the cervix. The vagina is a cylinder-shaped organ that opens to the outside of the body. The other end joins with the uterus. Sperm deposited in the vagina swim up through the cervix, into the uterus, and from there into a | text | null |
L_0406 | female reproductive system | T_2334 | When a baby girl is born, her ovaries contain all of the eggs they will ever produce. But these eggs are not fully developed. They develop only after the female reaches puberty at about age 12 or 13. Then, just one egg develops each month until she reaches her 40s or early 50s. | text | null |
L_0406 | female reproductive system | T_2335 | Human eggs are very large cells. In fact, they are the largest of all human cells. You can even see an egg without a microscope. Its almost as big as the period at the end of this sentence. Like a sperm cell, an egg cell is a haploid cell with half the number of chromosomes of other cells in the body. Unlike a sperm cell, the egg lacks a tail and contains a lot of cytoplasm. | text | null |
L_0406 | female reproductive system | T_2336 | Egg production takes place in the ovaries. It occurs in several steps: 1. Before birth, special cells in the ovaries go through mitosis to make identical daughter cells. 2. The daughter cells then start to divide by meiosis. However, they go though only the first of the two cell divisions of meiosis at this time. They remain in that stage until the girl goes through puberty. 3. After puberty, an egg develops in an ovary about once a month. As you can see in Figure 22.4, the egg rests in a nest of cells called a follicle. The follicle and egg grow larger and go through other changes. 4. After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation. After ovulation occurs, the moving fingers of the nearby fallopian tube sweep the egg into the tube. Fertilization may occur if sperm reach the egg while it is passing through the fallopian tube. If this happens, the egg finally completes meiosis. This results in two daughter cells that differ in size. The smaller cell is called a polar body. It soon breaks down and disappears. The larger cell is the fertilized egg, which will develop into a new human being. | text | null |
L_0406 | female reproductive system | T_2337 | Egg production in the ovary is part of the menstrual cycle. The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month on average. These changes include the development of an egg and follicle in the ovary. While the egg is developing, other changes are taking place in the uterus. It develops a thick lining that is full of tiny blood vessels. The lining prepares the uterus to receive a fertilized egg if fertilization actually takes place. If fertilization doesnt occur, the egg passes through the uterus and vagina and out of the body. The lining of the uterus also breaks down. Blood and other tissues from the lining pass through the vagina and leave the body. This is called menstruation. Menstruation is also called a menstrual period. It typically lasts about 4 days. When the menstrual period ends, the cycle begins repeats. | text | null |
L_0407 | reproduction and life stages | T_2338 | When a sperm penetrates the cell membrane of an egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote, contains the diploid number of chromosomes. Half of the chromosomes come from the egg, and half come from the sperm. You can watch the process of fertilization and the development of a baby until birth in this amazing video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0407 | reproduction and life stages | T_2339 | The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. As it travels, it divides many times by mitosis. It soon forms a tiny, fluid-filled ball of cells called a blastocyst. The blastocyst has an inner and outer layer of cells, as you can see in Figure 22.5. The inner layer, called the embryoblast, will develop into the new human being. The outer layer, called the trophoblast, will develop into other structures needed to support the new organism. | text | null |
L_0407 | reproduction and life stages | T_2340 | The blastocyst continues down the fallopian tube until it reaches the uterus, about 4 or 5 days after fertilization. When the outer cells of the blastocyst contact cells lining the uterus (the endometrium in Figure 22.5), the blastocyst embeds itself in the uterine lining. This process is called implantation. It generally occurs about a week after fertilization. | text | null |
L_0407 | reproduction and life stages | T_2341 | After implantation occurs, the blastocyst is called an embryo. The embryonic stage lasts from the end of the first week following fertilization through the end of the eighth week. During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues. Most organs also start to form. You can see some of the specific changes that take place during weeks four to eight of the embryonic period in Figure 22.6. By the end of week eight, the embryo is about 30 millimeters (just over 1 inch) in length. It may also have begun to move. | text | null |
L_0407 | reproduction and life stages | T_2342 | From the eighth week following fertilization until birth, the developing human being is called a fetus. Birth typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks. During this time, the organs complete their development. The fetus also grows rapidly in length and weight. Some of the specific changes that occur during the fetal stage are listed in Figure 22.7. By the 38th week, the fetus is fully developed and ready to be born. A 38-week fetus normally ranges from about 36 to 51 centimeters (1420 inches) in length and weighs between 2.7 and 4.6 kilograms (about 610 pounds). | text | null |
L_0407 | reproduction and life stages | T_2343 | The fetus could not grow and develop without oxygen and nutrients from the mother. Wastes from the fetus also must be removed in order for it to survive. The exchange of these substances between the mother and fetus occurs through the placenta. The placenta is a temporary organ that starts to form shortly after implantation. It forms from the trophoblast layer of cells in the blastocyst and from maternal cells in the uterus. The placenta continues to develop and grow to meet the needs of the growing fetus. A fully developed placenta, like the one in Figure 22.8, is made up of a large mass of blood vessels from both mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mothers and fetuss blood to exchange substances across their capillary walls without the blood actually mixing. The fetus is connected to the placenta through the umbilical cord. This is a long tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries. It exchanges gases and other substances with the mothers blood. Then it travels back to the fetus through the umbilical vein. Another structure that supports the fetus is the amniotic sac. This is a membrane that surrounds and protects the fetus. It contains amniotic fluid, which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury. | text | null |
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