the heart is divided into how many parts
four
title: Heart text: Heart The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, as well as assists in the removal of metabolic wastes. In humans, the heart is located between the lungs, in the middle compartment of the chest. In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the "right heart" and their left counterparts as the "left heart". title: Heart text: the aorta. Two small openings above the aortic valve carry blood to the heart itself, the left main coronary artery and the right coronary artery. The heart wall is made up of three layers: the inner endocardium, middle myocardium and outer epicardium. These are surrounded by a double-membraned sac called the pericardium. The innermost layer of the heart is called the endocardium. It is made up of a lining of simple squamous epithelium, and covers heart chambers and valves. It is continuous with the endothelium of the veins and arteries of the heart, and is joined to the myocardium with title: Heart text: and up to 1200 bpm in the ruby-throated hummingbird. Adult amphibians and most reptiles have a double circulatory system, meaning a circulatory system divided into arterial and venous parts. However, the heart itself is not completely separated into two sides. Instead, it is separated into three chambers – two atria and one ventricle. Blood returning from both the systemic circulation and the lungs is returned, and blood is pumped simultaneously into the systemic circulation and the lungs. The double system allows blood to circulate to and from lungs which deliver oxygenated blood directly to the heart. In reptiles, the heart title: Heart text: is usually situated around the middle of the thorax, and in snakes, usually between the junction of the upper first and second third. There is a heart with three chambers: two atria and one ventricle. The ventricle is incompletely separated into two halves by a wall (septum), with a considerable gap near the pulmonary artery and aortic openings. In most reptilian species, there appears to be little, if any, mixing between the bloodstreams, so the aorta receives, essentially, only oxygenated blood. The exception to this rule is crocodiles, which have a four-chambered heart. In the heart of lungfish, the septum title: Atrium (heart) text: the third chamber, the ventricle, before it reaches the conus anteriosus, which itself is connected to the ventral aorta. This is considered a primitive arrangement, and many vertebrates have condensed the atrium with the sinus venosus and the ventricle with the conus anteriosus. With the advent of lungs came a partitioning of the atrium into two parts divided by a septum. Among frogs, the oxygenated and deoxygenated blood are mixed in the ventricle before being pumped out to the body's organs; in turtles, the ventricle is almost entirely divided by a septum, but retains an opening through which some mixing