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What is Hypovolemia and explain in detail?
Hypovolemia, also known as volume depletion or volume contraction, is a state of abnormally low extracellular fluid in the body. This may be due to either a loss of both salt and water or a decrease in blood volume. Hypovolemia refers to the loss of extracellular fluid and should not be confused with dehydration.Hypovolemia is caused by a variety of events, but these can be simplified into two categories: those that are associated with kidney function and those that are not. The signs and symptoms of hypovolemia worsen as the amount of fluid lost increases. Immediately or shortly after mild fluid loss (from blood donation, diarrhea, vomiting, bleeding from trauma, etc.), one may experience headache, fatigue, weakness, dizziness, or thirst. Untreated hypovolemia or excessive and rapid losses of volume may lead to hypovolemic shock. Signs and symptoms of hypovolemic shock include increased heart rate, low blood pressure, pale or cold skin, and altered mental status. When these signs are seen, immediate action should be taken to restore the lost volume.

Signs and symptoms
Signs and symptoms of hypovolemia progress with increased loss of fluid volume.Early symptoms of hypovolemia include headache, fatigue, weakness, thirst, and dizziness. The more severe signs and symptoms are often associated with hypovolemic shock. These include oliguria, cyanosis, abdominal and chest pain, hypotension, tachycardia, cold hands and feet, and progressively altering mental status.

Causes
The causes of hypovolemia can be characterized into two categories:

Kidney
Loss of body sodium and consequent intravascular water (due to impaired reabsorption of salt and water in the tubules of the kidneys)
Osmotic diuresis: the increase in urine production due to an excess of osmotic (namely glucose and urea) load in the tubules of the kidneys
Overuse of pharmacologic diuretics
Impaired response to hormones controlling salt and water balance (see mineralocorticoids)
Impaired kidney function due to tubular injury or other diseases

Other
Loss of bodily fluids due to:Gastrointestinal losses; e.g. vomiting and diarrhea
Skin losses; e.g. excessive sweating and burns
Respiratory losses; e.g. hyperventilation (breathing fast)
Build up of fluid in empty spaces (third spaces) of the body due to:Acute pancreatitis
Intestinal obstruction
Increase in vascular permeability
Hypoalbuminemia
Loss of blood (external or internal bleeding or blood donation)

Pathophysiology
The signs and symptoms of hypovolemia are primarily due to the consequences of decreased circulating volume and a subsequent reduction in the amount of blood reaching the tissues of the body. In order to properly perform their functions, tissues require the oxygen transported in the blood. A decrease in circulating volume can lead to a decrease in bloodflow to the brain, resulting in headache and dizziness.Baroreceptors in the body (primarily those located in the carotid sinuses and aortic arch) sense the reduction of circulating fluid and send signals to the brain to increase sympathetic response (see also: baroreflex). This sympathetic response is to release epinephrine and norepinephrine, which results in peripheral vasoconstriction (reducing size of blood vessels) in order to conserve the circulating fluids for organs vital to survival (i.e. brain and heart). Peripheral vasoconstriction accounts for the cold extremities (hands and feet), increased heart rate, increased cardiac output (and associated chest pain). Eventually, there will be less perfusion to the kidneys, resulting in decreased urine output.

Diagnosis
Hypovolemia can be recognized by a fast heart rate, low blood pressure, and the absence of perfusion as assessed by skin signs (skin turning pale) and/or capillary refill on forehead, lips and nail beds.  The patient may feel dizzy, faint, nauseated, or very thirsty.  These signs are also characteristic of most types of shock.In children, compensation can result in an artificially high blood pressure despite hypovolemia (a decrease in blood volume). Children typically are able to compensate (maintain blood pressure despite hypovolemia) for a longer period than adults, but deteriorate rapidly and severely once they are unable to compensate (decompensate)