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).
Investigation
In a hospital, physicians respond to a case of hypovolemic shock by conducting these investigations:
Blood tests: U+Es/Chem7, full blood count, glucose, blood type and screen
Central venous catheter
Arterial line
Urine output measurements (via urinary catheter)
Blood pressure
SpO2 oxygen saturation monitoring

Stages
Untreated hypovolemia can lead to shock (see also: hypovolemic shock). Most sources state that there are 4 stages of hypovolemia and subsequent shock; however, a number of other systems exist with as many as 6 stages.The 4 stages are sometimes known as the "Tennis" staging of hypovolemic shock, as the stages of blood loss (under 15% of volume, 15–30% of volume, 30–40% of volume and above 40% of volume) mimic the scores in a game of tennis: 15, 15–30, 30–40 and 40. It is basically the same as used in classifying bleeding by blood loss. The signs and symptoms of the major stages of hypovolemic shock include:

Treatment
Field care
The most important step in treatment of hypovolemic shock is to identify and control the source of bleeding.Medical personnel should immediately supply emergency oxygen to increase efficiency of the patients remaining blood supply. This intervention can be life-saving.Also, the respiratory pump is especially important during hypovolemia as spontaneous breathing may help reduce the effect of this loss of blood pressure on stroke volume by increasing venous return.The use of intravenous fluids (IVs) may help compensate for lost fluid volume, but IV fluids cannot carry oxygen the way blood does—however, researchers are developing blood substitutes that can.