Hypovolemia

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, with signs and symptoms including increased heart rate, low blood pressure, pale or cold skin, and altered mental status. A person exhibiting these signs is experiencing a medical emergency and needs the lost fluid volume replaced.

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.^{[citation needed]}

The causes of hypovolemia can be characterized into two categories:

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.^{[citation needed]}

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.^{[citation needed]}