Loss of blood to the outside, in tissue or in body cavities or caverns, as well as plasma or fluid losses (e.g. when sweating , when suffering from burns , with persistent and severe diarrhea ) can be the cause of hypovolemia. This is then expressed by a small blood pressure amplitude (pulse pressure), drop in blood pressure , increase in pulse rate, a shock index - i.e. the ratio of pulse to systolic blood pressure - above 1.0 / min / mmHg , inadequate peripheral blood flow, low central venous pressure and decreased Urinary excretion. Hypovolemia can therefore also be a complication of diabetes mellitus , where the osmotic activity of glucose during urine excretion leads to a high loss of fluid and electrolytes .
The physical registration of hypovolemia takes place in vessels close to the heart via baroreceptors and in the kidneys via volume sensors. These initiate changes in the kidneys that aim to maintain the water reserve and lead to an increase in thirst . The mode of action is indirect via baroreceptors via an increased release of the antidiuretic hormone and directly via volume receptors.
Depending on the mechanism that triggers the shock, a distinction is made between absolute and relative hypovolemia. Absolute hypovolemia is triggered by the loss of circulating volume to the outside or inside; in relative hypovolemia, volume is redistributed in the body periphery due to cardiac or vascular insufficiency.
In both cases the venous return flow to the heart is reduced. Hypovolemia requires medical treatment, it can lead to hypovolemic shock , and 50% blood loss is almost always fatal without therapy.
If neither hypovolemia nor hypervolemia is present, this can be described as volume homeostasis, isovolemia or normovolemia , the organism has exactly the physiological amount of fluid available in the correct distribution within the compartments .