For the supply of tissues with oxygen and nutrients it is vital to maintain the circulation and in this regard the maintenance of intravascular volume is the single most important parameter.

A reduction of the circulating blood volume is called hypovolaemia and can be the consequence of a blood or plasma loss (absolute hypovolaemia) or a vasodilation (relative hypovalaemia). Hypovolaemia increases the sympathetic nervous system activity and activates the renin-angiotensin-aldosterone system. In the adult a satisfactory compensation can only be reached, however, if the lack of volume in the circulation does not surpass 10-15% of the normal volume. Hypovolaemia above this level can no longer be compensated for through these mechanisms alone and will result in a sustained fall of blood pressure and increased heart rate. A reduced cardiac preload as a consequence of diminished venous return reduces stroke volume and hence cardiac output. The ensuing disturbance of macro and microcirculation leads to hypoxia which itself induces endothelial damage with activation of blood coagulation and diminishing blood flow, especially in the microcirculation. The reduced blood flow allows rouleau formation of the red blood cells, further deteriorating the blood flow through the capillaries. The net result is a failure of the circulatory system to maintain adequate organ perfusion and oxygenation, organ failures and death.