In a male adult of 70 kg there will be approximately 42 L of fluid in the body. Of this, approximately 24 L will be contained in the intracellular compartment and 18 L will be in the extracellular compartments (Chapter 27). Loss of volume from any one of these compartments can lead either directly or indirectly to a reduction in circulating volume.

Hypovolaemia may occur for a number of different reasons depending on the type of body fluid lost (Table 35.1). Direct losses arise as a result of whole blood loss (as in haemorrhage), or plasma depletion (as in substantial burns). In contrast, indirect losses (where plasma has diffused from the intravascular to the extracellular space causing interstitial fluid loss) may develop following excessive sweating, severe diarrhoea and vomiting, or increased stoma or urinary output. Homeostatic responses will endeavour to compensate for all of these losses (Figure 35.1), but, depending on the severity and duration of the problem, they may be insufficient to prevent hypovolaemic shock.

Shock begins when the intravascular volume has been decreased by 15% or more. The result is reduced tissue perfusion and inadequate oxygen delivery to the cells, leading to impaired cellular metabolism. Aerobic (with oxygen) processes for the production of ATP (energy) will be replaced by the inferior anaerobic