Physiology of human fluid balance

The physiology of fluid balance in humans should be understood and applied in clinical practice. Fluid balance, when managed accurately and safely, can prevent significant morbidity and mortality. Anaesthesia and critical care patients are often fasted and under physiological stress. Therefore, homeostatic regulation of fluid balance is impaired. A disturbance in normal fluid balance induces a physiological ‘stress’ response via metabolic, neuroendocrine and immune-mediated systems. Critically unwell patients may suffer morbidity secondary to high-volume fluid losses or oedema. There are three fluid compartments discussed in relation to human fluid balance. The intracellular space is surrounded by extracellular fluid, separated by a water permeable cell membrane. Extracellular fluid (ECF) compartment volume and electrolyte concentration, majorly sodium, must be tightly regulated to avoid osmosis and cell damage. The renal system maintains ECF volume by regulating sodium and osmotic concentration by retaining or excreting water.