Osmolarity and partitioning of fluids

Inappropriate fluid therapy can lead to significant morbidity and mortality. The maintenance of normal cellular and organ function relies on body fluid homeostasis. The concept of total body water compartmentalization, and the essential principles behind fluid physiology such as osmolarity and osmolality, osmosis and tonicity will be discussed. Factors that can influence body fluid distribution across different compartments and capillary fluid dynamics, such as the Starling forces and the role of endothelial glycocalyx layer will be discussed. Sodium, which plays a major role in plasma osmolality and compartment volume, requires tight regulation through various neuroendocrine mechanisms including the renin–angiotensin–aldosterone system (RAAS) and vasopressin. In addition to the regulation of the extracellular environment, cells have a number of specialized transmembrane transporters such as the sodium–potassium pump and the aquaporin channels to modulate water movement in response to osmotic stress.