Important than preserving plasma tonicity.Ī rise in circulating angiotensin II following activation of the renin–angiotensin system in response to significant extracellular volume depletion can also stimulate sensation of thirst. If there is a concomitant fall in tonicity and pressure, baroreceptors will in effect “override” osmoreceptors, as maintenance of circulating volume is more The resulting dilution of the ECF acts as negative feedback on the hypothalamic osmoreceptors.īaroreceptors located in the walls of the aorta, the great veins, right atrium and carotid artery are less sensitive than osmoreceptors but will respond to a 10% fall in intravascular volume or to hypotension with a more pronounced rise in ADH and stimulation of thirst. ADH increases the water permeability of the collecting tubules in the kidney via expression of aquaporin2 and thereby improves water reabsorption. Thirst is defined as the “urge to drink water” and will prompt enteral intake of fluids. A rise above 295 mosmol/l will result in activation of the “Thirst Centre” located in the anterior hypothalamic structures and cause increased release of ADH.
Osmoreceptors in the hypothalamus react to minute changes in ECF tonicity. The two main factors in the human body that result in changes in water intake or excretion are the sensation of thirst and levels of antidiuretic hormone (ADH/ vasopressin). Physiological regulation of fluid balance As a result, proteins are the major determinant of water movement between the interstitial and the intravascular compartments (see Table 3). In contrast, the capillary endothelium is freely permeable to both water and ions, but is relatively impermeable to larger molecules such as proteins. If ECF tonicity is decreased, the reverse applies. An increase in ECF tonicity results in the movement of water out of cells into the extracellular space.
Variation in ECF tonicity is the main determinant of the relative distribution of total body water between the ICF and ECF compartments. Because the changes in chloride are largely secondary to changes in sodium, the amount of sodium in the ECF is the most important determinant of ECF volume.Ĭell membranes separating ICF and ECF compartments are selective to solutes but allow water to diffuse freely between the two compartments to ensure iso-osmolarity. The volume of ECF is determined mainly by the total amount of osmotically active solutes (sodium and chloride). The electrolyte concentration varies from one compartment to another, as illustrated in Table 2. The solutes found in body fluids include non-electrolytes (eg, protein, urea, glucose, oxygen, carbon dioxide and organic acids) and electrolytes (eg, sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate and sulphate). Interstitial fluid is mostly found in tissues adjacent to the microvascular circulation more distant connective tissues remain relatively dry. The ECF compartment is further divided into: the intravascular fluid or plasma compartment and the interstitial fluid compartment, which is outside the vascular system. The remaining one-third of TBW is found outside of cells, the extracellular fluid (ECF) compartment. In adults about two-thirds of TBW is contained within cells, the intracellular fluid (ICF) compartment (Figure 1). Because fat is essentially water-free while muscle has a high water content, TBW as a percentage of actual body weight varies between groups and individuals, as illustrated in Table 1. Total body water (TBW) is consistently 70% of the lean body weight across age and sex ranges. It is the solvent in which solutes in the body are either suspended or dissolved. Water is the largest single component of the body. It is therefore important for pharmacists to understand basic fluid physiology to be able to advise in safe prescribing and administration of parenteral fluid therapy.
An expert introduction for pharmacists to the elements of fluid physiology and the basis of fluid therapyĪdequate hydration is essential for the human body to maintain both cell metabolism and perfusion of vital organs.