Because thirst is the major factor controlling water intake, the physiological regulation of thirst is associated with the need to maintain a relatively stable volume of TBW. Although water is lost from the body continually, albeit usually in relatively small amounts, and hence the body is almost always developing a water deficit, water intake is intermittent. The amount of fluid usually ingested is in excess of that required to replace the losses incurred since the last water intake. The factors that initiate, maintain, and end the drinking response are various and are not fully understood. However, because the regulation of the volume and composition of the various water pools of the body play an essential role in controlling the perception of thirst, an understanding of the homeostatic mechanisms involved has given us the best insight we have into the complexities of the perception of thirst.
The total volume, distribution, and composition of body fluids must be regulated within narrow limits for normal cellular function to be maintained. Body water is passively distributed between the extracellular and intracellular pools according to osmotic, oncotic, and hydrostatic forces as shown in Figure 2. The sodium and chloride contents of the extracellular fluid constitute the two greatest osmo-tically active components of this fluid and are therefore important in maintaining its volume. Potassium, phosphate, and protein fulfill a similar role in regulating the intracellular fluid volume. The distribution of water between the intravascular and extravascular pools is dependent on the balance of hydrostatic and oncotic pressures across the capillaries and postcapillary venules.
Figure 2 Diagramatical representation of the forces that regulate the distribution of the body water pools. The volumes given are those determined in a single male subject with a lean body mass of 75.8 kg.
Variation in the water-to-solute ratio of a body fluid pool results in changes in the tonicity and hence effective osmolality of the fluid. Because the various body water pools are in dynamic equilibrium with each other (Figure 2) there is a tendency for adjustments to occur throughout the body as water moves from regions of low solute concentration to those of higher solute concentration. Changes in plasma osmolality are relatively easy to monitor; therefore, there is a tendency to equate changes in the circulation as the effector of fluid balance control. However, it is important to remember that any alteration in one body pool will affect the others and that receptors that initiate responses affecting water balance may reside at sites far removed from the circulation.
Loss of water from the body or an increase in the circulating solute concentration cause an increase in the osmolality of primarily the extracellular fluid; water then moves into the extracellular space from the cells producing a reduction in cell volume. Changes in plasma osmolality are therefore thought to be signaled to the effector mechanisms by changes in the cell volume of specific specialized cells, collectively termed osmoreceptors. Because the main solute determining the tonicity of the extracellular fluid is sodium, there has been debate about whether the receptor cells detect changes in osmolality or changes in sodium ion content. The evidence suggests that at least the majority of the receptors respond to osmolality rather than to sodium concentration. These osmoreceptors have a regulatory role not only in the perception of thirst but also in the maintenance of the circulating levels of hormones that regulate the excretion of water and solute by the kidneys (Figure 3). Because increases in the extracellular osmolality effectively decrease the volume of the cells in the body, this form of dehydration is termed cellular dehydration.
Alteration in the volume of the extracellular fluid pool without changes in its osmolality also affects the fluid balance hormone concentrations and the sensation of thirst. Changes in the volume of blood in the circulation affect the blood and capillary pressures and atrial filling pressure. The effect on capillary pressure will tend to redistribute body water and help to adjust the circulating fluid volume, and the change in venous return to the heart will alter the cardiopulmonary and arterial stretch receptor (baroreceptor) activity. The level of afferent activity from these baroreceptors directly affects both the sensation of thirst and the secretion of some fluid balance hormones. Additionally, modifications to the arterial blood pressure can directly affect renal perfusion, which together with baroreceptor activity to the kidneys regulates the renin-angiotensin system (Figure 4). Although the effect on the kidneys can influence the perception of thirst, the main renal response is to regulate urine water and solute excretion. A decrease in the volume of the extracellular pool with no concomitant change in plasma osmolality is termed extracellular dehydration.
When humans are given access to fluids after the development of a water deficit, their drinking response usually follows a pattern of rapid ingestion of more than 50% of the total intake followed by intermittent consumption of relatively small volumes of drink over a longer period. Although initiation of the response to drink is due mainly to osmotic or blood volume (volemic) changes, there appear to be
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