Mechanisms of Thirst Regulation

The sensation of thirst is regulated separately by both the osmotic pressure and the volume of the body fluids and as such is closely related to the control mechanisms that are responsible for the secretion of the fluid balance hormones, which affect water and solute reabsorption in the kidneys and play a role in blood pressure control. These hormones—arginine vasopressin, atrial natriuretic peptide, oxytocin, and the renin-angiotensin-aldosterone system—are central to the regulation of thirst. The hypothalamus and forebrain appear to be the main areas involved in the control of thirst and antidiuresis, and collectively these parts of the brain have been termed the

Hour of day

Figure 6 Changes in body weight during 13 h in the desert. The majority of the volume of drink ingested was associated with food intake. Sweat loss varied from 150 to 700ml/h, and total fluid intake was 3.051. At the end of the period body mass was essentially the same at the beginning and end of the day; therefore, water intake and output were equal. (From Adolph ED, Physiology of Man in the Desert. Copyright © 1947. Reprinted by permission of John Wiley & Sons Inc.)

Hour of day

Figure 6 Changes in body weight during 13 h in the desert. The majority of the volume of drink ingested was associated with food intake. Sweat loss varied from 150 to 700ml/h, and total fluid intake was 3.051. At the end of the period body mass was essentially the same at the beginning and end of the day; therefore, water intake and output were equal. (From Adolph ED, Physiology of Man in the Desert. Copyright © 1947. Reprinted by permission of John Wiley & Sons Inc.)

thirst control centres. Neurons that are responsive to changes in osmolality, intravascular volume (vole-mia), and blood pressure are found within these areas of the brain, as are other receptors that are responsive to many of the fluid balance hormones. Neural pathways from the thirst control centers and the kidneys may allow some direct integration between the control of thirst and excretion, whereas within the brain all of the major fluid balance hormones are present as neurohormones. Afferent input from systemic receptors monitoring osmolality, circulating sodium concentration, and changes in intravascular volume and pressure also have roles in controlling the feeling of thirst. Therefore, there appears to be a complex integrated system for both monitoring the status of the body water pools and controlling intake and excretion (Figures 3 and 4). Many of the regulatory mechanisms controlling water balance appear to overlap, with several stimuli appearing to subserve the same response; however, it is assumed that this effect is required in order to ensure that the blocking of one type of stimulus will not prevent homeostatic control.

Osmotic Regulation of Thirst

The osmolality of circulating plasma is normally maintained within a very narrow limit between 270 and 295 mosmol/kg, with the circulating levels of the antidiuretic hormone arginine vasopressin playing a major role in its homeostatic regulation. An increase of as little as 2 or 3% in plasma osmol-ality is sufficient to produce a strong sensation of thirst and a significant increase in circulating argi-nine vasopressin concentration (Figure 7). The osmoreceptors that monitor the tonicity of the body pools appear to reside mainly in an area of the brain that lacks a blood-brain barrier; therefore, they appear to respond mainly to changes that occur in the osmolality of the blood rather than in the cerebral interstitium. Although the changes in the circulating levels of arginine vasopressin and the perception of thirst appear to parallel one another, it is unlikely that the same receptors are responsible for both responses. It is likely that there are different neurons that react to the same stimulus. However, there may be some neurohormonal interaction between the osmotically activated thirst centers and the 'vasopressin-releasing center' in the brain, and arginine vasopressin-responsive neurons have been detected within the thirst centers (Figure 3).

The current theory of the osmotic control of thirst suggests that there is constant output of both inhibitory and excitatory neural activity from the o-o PAVP = 1.48 [Posm - 284.7], r= 0.977 Thirst = 9.06 [Posm - 293.5], r= 0.966

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