The hypothalamus maintains water balance by controlling both water intake and water loss. Electrical stimulation or injection of hypertonic saline into the anterior hypothalamus leads to the desire to drink. Drinking is regulated by changes in plasma osmolality and extracellular fluid (ECF) volume. Depletion in ECF volume leads to thirst. The thirst sensation is mediated partly by vasopressin release from the hypothalamus and also via the renin-angiotensin system. When an individual is dehydrated, plasma osmolality increases and volume of the ECF (therefore plasma volume) decreases. As a consequence, the hypothalamic osmoreceptors and stretch receptors in the large vessels are stimulated and vasopressin is released from the hypothalamus. However, the stimuli for vasopressin release as a result of changes in plasma osmolality and plasma volume may override one another. For instance, during haemorrhage the resulting hypovolaemia increases vasopressin release even when the plasma is hypotonic.
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