Describe the different types of thirst and their basic biological mechanisms.
What will be an ideal response?
Thirst can be initiated through a detection of low fluid volume in body cells, called osmotic thirst, or through a detection of low blood volume due to inadequate water, called hypovolemic thirst. Third ventricle brain areas, especially the organum vasculosum lamina terminalis, detect osmotic thirst and communicate the need to drink to the median preoptic nucleus of the hypothalamus, which prompts drinking. Since hypovolemic thirst involves reductions in blood volume, this type of thirst is first detected in the body - mainly where large veins connect to the heart - or in the kidneys. Hypovolemia signals from the heart are sent to the nucleus of the solitary tract of the medulla via the vagus nerve. The median preoptic nucleus of the hypothalamus then receives the signal and, like in osmotic thirst, prompts drinking. Signals from the kidneys are sent via hormones; reduced blood volume results in release of renin, which then prompts production of angiotensin II. The subfornical organ in the brain receives the hormone signal and signals the median preoptic nucleus of the hypothalamus, which again prompts drinking.
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