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Normally we feel thirsty when our mouths and throats are dry. But this is not the whole story. Research has demonstrated the importance of the kidneys and hypothalamus in maintaining thirst homeostasis.
Cells in the lateral preoptic area of the hypothalamus called osmoreceptors are sensitive not only to the relative amount of water in the blood, but, more particularly, to cellular dehydration due to the amount of salt that becomes concentrated as the body dehydrates from loss of water (Peck & Novin, 1971). An increase in the amount of salt in the blood will thus cause the anterior hypothalamus to signal the pituitary gland to secrete antidiuretic hormone (ADH), which causes the kidneys to slow down their production of urine. This restores body fluid. At the same time, the hypothalamus signals the brain's cortex to create a thirst drive to force the increased water intake needed to restore the normal salt level (Epstein, 1982). Prolonged water deprivation also results in the stimulation of receptors in the kidneys. The kidneys then secrete the hormone renin that produces angiotensin in the bloodstream. This causes the kidneys to retain more water and stimulates the subfornical and cortex areas of the brain to create a thirst drive (Simpson & Routtenberg, 1973). Drinking more water then restores homeostasis. Often we drink when we are not particularly thirsty. Kraly (1984) pointed out that we typically drink during mealtime. He suggested that perhaps food causes the gastrointestinal tract to stimulate the thirst drive. It is also likely that we learn the habit of drinking whenever we eat (Logue, 1991). For example, health experts tell us that we should drink 8 glasses of water a day. Thus, although thirst serves as an example of homeostasis, at times nonhomeostatic influences are important in thirst motivation. |
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