When a human being's water intake is lower than its level of water loss, two bodily processes are set in motion. First, the person becomes thirsty and drinks water (provided it is available). Second, the kidneys start to retain water by reabsorbing it and concentrating the urine. Thus, the kidneys can conserve the water that is already in the body. These processes are set in motion by the central nervous system (CNS).
The CNS responds to two primary internal bodily mechanisms. One is cellular dehydration thirst, and the other is hypovolemic thirst (a change in the volume of water in the body). In order to understand these mechanisms, one must realize that the body contains two main supplies of water. One supply, the intracellular fluid, is in the cells; the other supply consists of the extracellular fluid surrounding the cells and tissues and the fluid in the circulatory system. Water moves between these two areas by means of a process called osmosis, which causes it to move from an area of higher concentration to an area of lower concentration.
A person who is deprived of water will experience cellular dehydration thirst as a result of water loss caused by perspiration and excretion through the urine. This increases the salt concentration in the extracellular fluid, thereby lowering the water concentration. Thus, the cells lose their water to the surrounding extracellular fluid. The increasing salt concentration triggers specialized osmoreceptors located in the hypothalamic region of the brain. Two events occur: First, drinking is stimulated; second, antidiuretic hormone (ADH) is secreted from the pituitary gland in the brain. The ADH helps to promote the reabsorption of water into the kidneys.
The second kind of thirst, hypovolemic thirst, occurs when there is a decrease in the volume of the extracellular fluid as a result of bleeding, diarrhea, or vomiting. This produces a decrease in the salt concentration of the extracellular fluid, which lowers the blood pressure, which in turn stimulates the kidney cells to release a chemical. Eventually, the thirst receptors in the hypothalamus are stimulated; these cause the organism to consume water. In addition, ADH is secreted in this process, which promotes the conservation of water.
The regulation of water intake in humans is thus related to a number of factors and is quite complex. Though cellular dehydration thirst and hypovolemic thirst play a role, it appears that in humans, peripheral factors such as dry mouth play an even larger role. Humans can drink rapidly, replacing a twenty-four-hour water deficit in two to three minutes. This occurs even before the cellular fluid has replaced the water, which takes approximately eight to twelve minutes.
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