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Water balance must be regulated because equal amounts of water consumed by eating and drinking must be off-set by the amount that is excreted. The amount of the solvent water relative to dissolved solute like salts, must be closely monitored by organs in the body to help maintain homeostasis.
Kidneys are the most important organs for this function. They can either re-absorb water back into the circulating blood when water is needed or they can excrete excess water by producing a more dilute solution of urine. The hormone ADH, secreted by the thalamus plays a role by facilitating the transport of water into water channels in the cell membranes lining collecting ducts in the kidneys for the process of reabsorption. If no ADH is present, water is excreted in the urine. This is an example of feedback-if the plasma is too concentrated, ADH is released. When a normal blood volume occurs, ADH secretion is inhibited.
Sodium is a solute that is very closely regulated by the body. Cells can swell if the solvent is too high relative to the solute, or can shrink, if the solute levels are too high. Sodium is the major solute of extracellular fluids. Sodium enters the body from sodium chloride salts found in foods.
Osmolarity, the amount of solute per volume, must be regulated. For example, when you become dehydrated and lose water from the body, you lose more solvent(water) than solute(sodium) and the osmolarity of your body fluids increases. You must conserve water in this case. ADH helps to lower osmolarity by reabsorbing more water back into the blood stream from the kidneys. This in turns regulates the levels of salt vs. water in the body fluids.
Another organ of salt excretion is the sweat glands, located in the skin. These small tubular-- shaped organs produce sweat which contains salt, water and urea. Exocrine sweat glands are found all over the body and apocrine sweat glands are in the arm pits. Even ciliary glands in the eyelids are modified apocrine glands. Although sweat is mainly water, it has some electrolytes including the salt sodium chloride, because it comes from blood plasma. Perspiration helps the body to cool down as the water in sweat evaporates, but it also allows for the excretion of excess salt from the body plasma.
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