The skin is another site through which water is lost. The sweating which occurs in warm weather is an obvious example. Even during the winter, when the air is cool, "insensible perspiration" occurs as water diffuses through the skin. Insensible perspiration occurs at all times. The perspiration that can be sensed comes from eccrine glands. They secrete a watery fluid that cools the skin by using body heat to evaporate the liquid. The amount of salts and nitrogenous wastes in perspiration is not large; however, when one is working in a hot environment, the loss may be significant.
Primarily, though, it is water that is lost and must be replaced.
The apocrine sweat glands are located in the armpits and groin. They become active during emotional stimulation. These are the glands associated with the musky odor that some animals exude. These glands are not important in regulating body temperature, and their evaporation of water is not a major component of water balance mechanisms. The losses of water from the respiratory tract and the skin are obligatory, usually amounting to about 850 milliliters a day.
The intestinal tract is a source of water gain, as it ingests both liquids and food (with its associated water). Some water, however, is also lost because the copious intestinal secretions contain water. In fact, one day's intestinal secretions may amount to twice the body's plasma volume (from which it is derived). Not all water is reabsorbed in the passage through the stomach, small intestine, and large intestine: About one hundred milliliters are lost in the feces each day.
In a normal human diet, ingested fluid tends to exceed the minimum required by about one liter. Whatever excess is not used in evaporative cooling and lost from expired air or in feces is excreted by the kidney. The minimum water uptake required for balance is defined as that required to provide minimum urine volume without weight loss. The stomach and the small intestine reabsorb most of the ingested and secreted water. Only 35 percent reaches the large intestine. The large intestine is specialized to absorb water and produce semisolid feces for excretion. The maximum rate of water absorption by the intestines lies well above what is normally required.
The body fluid compartments provide an excellent example of the steady-state system characteristic of living things. Intracellular fluid must maintain a composition that promotes chemical reactions and diffusion despite the changes that those reactions bring. Extracellular compartments must retain their individual characteristics even though they communicate with one another. Hormones and nerves coordinate the interactions of the digestive, respiratory, integumentary, and uri nary systems, which contribute to the constant conditions of volume and composition of the body fluid compartments.
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