![]() The body monitors the amount of water that it contains through changes in the osmolality of ECF. The body maintains steady-state water balance by ensuring that the amount of water added to the body each day is exactly balanced by the amount lost or excreted from the body. The only regulated route for excretion of water from the body is the kidneys. The amount of water lost by these routes also varies over time, depending on the respiratory rate, physical activity, ambient temperature, and the presence or absence of diarrhea. In addition, water is lost from the body via respiration, sweating, and feces. Importantly, the amount of water added to the body each day is not constant, although it can be regulated to a degree by the thirst mechanism. Each day various volumes of liquid are ingested, and water is produced through cellular metabolism. In keeping with the dam and lake analogy, consider the maintenance of steady-state water balance in humans (see Chapter 34 for details). In this example, the dam operator, who controls the spillways, is that effector.įor virtually every substance in the body, the amount or concentration of which must be maintained within a narrow range, there is a set point and mechanism for monitoring deviations from that set point and effector mechanisms to maintain amounts or concentrations of that substance within the body constant, or in steady-state balance. Finally, there must be a mechanism, or “effector,” that regulates the amount of water that leaves the lake through the spillway. There must also be some way to measure deviations from the set point, such as a measure of the depth of the lake. For such a system to work, there must be a “set point,” or a determination of what the optimal level of water in the lake should be. Because the addition of water and loss by evaporation are not easily controlled, the only way to maintain the level of the lake constant is to regulate the amount that is allowed through the spillways. For the level of the lake to remain constant (i.e., steady-state balance), the rate at which water is added, regardless of the source, must be exactly matched by the amount of water lost, again by whichever route. At the same time, water is lost through the spillways of the dam and by the process of evaporation. In addition, water is added by rain and snow. ![]() Each day, water enters the lake from the various streams and rivers that feed it. The concept of steady-state balance can be illustrated by considering a river on which a dam is built to create an artificial lake. Finally, because epithelial cells are so central to the process of regulating the volume and composition of body fluids, the principles of solute and water transport by epithelial cells are reviewed. Included is a presentation on how cells generate and maintain a membrane potential, which is fundamental to understanding the function of excitable cells (e.g., neurons and muscle cells). To provide background for further study of the organ systems, this chapter presents an overview of the concept of steady-state balance, reviews the normal volume and composition of body fluids, and describes how cells maintain their intracellular composition and volume. Finally, the nervous and endocrine systems provide regulation and integration of these important functions. The cardiovascular system delivers nutrients to and removes waste products from cells and tissues. ![]() Transport by the epithelial cells of the gastrointestinal tract, kidneys, and lungs controls both the intake and excretion of numerous substances and water. The ability of the body to maintain constant volume and composition of the intracellular fluid (ICF) and ECF is a complex process that involves all organ systems of the body. The ECF, in turn, helps cell maintain a constant intracellular environment. These cells, which constitute the interface between the internal environment of the body and the external world, maintain the volume and composition of the fluid bathing all cells (i.e., the extracellular fluid ) constant. Such maintenance of steady-state balance, where the volume and composition of body fluids remain constant despite the addition and elimination of water and solutes from the body, to a large degree reflects the function of epithelial cells. In a healthy individual this occurs without significant changes in either the volume of body fluids or their composition. In addition, each day food and water are ingested and waste products are excreted from the body. ![]() This is accomplished by the transport of many substances and water into and out of the cell with the use of membrane transport proteins as described in Chapter 1. Normal cellular function requires that the intracellular composition of ions, small molecules, water, pH, and a host of other substances be maintained within a narrow range. ![]()
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