![]() ![]() The remaining water that must be lost to account precisely for the total intake is excreted by the kidneys. These two routes of water loss are relatively constant under conditions of normal activity in a moderate climate. ![]() A small amount of water (100-200 cc) is lost daily in the feces, and a rather substantial amount of water (approximately 900 cc) is lost by vaporization from the respiratory tract and skin the latter is referred to as insensible water loss and must be distinguished from perspiration. 2, water is gained by the body via three sources: (1) water ingested in the form of liquids, (2) water contained in solid foods, and (3) water derived from oxidative metabolism of carbohydrates. ![]() But, for the moment, the precision of these homeostatic processes can be illustrated by water balance in the normal human. A large portion of this text is devoted to considering how these organs perform those essential tasks. It is the precise interplay of these interfaces (particularly the alimentary canal, lungs, and kidneys) and the external environment that maintains the constancy of our milieu interieur and permits us to live a relatively free and independent life. The final organ in contact with the external environment is, of course, the skin, which plays a primary role in temperature regulation, but is also a source of water loss. In short, the composition of our body fluids is determined not by what the mouth takes in but by what our kidneys keep (Smith, 1961). The third organ is the kidneys, which turn over the sea within us many times each day and correct for the indiscretions of our gastrointestinal tracts. The second organ is the lungs, which are responsible for exchanging oxygen and carbon dioxide with the environment and, as discussed below, are a source of water loss. It is responsible for absorbing water, essential elements, and the metabolites that form our building blocks and fuel our activities for the most part, however, it is indiscriminant with respect to what it will permit to enter the body. ![]() Total body water makes up approximately 60% of body weight.Īnus-the alimentary canal. One is a tube that runs from mouth to FIGURE 1 Sizes of the major body fluid compartments and the four organs that interface the extracellular fluid and the external environment. The ECF of all vertebrates is in intimate contact with four organs that interface with the external environment. The transcellular fluid compartment in such an individual comprises approximately 2 to 4% of the TBW and contains approximately 1 to 2 L of water. 1, and the way in which the sizes of these compartments are determined is discussed below. The distribution of this water among the plasma ISF and ICF compartments is shown in Fig. In an average human adult weighing approximately 70 kg, the TBW makes up approximately 60% of body weight or about 40 L. A fourth, small compartment, called the transcellular fluid compartment, consists primarily of fluid in transit in the lumina of epithelial organs (e.g., the gall bladder, stomach, intestines, and urinary bladder), as well as the cerebrospinal fluid and the intraocular fluid. This compartment is separated from the ICF compartment by cell membranes, which in most instances, as discussed in Chapter 3, are highly permeable to water but very selective with respect to the passage of solutes. The plasma is separated from the ISF compartment by highly permeable capillaries together, plasma and ISF constitute the extracellular fluid (ECF) compartment. The total body water (TBW) in higher animals is distributed among three major compartments: the blood plasma, the interstitial fluid (ISF), and the intracellular fluid (ICF). ![]()
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