Body Fluid Compartments And Their Contacts With The Outside World

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). The plasma is separated from the ISF compartment by highly permeable capillaries; together, plasma and ISF constitute the extracellular fluid (ECF) compartment. 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. 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.

In an average human adult weighing approximately 70 kg, the TBW makes up approximately 60% of body weight or about 40 L. The distribution of this water among the plasma ISF and ICF compartments is shown in Fig. 1, and the way in which the sizes of these compartments are determined is discussed below. 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.

The ECF of all vertebrates is in intimate contact with four organs that interface with the external environment. One is a tube that runs from mouth to

Transcellular Space Fluid Compartments
FIGURE 1 Sizes of the major body fluid compartments and the four organs that interface the extracellular fluid and the external environment. Total body water makes up approximately 60% of body weight.

anus—the alimentary canal. 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. 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. 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. 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 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.

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. A large portion of this text is devoted to considering how these organs perform those essential tasks. But, for the moment, the precision of these homeostatic processes can be illustrated by water balance in the normal human. As illustrated in Fig. 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. 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. These two routes of water loss are relatively constant under conditions of normal activity in a moderate climate. The remaining water that must be lost to account precisely for the total intake is excreted by the kidneys. Were it not for this fine balance, an individual might gradually enter a state of negative water balance and become dehydrated or a state of positive water balance and become water loaded, both potentially serious conditions.

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  • christopher
    What are body fluid compartments and their contacts with the outside world?
    6 years ago

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