Body Fluid Compartments
The total body fluid is distributed mainly between two compartments: the extracellular fluid and the intracel- lular fluid (Figure 25–1). The extracellular fluid is divided into the interstitial fluid and the bloodplasma.
There is another small compartment of fluid that is referred to as transcellular fluid. This compartment includes fluid in the synovial, peritoneal, pericardial, and intraocular spaces, as well as the cerebrospinal fluid; it is usually considered to be a specialized type of extracellular fluid, although in some cases, its com- position may differ markedly from that of the plasma or interstitial fluid. All the transcellular fluids together constitute about 1 to 2 liters.
In the average 70-kilogram adult human, the total body water is about 60 per cent of the body weight, or about 42 liters. This percentage can change, depending on age, gender, and degree of obesity. As a person grows older, the percentage of total body weight that is fluid gradually decreases. This is due in part to the fact that aging is usually associated with an increased percentage of the body weight being fat, which decreases the percentage of water in the body. Because women normally have more body fat than men, they contain slightly less water than men in proportion to their body weight. Therefore, when discussing the “average” body fluid compartments, we should realize that variations exist, depending on age, gender, and percentage of body fat.
About 28 of the 42 liters of fluid in the body are inside the 75 trillion cells and are collectively called the intra-cellular fluid. Thus, the intracellular fluid constitutesabout 40 per cent of the total body weight in an “average” person.
The fluid of each cell contains its individual mixture of different constituents, but the concentrations of these substances are similar from one cell to another. In fact, the composition of cell fluids is remarkably similar even in different animals, ranging from the most primitive microorganisms to humans. For this reason, the intracellular fluid of all the different cells together is considered to be one large fluid compartment.
All the fluids outside the cells are collectively called the extracellular fluid. Together these fluids account for about 20 per cent of the body weight, or about 14 liters in a normal 70-kilogram adult. The two largest com-partments of the extracellular fluid are the interstitialfluid, which makes up more than three fourths of theextracellular fluid, and the plasma, which makes up almost one fourth of the extracellular fluid, or about 3 liters. The plasma is the noncellular part of the blood; it exchanges substances continuously with the intersti-tial fluid through the pores of the capillary mem-branes. These pores are highly permeable to almost all solutes in the extracellular fluid except the proteins. Therefore, the extracellular fluids are constantly mixing, so that the plasma and interstitial fluids have about the same composition except for proteins, which have a higher concentration in the plasma.
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