The total body fluid is distributed mainly between two compartments: the extracellular fluid and the intracellular fluid (Figure 1). The extracellular fluid is divided into the interstitial fluid and the blood plasma.

Fig1. Summary of body fluid regulation, including the major  body fluid compartments and the membranes that separate these  compartments. The values shown are for an average 70-kilogram  adult man.

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 composition may differ markedly from that of the plasma or interstitial fluid. All the transcellular fluids together constitute about 1 to 2 liters.

In a 70kilogram adult man, the total body water is about 60 percent of the body weight, or about 42 liters. This percentage depends on age, gender, and degree of obesity. As a person grows older, the percentage of total body weight that is fluid gradually decreases. This decrease 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 a greater percentage of body fat compared with men, their total body water averages about 50 percent of the body weight. In premature and newborn babies, the total body water ranges from 70 to 75 percent of body weight. Therefore, when discussing “average” body fluid compartments, we should realize that variations exist, depending on age, gender, and percentage of body fat.

In many other countries, the average body weight (and fat mass) has increased rapidly during the past 30 years. Currently, the average body weight for men older than 20 years in the United States is estimated to be approximately 86.4 kg, and for women, it is 74.1 kg. Therefore the data discussed for an “average” 70 kg man in this chapter (as well as in other chapters) would need to be adjusted accordingly when considering body fluid compartments in most people.

INTRACELLULAR FLUID COMPARTMENT

 About 28 of the 42 liters of fluid in the body are inside the 100 trillion cells and are collectively called the intra cellular fluid. Thus, the intracellular fluid constitutes about 40 percent 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.

EXTRACELLULAR FLUID COMPARTMENT

 All the fluids outside the cells are collectively called the extracellular fluid. Together these fluids account for about 20 percent of the body weight, or about 14 liters in a 70kilogram man. The two largest compartments of the extracellular fluid are the interstitial fluid, which makes up more than three fourths (11 liters) of the extracellular 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 interstitial fluid through the pores of the capillary membranes. These pores are highly permeable to almost all solutes in the extracellular fluid except the proteins. Therefore, the extracellular fluids are constantly mixing, so the plasma and interstitial fluids have about the same composition except for proteins, which have a higher concentration in the plasma.

BLOOD VOLUME

Blood contains both extracellular fluid (the fluid in plasma) and intracellular fluid (the fluid in the red blood cells). However, blood is considered to be a separate fluid compartment because it is contained in a chamber of its own, the circulatory system. The blood volume is especially important in the control of cardiovascular dynamics.

The average blood volume of adults is about 7 percent of body weight, or about 5 liters. About 60 percent of the blood is plasma and 40 percent is red blood cells, but these percentages can vary considerably in different people, depending on gender, weight, and other factors.

Hematocrit (Packed Red Blood Cell Volume). The hematocrit is the fraction of the blood composed of red blood cells, as determined by centrifuging blood in a “hematocrit tube” until the cells become tightly packed in the bottom of the tube. Because the centrifuge does not completely pack the red blood cells together, about 3 to 4 percent of the plasma remains entrapped among the cells, and the true hematocrit is only about 96 percent of the measured hematocrit.

In men, the measured hematocrit is normally about 0.40, and in women, it is about 0.36. In persons with severe anemia, the hematocrit may fall as low as 0.10, a value that is barely sufficient to sustain life. Conversely, in persons with some conditions excessive production of red blood cells occurs, resulting in polycythemia. In these persons, the hematocrit can rise to 0.65.