The total body fluid is distributed mainly between two compartments, the extracellular fluid and the intracellular fluid. The extracellular fluid is divided into the interstitial fluid and the blood plasma.
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 70-kg man, the total body water is about 60% of the body weight, or about 42 liters. This percentage depends on age, 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% of the body weight. In premature and newborn babies, the total body water ranges from 70% to 75% of body weight.
Therefore, when discussing average body fluid compartments, we should realize that variations exist, depending on age, and percentage of body fat.
In many other countries, the average body weight (and fat mass) has increased rapidly during the past 30 years. The average body weight for men older than 20 years in the United States is estimated to be approximately 88.8 kg (~196 pounds), and for adult women it is 77.4 kg (~170 pounds).
Therefore, data discussed for an average 70-kg man in this and other chapters would need to be adjusted accordingly when considering body fluid compartments in most people.
About 28 of the 42 liters of fluid in the body are inside the trillions of cells and is collectively called the intracellular fluid. Thus, the intracellular fluid constitutes about 40% 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 intracel lular 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% of the body weight, or about 14 liters in a 70-kg 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 extra- cellular fluid, or about 3 liters.
The plasma is the noncellu- lar 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 extracellular fluid (the fluid in plasma) and intracellular fluid (the fluid in the red blood cells). How- ever, 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% of body weight, or about 5 liters. About 60% of the blood is plasma and 40% is red blood cells, but these percentages can vary considerably in different people, depending on weight, and other factors.
Hematocrit (Packed Red Blood Cell Volume). The he- matocrit 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% of the plasma remains entrapped among the cells, and the true hematocrit is only about 96% 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.
Similar lonic Composition of Plasma and Interstitial Fluid
The plasma and interstitial fluid are separated only by highly permeable capillary membranes, their ionic composition is similar. The most important difference between these two compartments is the higher concentration of protein in the plasma; because the capillaries have a low permeability to the plasma proteins, only small amounts of proteins are leaked into the interstitial spaces in most tissues.
Because of the Donnan effect, the concentration of positively charged ions is slightly greater (~2%) in plasma than in interstitial fluid. Plasma proteins have a net negative charge and therefore tend to bind cations such as sodium and potassium ions, thus holding extra amounts of these cations in the plasma, along with the plasma proteins.
Conversely, negatively charged ions (anions) tend to have a slightly higher concentration in interstitial fluid compared with plasma because the negative charges of the plasma proteins repel the negatively charged anions. For practical purposes, however, the concentrations of ions in interstitial fluid and plasma are considered to be about equal.
One can see that the extracellular fluid, including the plasma and interstitial fluid, contains large amounts of sodium and chloride ions and reasonably large amounts of bicarbonate ions but only small quantities of potassium, calcium, magnesium, phosphate, and organic acid ions.
The composition of extracellular fluid is carefully regulated by various mechanisms, but especially by the kidneys, as discussed later. This regulation allows the cells to remain continually bathed in a fluid that contains the proper concentration of electrolytes and nutrients for optimal cell function.
INTRACELLULAR FLUID CONSTITUENTS
The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly permeable to water but is not permeable to most electrolytes in the body. In contrast to the extracellular fluid, the intracellular fluid contains only small quantities of sodium and chloride ions and almost no calcium ions.
Instead, it contains large amounts of potassium and phosphate ions plus moderate quantities of magnesium and sulfateions, all of which have low concentrations in the extracellular fluid. Also, cells contain large amounts of protein-almost four times as much as in the plasma.