Comparisons of the composition of the extracellular fluid, including the plasma and interstitial fluid, and the intra cellular fluid are shown in Figures 1 and 2 and in Table 1.
Fig1. Major cations and anions of the intracellular and extra cellular fluids. The concentrations of Ca++ and Mg++ represent the sum of these two ions. The concentrations shown represent the total of free ions and complexed ions.
Fig2. Nonelectrolytes of the plasma.
Table1. Osmolar Substances in Extracellular and Intracellular Fluids
IONIC COMPOSITION OF PLASMA AND INTERSTITIAL FLUID IS SIMILAR
Because 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 con centration 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 (cations) is slightly greater (~2 percent) in the plasma than in the interstitial fluid. The 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 the interstitial fluid compared with the plasma, because the negative charges of the plasma proteins repel the negatively charged anions. For practical purposes, however, the concentration of ions in the interstitial fluid and in the plasma is considered to be about equal.
Referring again to Figure 1, one can see that the extracellular fluid, including the plasma and the interstitial fluid, contains large amounts of sodium and chloride ions, 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 of the 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 sulfate ions, 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.
