Comparisons of the composition of the extracellular fluid, including the plasma and interstitial fluid, and the intracellular fluid are shown in Figures 25-2 and 25-3 and in Table 25-2.
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 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 (cations) is slightly greater (about 2 per cent) 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
- Phospholipids - 2S0 mg/dl
-Cholesterol - 150 mg/dl
-Urea - 15 mg/dl -Lactic acid - 10 mg/dl -Uric acid - 3 mg/dl -Creatinine - 1.5 mg/dl 'Bilirubin - 0.5 mg/dl 'Bile salts - trace
Major cations and anions of the intracellular and extracellular 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.
Nonelectrolytes of the plasma.
Osmolar Substances in Extracellular and Intracellular Fluids
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