alents are calculated as the product of moles times valence and represent the concentration of charged species. For singly charged (univalent) ions, such as Na+, K+, Cl—, or HCO 3 , 1 mmol is equal to 1 mEq. For doubly charged (divalent) ions, such as Ca2+, Mg2+, or SO42-, 1 mmol is equal to 2 mEq. Some electrolytes, such as proteins, are polyvalent, so there are several mEq/mmol. The usefulness of expressing concentrations in terms of mEq/L is based on the fact that in solutions, we have electrical neutrality,- that is

If we know the total concentration (mEq/L) of all cations in a solution and know only some of the anions, we can easily calculate the concentration of the remaining anions. This was done in Table 24.2 for the anions labeled "Others." Plasma concentrations are listed in the first column of Table 24.2. Na+ is the major cation in plasma, and Cl— and HCO3 — are the major anions. The plasma proteins (mainly serum albumin) bear net negative charges at physiological pH. The electrolytes are actually dissolved in the plasma water, so the second column in Table 24.2 expresses concentrations per kg H2O. The water content of plasma is usually about 93%,- about 7% of plasma volume is occupied by solutes, mainly the plasma proteins. To convert concentration in plasma to concentration in plasma water, we divided the plasma concentration by the plasma water content (0.93 L H2O/L plasma). Therefore, 142 mEq Na+/L plasma becomes 153 mEq/L H2O or 153 mEq/kg H2O (since 1 L of water weighs 1 kg).

Interstitial fluid (Column 3 of Table 24.2) is an ultrafil-trate of plasma. It contains all of the small electrolytes in essentially the same concentration as in plasma, but little protein. The proteins are largely confined to the plasma because of their large molecular size. Differences in small ion concentrations between plasma and interstitial fluid (compare Columns 2 and 3) occur because of the different protein concentrations in these two compartments. Two factors are involved. The first is an electrostatic effect: Because the plasma proteins are negatively charged, they cause a redistribution of small ions, so that the concentrations of diffusible cations (such as Na+) are lower in inter stitial fluid than in plasma and the concentrations of diffusible anions (such as Cl—) are higher in interstitial fluid than in plasma. Second, Ca2+ and Mg2+ are bound to some extent (about 40% and 30%, respectively) by plasma proteins, and it is only the unbound ions that can diffuse through capillary walls. Hence, the total plasma Ca2+ and Mg2+ concentrations are higher than in interstitial fluid.

ICF composition (Table 24.2, Column 4) is different from ECF composition. The cells have a higher K+, Mg2+, and protein concentration than in the surrounding interstitial fluid. The intracellular Na+, Ca2+, Cl—, and HCO3 — levels are lower than outside the cell. The anions in skeletal muscle cells labeled "Others" are mainly organic phosphate compounds important in cell energy metabolism, such as creatine phosphate, ATP, and ADP. As described in Chapter 2, the high intracellular [K+] and low intracellular [Na+] are a consequence of plasma membrane Na+/K+-ATPase activity,- this enzyme extrudes Na+ from the cell and takes up K+. The low intracellular [Cl—] and [HCO3 —] in skeletal muscle cells are primarily a consequence of the inside negative membrane potential ( — 90 mV), which favors the outward movement of these small, negatively charged ions. The intracellular [Mg2+] is high,- most is not free, but is bound to cell proteins. Intracellular [Ca2+] is low,- as discussed in Chapter 1, the cytosolic [Ca2+] in resting cells is about 10—7M (0.0002 mEq/L). Most of the cell Ca2+ is sequestered in organelles, such as the sarcoplasmic reticulum in skeletal muscle.

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