| Regulation of cell osmolality and volume
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| The cellular mechanism responsible for maintaining its volume is the sodium-potassium pump. The inhibition of the pump, for example by a poisonous glycoside such as ouabain, results in a decrease of intracellular potassium concentration and an increase in sodium. This decreases the membrane potential (depolarizes the cell membrane). To maintain electrical neutrality, chloride ions also enter the cells as counterions to sodium.
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| Intracellular sodium concentration regulates the activity of the sodium-potassium pump
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| EDEMA RESULTS FROM A LOSS OF PROTEIN |
| An 8-year-old girl was referred to a nephrologist after she had been noticed to have puffiness of her face and swollen ankles over a period of about 2 weeks. Dipstick test for urine protein yielded a strongly positive (++++) result and measurement in a 24-hour collection showed protein excretion of 7g/day. Reference value for urinary protein excretion is less than 0.15g/day. |
| Comment. Renal biopsy showed so-called minimal change disease. The damage to the renal filtration barrier was the cause of the proteinuria, with the urinary protein loss causing in turn a decrease in the plasma oncotic pressure and retention of water in the ECF, leading to edema. The condition went into remission after therapy with a glucocorticoid. |
When the intracellular concentration of sodium increases, the activity of the sodium-potassium pump also increases, and sodium ions are extruded from the cell. In this way, cells protect themselves from changes in the fluid volume. Another mechanism which protects cell volume is the intracellular generation of osmotically active substances. For instance, the brain cells adapt to the increased ECF osmolality by increasing
their amino acid concentration, and cells in the renal medulla exposed to a hyperosmotic environment (see below) produce an osmotically active alcohol, sorbitol , and increase the concentration of an amino acid taurine.
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