In a kidney dialysis machine, waste is removed from the patient's blood because
- A. the dialysis fluid contains enzymes that destroy urea
- B. the blood is heated so that urea evaporates
- C. urea is actively pumped across the membrane
- D. the dialysis fluid contains no urea, so urea diffuses out across a partially permeable membrane
Explanation
Dialysis fluid is made with the same concentrations of glucose and mineral ions as normal blood plasma but with no urea at all, so only the wastes follow a concentration gradient out while useful substances have no gradient and stay. The fluid flows in the opposite direction to the blood, keeping the gradient steep along the whole membrane. Nothing is pumped, since the whole process is diffusion.
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