In a kidney dialysis machine, waste is removed from the patient's blood because
Correct answer: D. the dialysis fluid contains no urea, so urea diffuses out across a partially permeable membrane
- 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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About Excretion
Excretion removes metabolic wastes such as carbon dioxide, urea and excess salts, and differs from egestion, which removes undigested food. Questions cover kidney structure, nephron ultrafiltration, selective reabsorption and urine formation, along with the excretory roles of the lungs, skin and liver in maintaining homeostasis.
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