Which of the following is not reabsorbed by the proximal convoluted tubule?
(a) Potassium
(b) Water
(c) Urea
(d) Sodium chloride
(e) Hydrogen ion
Answer: (c) Urea
In the nephron, the proximal convoluted tubule (PCT) plays an important role in the reabsorption of a large volume of filtrate that contains water, ions, and many organic substances. However, urea in this segment of the nephron is not reabsorbed as the filtered product is permitted to continue its passage into the collecting duct. Although some amount of urea may be reabsorbed back in the blood, the major part advances through the tubule. All these are actively or passively reabsorbed in the PCT to help maintain fluid and electrolyte balance; the substances include potassium, water, sodium chloride, and hydrogen ions. Urea is a toxic substance in the sense that it is a waste product resulting from the digestion of proteins and is mostly eliminated in the urine. It rises as it passes through the nephron, most densely in the loop of Henle and the collecting duct where this property of the kidney to concentrate the urine is achieved.
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