Question

Imagine you take a drug that increases the rate of Na reabsorption by your kidneys. All else equal, which of the following will be true?

A The volume of urine you produce per unit time will be lower after taking the drug.

B The volume of urine you produce per unit time will be the same after taking the drug.

C The volume of urine you produce per unit time will be higher after taking the drug.

Answer 1

Imagine that you take a drug that increases the rate of Na reabsorption by your kidneys. All else equal, which of the following will be true?

Answer: The volume of urine you produce per unit time will be lower after taking the drug.

Explanation: The given drug is increasing the reabsorption of Na (sodium) from the kidneys. This means that there is increased reabsorption of sodium ions from the tubular fluid of the nephrons of the kidneys into the peritubular capillaries surrounding the tubules of the nephrons. This increases the oncotic pressure in the peritubular capillaries. Due to increased oncotic pressure in the peritubular capillaries, there is movement of water from the tubular fluid into the peritubular capillaries to maintain its oncotic pressure within optimum range. As more water is reabsorbed from the tubules into the peritubular capillaries, there is decreased urine formation per unit time.

Thus, if the drug is increasing the rate of sodium reabsorption by the kidneys, it will decrease the amount of urine formed per unit time as water will also be reabsorbed more which will reduce the amount of urine formation. Water follows sodium ions during reabsorption from the tubular fluid into the peritubular capillaries to maintain the oncotic pressure.