Question

what is actively transported back into the bloodstream by the proximal convoluted tubule?

chloride ions.

potassium ions.

sodium ions.

magnesium ions.

calcium ions.

Explanation:

The proximal convoluted tubule (PCT) is a part of the nephron in the kidney. Active transport is a process that requires energy (usually in the form of ATP) to move substances against their concentration gradient. Sodium ions (\(Na^{+}\)) are actively transported back into the bloodstream by the PCT. The sodium - potassium pump (\(Na^{+}/K^{+}\) - ATPase) is an important protein involved in this process. It pumps \(Na^{+}\) out of the PCT cells (into the interstitial fluid and then into the bloodstream) and \(K^{+}\) into the PCT cells. Chloride ions (\(Cl^{-}\)) are mainly reabsorbed passively (following the electrical gradient created by \(Na^{+}\) reabsorption in many cases). Potassium ions (\(K^{+}\)) have a more complex reabsorption pattern, and in the PCT, some \(K^{+}\) is reabsorbed passively. Magnesium (\(Mg^{2+}\)) and calcium (\(Ca^{2+}\)) reabsorption occurs more significantly in other parts of the nephron (such as the loop of Henle for \(Mg^{2+}\) and both the loop of Henle and distal convoluted tubule for \(Ca^{2+}\) depending on hormonal regulation).

Answer:

Sodium ions.