Which term describes the process that reclaims molecules such as glucose, vitamins, ions (especially sodium), and amino acids for circulation in the blood, often against a concentration gradient?

• A. Passive tubular reabsorption: water moves into the bloodstream
• B. Glomerular filtration: bulk filtration of plasma
• C. Active tubular reabsorption: reclaiming glucose, vitamins, ions, and amino acids for circulation in the blood (against concentration gradient)
• D. Tubular secretion: movement of substances from blood into urine

Answer: (C)

Active tubular reabsorption is the process by which essential solutes are reclaimed from the filtrate and returned to the blood, and it does this using energy to move substances against their concentration gradients. In the nephron, especially in the proximal tubule, transport proteins couple the movement of solutes like glucose, amino acids, vitamins, and many ions to a sodium gradient created by the Na+/K+-ATPase pump. This setup allows these substances to be moved from the tubular fluid into the epithelial cells and then into the bloodstream, often against their gradient. Water tends to follow these reabsorbed solutes osmotically, helping restore blood volume and electrolyte balance.

This differs from glomerular filtration, which is the bulk movement of plasma from blood into the filtrate to begin urine formation, and from tubular secretion, which moves substances from blood into the filtrate to be excreted. It also differs from passive tubular reabsorption, where substances move down their gradient without active energy expenditure, which is more typical for water movement driven by osmotic forces rather than transporter-driven reclaiming of nutrients.