Which statement correctly distinguishes tubular reabsorption from tubular secretion?

Which statement correctly distinguishes tubular reabsorption from tubular secretion?

• A. Reabsorption moves substances from blood to tubular fluid, while secretion moves from tubular fluid to blood.
• B. Both processes move substances in the same direction.
• C. Reabsorption moves substances from tubular fluid to blood, while secretion moves from blood to tubular fluid.
• D. Both occur only in the glomerulus.

Answer: (C)

The moving direction of substances across the nephron tubule determines this concept. In tubular reabsorption, substances move from the filtrate in the tubular lumen back into the blood of the peritubular capillaries. This is how the body reclaims water, glucose, amino acids, electrolytes, and bicarbonate to maintain volume and composition. In tubular secretion, substances move from the blood into the tubular filtrate, allowing the kidney to excrete wastes and adjust pH and ion balance—think of hydrogen ions, potassium, and various drugs being added to the filtrate for elimination. So the correct statement is that reabsorption moves substances from tubular fluid to blood, while secretion moves substances from blood to tubular fluid. The glomerulus is the filtration site, not where reabsorption or secretion primarily occur, which is why a choice claiming both happen only there is not accurate.

The moving direction of substances across the nephron tubule determines this concept. In tubular reabsorption, substances move from the filtrate in the tubular lumen back into the blood of the peritubular capillaries. This is how the body reclaims water, glucose, amino acids, electrolytes, and bicarbonate to maintain volume and composition. In tubular secretion, substances move from the blood into the tubular filtrate, allowing the kidney to excrete wastes and adjust pH and ion balance—think of hydrogen ions, potassium, and various drugs being added to the filtrate for elimination.

So the correct statement is that reabsorption moves substances from tubular fluid to blood, while secretion moves substances from blood to tubular fluid. The glomerulus is the filtration site, not where reabsorption or secretion primarily occur, which is why a choice claiming both happen only there is not accurate.