Which process adds substances to the tubular fluid from blood?

Which process adds substances to the tubular fluid from blood?

• A. Secretion
• B. Filtration
• C. Re-absorption
• D. Excretion

Answer: (A)

Secretion is the process by which certain substances are moved from the blood into the tubular lumen, adding them to the tubular fluid. This happens as the tubule cells actively transport ions, organic ions, and other solutes from the peritubular blood into the nephron lumen, often using transporters powered by the Na+/K+-ATPase gradient. Examples include hydrogen ions, potassium, and various drugs or organic acids and bases. Filtration creates the initial filtrate at the glomerulus, but it’s secretion along the tubule that specifically adds additional substances from blood into the tubular fluid; reabsorption moves substances the other way, from tubule to blood, and excretion is the final elimination of waste in urine.

Secretion is the process by which certain substances are moved from the blood into the tubular lumen, adding them to the tubular fluid. This happens as the tubule cells actively transport ions, organic ions, and other solutes from the peritubular blood into the nephron lumen, often using transporters powered by the Na+/K+-ATPase gradient. Examples include hydrogen ions, potassium, and various drugs or organic acids and bases. Filtration creates the initial filtrate at the glomerulus, but it’s secretion along the tubule that specifically adds additional substances from blood into the tubular fluid; reabsorption moves substances the other way, from tubule to blood, and excretion is the final elimination of waste in urine.