Calcium absorption along the small intestine occurs by two routes. Which statement correctly describes the active, regulated component?

• A It predominates in the jejunum and ileum and is inversely related to dietary calcium intake
• B It is confined to the colon and proceeds independently of any hormonal regulation
• C It requires the same Na+-dependent cotransporter used for glucose absorption
• D It occurs mainly in the duodenum, involves TRPV6 uptake, cytosolic calbindin transport and basolateral Ca2+-ATPase, and is induced by calcitriol ✓

Explanation

Transcellular calcium absorption is concentrated in the duodenum, is saturable, and adapts to body needs. Calcitriol induces TRPV6 on the apical membrane, cytosolic calbindin-B9k that buffers and shuttles calcium, and the basolateral plasma membrane Ca2+-ATPase that extrudes it. When dietary calcium is abundant, passive paracellular absorption along the whole small intestine suffices, which is why the regulated route matters most at low intakes. No Na+-coupled transporter carries calcium.

Reference: Berne and Levy Physiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.