In acidemia (acidosis), what happens to ionized calcium?

• A. Ionized calcium increases
• B. Ionized calcium decreases
• C. No change in ionized calcium
• D. Ionized calcium becomes bound to protein

Answer: (A)

In acidemia, higher hydrogen ion concentration competes with calcium for albumin binding sites. When H+ binds to albumin, it reduces the albumin’s ability to hold calcium, displacing calcium from the protein and increasing the fraction that remains as free, ionized Ca2+. Ionized calcium is the physiologically active form, so its level rises in acidosis. (In contrast, alkalosis promotes more calcium binding to albumin, lowering the ionized fraction.)