Which of the following describes serum Ca2+?

a. Approximately 30% of the serum Ca2+ is ionized and combined with plasma anions.
b. Acidemia decreases the serum levels of ionized Ca2+.
c. More than half of the serum Ca2+ is nonionized and bound to plasma albumin.
d. Serum Ca2+ is present in three forms: ion-ized, protein bound, and complex.

ANS: D
Calcium is present in the blood in the following three forms: ionized, protein bound, and com-plex. Approximately 50% of serum calcium is ionized (Ca2+) and is physiologically active. An additional 10% forms calcium anion complexes. The remaining 40% is bound to plasma proteins, primarily albumen. Ionized calcium is physiologically active in processes such as enzyme activity, blood clotting, neuromuscular irritability, and bone calcification. Acidemia increases, and alkale-mia decreases, the concentration of Ca2+ in the serum.