American Board of Orthodontics (ABO) Practice Exam

Albumin is the most common protein that plasma calcium is bound to because it functions as a major carrier protein in the bloodstream. Calcium, which is vital for many physiological processes including muscle contraction, neurotransmitter release, and blood clotting, is predominantly found in its ionized form. However, a significant portion circulates in a bound state to prevent fluctuations that could disrupt homeostasis.

Albumin, in particular, has a high binding capacity for calcium ions due to its structure and the presence of specific binding sites. By binding to albumin, calcium is more efficiently transported in the bloodstream and maintained at appropriate levels, ensuring that physiological functions relying on calcium can occur without disruption.

Other proteins such as globulin, fibrinogen, and collagen do not play a primary role in the binding of plasma calcium. Globulins can bind other substances like hormones and vitamins but are not the main carriers of calcium. Fibrinogen is mainly involved in blood clotting, and collagen is primarily a structural protein providing support and strength to tissues, not a transporter of calcium in plasma. Thus, albumin's role as the dominant protein for calcium binding is critical for maintaining calcium homeostasis in the body.