Biological Systems MCAT Practice Exam

During capillary exchange, what maintains oncotic pressure to keep water in blood vessels?

1. Hemoglobin

2. Albumin

3. Platelets

4. Fibrinogen

The correct answer is: Albumin

Albumin is the primary protein responsible for maintaining oncotic pressure within the blood vessels. It is a type of plasma protein synthesized in the liver, and its presence in the blood contributes significantly to the osmotic balance between the blood and the surrounding tissues. Oncotic pressure, a form of osmotic pressure, is created by these proteins, and it helps retain water in the bloodstream by opposing hydrostatic pressure, which tends to push fluid out of the capillaries. The importance of albumin is particularly evident during capillary exchange, where it prevents excessive fluid loss from the capillaries into the interstitial fluid. Without sufficient levels of albumin, the oncotic pressure would decrease, leading to conditions such as edema, where fluid accumulates in tissues, resulting in swelling. Thus, albumin plays a vital role in fluid homeostasis in the circulatory system. In contrast, while hemoglobin is essential for oxygen transport in red blood cells, it does not contribute to oncotic pressure. Platelets are involved in blood clotting and not in maintaining oncotic pressure, while fibrinogen is a precursor to fibrin and is primarily involved in the coagulation process rather than fluid balance.