Master the Biology Bash: 2025 MCAT Biological Systems Practice Challenge!

Capillaries are the smallest blood vessels in the body, and their primary function revolves around facilitating the exchange of nutrients and waste products between the blood and surrounding tissues. Their structure is uniquely designed for this role; capillaries have thin walls comprising only a single layer of endothelial cells, which allows for easy diffusion of oxygen, carbon dioxide, glucose, and other small molecules.

As blood flows through the capillaries, oxygen and nutrients such as glucose diffuse from the blood into the tissues, where they are utilized for cellular respiration and metabolic processes. Simultaneously, waste products generated by these cells, including carbon dioxide and urea, diffuse back into the capillary blood for removal from the body. This dual exchange process is essential for maintaining cellular health and homeostasis throughout the organism.

Other options, while related to the circulatory system, do not describe the primary role of capillaries. For instance, although oxygen transport occurs in capillaries, it is not their primary function; rather, it is a part of the overall process that supports nutrient and waste exchange. Similarly, blood storage is primarily the function of larger vessels and organs like the spleen. Regulating blood pressure is mainly carried out by arterioles and the overall vascular system