Biological Systems MCAT Practice Exam

Endothelial cells line the interior surface of blood vessels and serve several important functions within the circulatory system. The primary function of these cells is to help maintain vascular tone, which refers to the regulation of blood vessel diameter through their ability to respond to various stimuli. This process is crucial for controlling blood flow and ensuring proper distribution of blood to tissues according to their needs. Endothelial cells achieve vascular tone maintenance by secreting various signaling molecules, such as nitric oxide, which promotes vascular relaxation and increases blood flow. They also play a role in the constriction of blood vessels when needed, thus demonstrating their ability to regulate both blood vessel dilation and constriction. This function is vital for overall cardiovascular health and the body's response to changes in metabolic demand. In contrast, other options such as oxygen transport and producing red blood cells are functions associated more directly with red blood cells, while nutrient absorption is primarily associated with the cells lining the intestines. Therefore, vascular tone maintenance is the key function that accurately describes the role of endothelial cells in the context of vascular biology.