Anatomy and Physiology II Practice Test

Capillaries serve a crucial role in the circulatory system primarily focused on the exchange of substances. They are the smallest blood vessels in the body, typically only one cell thick, which facilitates the diffusion of materials between the blood and surrounding tissues. This is where oxygen, carbon dioxide, nutrients, and waste products are exchanged.

Oxygen and nutrients from the blood pass through the capillary walls into the tissues, while waste products like carbon dioxide move from the tissues into the blood. This function is vital because it ensures that cells receive the substances they need for metabolism while also removing waste to maintain homeostasis.

The other options reflect functions associated with larger blood vessels, like arteries and veins, rather than the specific functions of capillaries. For instance, arteries transport blood away from the heart, veins contain valves to prevent backflow, and blood storage is typically a function of larger veins or the spleen, not capillaries. Thus, the primary and defining function of capillaries is indeed the exchange of substances, making that answer the correct choice.