What is the main function of capillaries?

The main function of capillaries is to facilitate the exchange of oxygen, carbon dioxide, nutrients, and waste between blood and tissues. Capillaries are the smallest and thinnest blood vessels in the circulatory system, providing a large surface area that allows for efficient diffusion of substances. Their walls are only one cell thick, which enables the direct transfer of gases and nutrients into surrounding tissues while simultaneously allowing for the removal of metabolic waste products. This process is crucial for cellular respiration and maintaining homeostasis within the body.

In contrast, other options refer to different functions related to other types of blood vessels or processes. For instance, transporting oxygenated blood is primarily the role of arteries, not capillaries. Storing excess blood during physical activity involves veins and the body's regulatory mechanisms, not the capillary network. Similarly, regulating blood flow from arteries to veins is a function attributed to arterioles and the overall control of the circulatory system, rather than a direct role of capillaries.