Final answer:
The capillaries are the smallest blood vessels that connect arterioles to venules, providing a surface area for microcirculation and the exchange of substances such as oxygen and nutrients with the surrounding tissues.
Step-by-step explanation:
The capillaries are the microscopic blood vessels in which substances are exchanged between the blood and tissues. These smallest of blood vessels connect arterioles and venules forming extensive networks called capillary beds. Through the thin walls of capillaries, physical exchange occurs as they allow the passage of individual blood cells, and they are the site for the diffusion of oxygen and nutrients between the blood and surrounding tissues.
The diameter of a capillary lumen ranges from 5-10 micrometers, allowing red blood cells to pass through one at a time, sometimes requiring deformation. This efficient design creates a large surface area for microcirculation, where gases and other substances like nutrients and metabolic waste products freely exchange through the capillary walls surrounded by tissue fluid (interstitial fluid).
Functionally, capillaries play a key role in the body's ability to maintain homeostasis. They adapt to the body's needs by either receiving more blood flow through dilation or less blood flow through constriction, all of which are controlled by signals from the autonomic nervous system or the endocrine system.