Capillaries are the smallest type of blood vessel, forming an extensive network that connects arterioles and venules within tissues. These vessels are characterized by extremely narrow lumen diameters, often only large enough for a single red blood cell to pass through at a time. This unique structure maximizes the surface area for exchange, allowing for the efficient passage of nutrients, gases, and waste products between the circulating blood and the surrounding interstitial fluid. Capillary walls are composed of a single layer of endothelial cells, facilitating the rapid process of diffusion and filtration vital for metabolic function throughout the body.