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Capillaries are the smallest of the body's blood vessels, forming a network that connects arterioles and venules. Their primary function is to facilitate the exchange of substances between the blood and the surrounding tissues. This exchange includes the delivery of oxygen and nutrients to cells and the removal of carbon dioxide and waste products. The walls of capillaries are extremely thin, typically only one cell thick, which allows for efficient diffusion. This thinness is crucial for their role in capillary exchange.

The capillary network is extensive, with an estimated total length of tens of thousands of miles in the human body. This vast surface area maximizes the efficiency of nutrient and gas exchange throughout all tissues. Blood flows through capillaries at a relatively slow rate, which further aids in the process of diffusion. The pressure within capillaries is also lower than in arteries, which helps to prevent damage to their delicate walls.

There are different types of capillaries, including continuous, fenestrated, and sinusoids, each adapted to the specific needs of the tissue they supply. Continuous capillaries are the most common and have a continuous wall of endothelial cells. Fenestrated capillaries have pores or fenestrations in their walls, allowing for more rapid passage of small molecules. Sinusoids are larger in diameter and have gaps between endothelial cells, permitting the passage of larger molecules and even cells. Understanding the structure and function of capillaries is fundamental to comprehending circulatory physiology and the maintenance of tissue homeostasis.