What is the smallest blood vessel in the human body?

The smallest blood vessels in the human body are capillaries. These tiny, thin-walled vessels are crucial for the exchange of substances between the blood and surrounding tissues. Capillaries connect arterioles (small arteries) to venules (small veins), facilitating the delivery of oxygen and nutrients to cells while also allowing for the removal of carbon dioxide and waste products.

The key characteristic that defines capillaries is their microscopic size, which allows them to penetrate tissues and form networks, thereby maximizing surface area for efficient exchange. Their walls are composed of a single layer of endothelial cells, making them highly permeable and ideal for diffusion. This structural feature distinguishes them from arteries, veins, and venules, which are generally larger and have more complex structures to accommodate higher blood pressures and volumes.