Capillaries are the smallest blood vessels in the human body, connecting arteries and veins. They play a crucial role in the circulatory system by facilitating the exchange of oxygen, nutrients, and waste products between blood and surrounding tissues. Their thin walls, made up of a single layer of cells, allow for efficient diffusion of substances. These tiny vessels are found throughout the body, reaching almost every cell. The extensive network of capillaries ensures that all tissues receive adequate blood supply. Conditions affecting capillaries, such as diabetes or hypertension, can lead to serious health issues, highlighting their importance in maintaining overall health.