A capillary is a small, thin-walled blood vessel that connects arterioles to venules. Capillaries are the smallest blood vessels in the body and are responsible for the exchange of nutrients, oxygen, and waste products between the blood and the tissues. Capillaries have a semipermeable membrane that allows for the passage of small molecules, such as water, electrolytes, and gases, while preventing the passage of larger molecules, such as proteins and cells. Capillaries are also involved in the regulation of blood pressure and body temperature.