A capillary is an extremely narrow, thin-walled blood vessel forming the network that connects small arteries to small veins throughout the body’s tissues. With walls a single cell thick, capillaries are the site where oxygen, nutrients, hormones, and waste products exchange between blood and cells. Their vast total surface area makes this exchange efficient, and their permeability is finely regulated. In nutrition, capillary function underlies the delivery of absorbed nutrients from the gut to tissues and the removal of metabolic waste. Conditions such as diabetes damage capillaries over time, impairing circulation in the eyes, kidneys, and extremities.