The capillaries are a network of tiny, oxygen-rich blood vessels, supplying oxygenated blood from the arteries to the different body tissues. The exchange of gases and substances between the blood and the tissues is facilitated by the large cross-sectional area of all the capillaries and by slowing down of the blood flow (to about 0.3mm/s as compared to the 320mm/s in the aorta). All organs have capillaries, except the stratified squamous epithelia, the cornea and lens of the eye, and cartilage. Capillaries are usually 1mm long and 5-15 microms in diameter. Capillaries form three dimensional networks that are supplied by several arteries. Occlusion of one artery may, therefore, be of no consequence to the organ in question. Nevertheless, if a particular capillary area depends on a single artery, such as an end artery without adequate cross communications with other vessels, occlusion to that artery results in necrosis (death) of the affected tissue, which is known as infarct. The branches of the arteries to the liver, kidneys, spleen, brain, retina, and the heart coronary vessels are end arteries.