TEM of capillaries are the smallest of a body's blood vessels (and lymph vessels) that make up the microcirculation. Their endothelial linings are only one cell layer thick. These microvessels, measuring around 5 to 10 micrometers in diameter, connect arterioles and venules, and they help to enable the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste substances between the blood and the tissues surrounding them. Lymph capillaries connect with larger lymph vessels to drain lymph collected in the microcirculation. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart. Magnification unknown.