EM of blood vessels. An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries. 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 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. Precapillary resistance is the modulation of blood flow by capillaries through vasomotion, either opening (dilating) and letting blood pass through, or by constricting their lumens, reducing bloodflow through the capillary bed (occluding the passage of blood). A venule is a very small blood vessel in the microcirculation that allows blood to return from the capillary beds to drain into the larger blood vessels, the veins.