Arteries, veins and capillaries

Capillaries are so narrow that red blood cells pass through them one behind the other.

Three kinds of tubes

Blood leaves the heart in arteries, does its work in capillaries and comes back in veins. Arteries branch into smaller arteries and then into arterioles. Arterioles lead into capillaries, the smallest and most numerous vessels. Capillaries join into venules, venules join into veins, and the veins grow larger as they merge until they reach the heart. The three kinds of vessel are built differently because they face very different conditions.

Arteries: built for pressure

Each squeeze of the ventricles sends a surge of blood into the arteries at high pressure. Artery walls have three layers: a smooth inner lining, a thick middle layer of smooth muscle and elastic fibres, and an outer layer of connective tissue. The elastic wall stretches with each surge and springs back, which is the pulse you can feel at your wrist or neck. The muscle in the walls of the smallest arteries, the arterioles, can narrow or widen them to control how much blood flows to each part of the body.

Capillaries: where the exchange happens

Capillaries connect the vessels that carry blood away from the heart with the vessels that return it. Their main job is exchange between the blood and the tissue cells around them. The wall of a capillary is extremely thin, about 1 micrometer, so oxygen and carbon dioxide can cross it by diffusion: they move from where there is more of them to where there is less. Nutrients and fluid leave the blood at one end of a capillary, and much of the fluid returns at the other end. Tissues that use a lot of energy, such as muscles, are packed with capillaries; tissues that use little have fewer.

Veins: built for volume

By the time blood reaches the veins, most of the pressure from the heart is gone. Vein walls have the same three layers as arteries, but with less smooth muscle and connective tissue, so they are thinner and less rigid. Because they are thinner and floppier, veins can hold more blood than arteries. Systemic veins carry blood with less oxygen back to the right atrium, while the pulmonary veins carry oxygen-rich blood from the lungs to the left atrium.

A quick way to keep them apart: arteries are the thick, springy pipes that carry the push, capillaries are the thin-walled meeting places, and veins are the wide, low-pressure return pipes.

Names for the layers

Anatomists give the three layers of a vessel wall Latin names. The inner lining is the tunica intima, the middle muscular and elastic layer is the tunica media, and the outer connective tissue is the tunica externa, also called the adventitia. In large arteries the tunica media is especially thick. In veins it is thin, which is the main reason a vein wall is thinner. A capillary has only the thin inner lining, which is exactly what makes it so good at exchange.

In the heart model the big vessels show the same pattern on a large scale. The aorta leaving the left ventricle is the widest artery in the body. The superior and inferior venae cavae arriving at the right atrium are the largest veins. All the tiny capillaries in between are far too small to draw, so the model shows only where the large vessels begin and end.