Blood vessels form a closed loop: arteries carry blood away from the heart, capillaries exchange with the tissues and veins bring blood back. Each type has a wall built for its job.
Arteries
Arteries have thick, muscular and elastic walls that handle high pressure. Large arteries stretch with each beat, which you feel as the pulse, and small arterioles tighten or relax to control flow and pressure.
Capillaries
Capillaries have walls only one cell thick, so oxygen, nutrients and wastes can cross. They form dense networks so that almost every cell lies close to one.
Veins
Veins have thinner walls, low pressure and one-way valves, mostly in the limbs. Leg muscles squeeze the veins when they contract and push blood back toward the heart.
Where the blood is
About two thirds of the blood volume sits in the veins at any moment. Veins can tighten to send this reserve back to the heart after blood loss.
| Vessel | Wall | Pressure |
|---|---|---|
| Artery | Thick, muscular, elastic | High, pulsatile |
| Capillary | One cell thick | Low |
| Vein | Thin, with valves | Very low |
At the bedside
Veins are used for IV lines and blood draws because they are near the surface, fill easily under a tourniquet and have low pressure. Bright red, pulsing blood means you are in an artery: remove the needle and hold firm pressure.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).