Venules collect blood from the capillary beds. They are small vessels with very thin walls containing little or no muscle. Venules join together to form veins, which carry blood back to the heart. Most veins carry deoxygenated blood; the pulmonary vein is the exception, carrying oxygenated blood from the lungs to the left atrium.By the time blood reaches the veins it has passed through the arterioles and capillaries, so it is at low pressure and flows smoothly without pulses. The structure of veins reflects this:
Thin wall. Veins have the same three layers as arteries, but the middle layer has much less smooth muscle and elastic tissue. The blood pressure is low, so a thick, elastic wall is not needed.Wide lumen. The lumen is large compared with the thickness of the wall. This reduces resistance, so blood can flow easily at low pressure.Valves. Veins contain pocket-shaped valves. Blood flowing towards the heart pushes them open; if blood starts to flow backwards, it fills the pockets and closes the valves. This prevents backflow.Because the pressure is so low, blood in veins, especially in the legs, needs help to return to the heart against gravity. When the skeletal muscles around a vein contract, they squeeze the thin-walled vein and push blood along it. The valves make sure the blood can only be pushed towards the heart. Breathing movements of the chest also help draw blood back to the heart.
Thin wall. Veins have the same three layers as arteries, but the middle layer has much less smooth muscle and elastic tissue. The blood pressure is low, so a thick, elastic wall is not needed.Wide lumen. The lumen is large compared with the thickness of the wall. This reduces resistance, so blood can flow easily at low pressure.Valves. Veins contain pocket-shaped valves. Blood flowing towards the heart pushes them open; if blood starts to flow backwards, it fills the pockets and closes the valves. This prevents backflow.Because the pressure is so low, blood in veins, especially in the legs, needs help to return to the heart against gravity. When the skeletal muscles around a vein contract, they squeeze the thin-walled vein and push blood along it. The valves make sure the blood can only be pushed towards the heart. Breathing movements of the chest also help draw blood back to the heart.