The function of Arteries:

Arteries are the blood vessels that carry blood away from the heart. In adults, all arteries except the pulmonary arteries, carry oxygenated blood.

 Structure of an artery:

Their structure shows that arteries are well adapted to their function Layers of an artery: The walls of an artery are composed of three layers.

1. Tunica externa:

The outermost layer is known as the tunica externa and it is composed of connective tissue.

2. Tunica media:

The middle layer is the tunica media and is made up of smooth muscles and elastic tissue.

3. Tunica intima:

The innermost layer is the tunica intima and is made up of mainly endothelial cells

The function of Lumen:

The hollow internal cavity in which the blood flows is called the lumen.

Arterioles:

When arteries enter body organs, they divide into smaller vessels known as arterioles. The arterioles enter tissues and divide into capillaries.

The function of Capillaries:

Capillaries are the smallest blood vessels, which are formed by the divisions of arterioles. The exchange of materials between blood and tissue fluid is carried out through the capillaries.

Structure of Capillaries:

The walls of capillaries are composed of only a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water, and lipids can pass through them and enter the tissue fluid. Waste products such as carbon dioxide and urea can diffuse from tissue fluid into the blood.

Note:

Capillaries are so small that the red blood cells need to partially fold into bullet-like shapes to pass through them in a single file.

The function of Veins:

A vein is a blood vessel that carries blood toward the heart. In adults, all veins except the pulmonary veins, carry deoxygenated blood. Veins are also well adapted to their function.

Structure of Veins:

The walls of the vein are composed of the same three layers as are present in the artery wall. The tunica externa and the tunica intima have the same composition, but the tunica media of a vein is comparatively thin as compared to that of an artery. It has fewer smooth muscles and elastic tissue. The lumen of the veins is broader than that of arteries.

The function of Valves:

In a tissue, capillaries join to form small venules, which join to form veins. Most veins have flaps called valves that prevent the backflow of blood.