What are the General Features of Blood Vessels?

Arteries carry blood from the heart to different body parts. Veins bring blood from different body parts to the heart. In the organs both arteries and veins divide to form arterioles and venules respectively. The arterioles and venules further divide into the thin walled vessels called capillaries. The capillaries of arteries and veins are united. The nutrient, hormones, gases, etc. can diffuse into the tissue cells through the walls of the capillaries and vice versa. The veins have valves to prevent backward flow of blood.

The wall of an artery and a vein consists of three coats:

(i) Tunic external: It is the outermost coat which is formed of connective tissues. Tunica external is also called topical adventitia.

(ii) Tunica media: It is a middle coat which is chiefly formed of elastic connective tissues and smooth muscle fibres.

(iii) Tunica internal: It is the innermost coat which is made up of two parts:

(a) Elastic membrane: This membrane is made up of elastic tissues.

(b) Endothelium: It is made up of flattened epithelial cells lining the lumen. Blood capillaries consist of endothelium only.