BLOOD VESSELS AND COURSE OF CIRCULATION OF BLOOD

Arterioles:

Arterioles are small arteries that deliver blood to capillaries. Arterioles also have smooth muscles on their walls.

Contraction and relaxation of these muscles alter the diameter of arterioles and thereby respectively reduce and increase the blood flow through them.

Capillaries:

Capillaries were discovered by Malpighi in 1661.

Capillaries are the smallest blood vessels in the body.

Capillary has no muscular wall.

Its wall is made of a single layer of flat endothelial cells and is consequently very permeable to water and small solutes, but not to proteins and other macromolecules.

The diameter of lumen of capillaries is from 7.5 µm to 75 µm. Only about 5 to 7% of the total volume of blood is contained in the capillaries.

Venules:

Venules are small vessels that continue from capillaries and merge to form veins. They drain blood from capillaries into veins.

Veins:

Veins have less elastic tissue and smooth muscle than arteries.

One major difference between an artery and vein is that vein has a thin muscular wall.

Veins contain valves to prevent backflow of blood.

Valves are necessary in veins but not in arteries because pressure in veins is low to push the blood.

Weak valves can lead to varicose veins or haemorrhoids.

All veins carry deoxygenated blood except pulmonary veins.

Pulmonary veins carry oxygenated blood from lungs back to heart.

Blood vessels that carry blood from lungs to heart are called pulmonary veins.

Wall of veins is collapsible (non collapsible in arteries).

Lumen of vein is wider and narrow than arteries.

Most veins are located close to skin (deep in body in case of arteries).

Vasa Vasorum : Blood vessels which supply blood to blood vessel are 'vasa vasorum'.

Course of Circulation of Blood

There are three types of circulation :

(a) Pulmonary circulation: from heart to the lungs and back.

(b) Systemic circulation: when blood is passed to different parts of the body.

(c) Portal circulation

Schematic plan of blood circulation in human