Blood Vascular System consists of a series of arteries that carry blood into all parts of the body. They divide into smaller arterioles, which in turn divide on entering the tissues into minute microscopic tubes, the capillaries.
The venous capillaries originate from the arterial capillaries and are interposed between the arterial capillaries and the beginning of the veins. The veins in turn carry impure blood to the heart.
ARTERIES
- The tubes which convey blood from the heart to various organs are called arteries. These are pulmonary and systemic arteries.
- Pulmonary artery leaves the heart by a single trunk, subdivides and reaches the lungs. This is called as pulmonary circulation.
- The systemic arteries originate from the aorta, which arise from the left ventricle and conveys pure blood to all parts of the body.
- The branches which result from the division of an artery at the termination of its course are named as terminal branches.
- The branches arising from an artery at varying intervals along its course are called collateral branches.
- The arteries are surrounded by a layer of connective tissue and may become related to the structures like veins, nerves, muscles, bones and skin.
- In bone marrow, heart, liver and small blood vessels differ from the capillaries as they are wider with irregular lumen and have no connective tissue covering.
- Their endothelial cells are in direct contact with cells of the organs and their walls are often incomplete. These vessels are called as sinusoids.
Anastomoses
- Branches of adjacent arteries communicate with each other and form anastomoses.
- When two vessels converge and unite with each other at their termination it is termed as anastomoses by convergence.
- When two branches are directed towards each other and unite to form a single curved tube it is called as anastomoses by arches.
- When the transverse branches connect the two adjacent parallel arteries it is termed as anastomoses by transverse communication.
Histology
- The structure of the artery composed of three coats as tunica intima, tunica media and tunica adventitia.
- The tunica intima is an endothelial lining with delicate layer of connective tissue and internal elastic lamina.
- The tunica media consist of smooth muscle cells with varying amount of collagenous and elastic tissue.
- The tunica adventitia consists of external elastic lamina and connective tissue.
VEINS
- Veins are thin walled structures compared to arteries that carry impure blood to the heart.
- The structure of veins resembles the arteries with few differences.
- The venous wall is composed of all the three layers but much thinner due to the reduction of muscular and elastic tissue components and lack of limiting membrane.
- The veins collapse when empty due the fact that they are thinner.
- Most of the veins excepting the trunk veins are provided with valves. These valves are formed by in folding of the tunica intima.
Blood supply and innervation
- The walls of the arteries and veins are supplied with blood by a large number of small arteries termed vasa vasorum.
- The lymphatics also ramify chiefly in the external coat.
- The nerves accompany the vessels, forming plexus around them and terminate in the muscular layer of the middle coat.
