They form a network of tubes that carry blood from the heart to body tissues and back to the heart. Blood vessels can be divided into arterial and venous system:
It is a set of vessels that start from the heart, branching out, each branch smaller in size, until they reach the capillaries.
They form a set of vessels that start from the tissues, forming larger branches until they reach the heart.
The blood vessels that carry blood out of the heart are the arteries. These branches branch out, become progressively smaller, and end up in small vessels of certain arterioles. From these vessels, the blood is able to perform its nutrition and absorption functions through a network of microscopic channels, called capillaries, which allow to the blood to exchange substances with the tissues.
From the capillaries, blood is collected in venules; then through the larger diameter veins it reaches the heart again. This passage of blood through the heart and blood vessels is called the BLOOD CIRCULATION.
1- External Tunic: It is basically composed of connective tissue. In this tunic we find small nerve and vascular fillets that are intended for the innervation and irrigation of the arteries. Found in large arteries only.
2- Medium tunic: is the middle layer composed of smooth muscle fibers and small amount of elastic connective tissue. Found in most arteries of the body.
3- Intimate Tunic: lines the arteries internally and without interruption, including capillaries. They are made up of endothelial cells.
Blood vessels are composed of several anastomoses, mainly in the cerebral vessels.
Anastomosis: means connection between arteries, veins and nerves which establish communication with each other. The connection between two arteries occurs in arterial branches, never in major trunks. Sometimes two small arteries anastomose to form a larger caliber vessel. Frequently the connection is made by long vessels, through thin vessels, ensuring a collateral circulation.
The Willis Polygon (best studied in & #8220; CNS Vascularization & #8221;) is an example of anastomosed vessels forming a polygon. This process occurs in the brain to ensure adequate oxygen demand from nerve cells, ie if a cerebral artery is blocked, the region irrigated by the injured vessel will still receive blood from another polygon artery, preserving the nerve tissue.