Blood circulates around the body through different blood vessels like Arteries, Veins and Capillaries.
Type # 1. Arteries:
Arteries carry oxygenated blood away from the heart. Only pulmonary artery is an exception which carries deoxygenated blood. These arteries are divided into smaller branches which are known as arterioles. The walls of the arteries are thick, strong and elastic Blood passes through this with a greater force.
Arteries are composed of three coats:
1. Tunica Adventitia:
This is the outer coat of the artery which is composed of fibrous connective tissue. The outer coat is protective. It gives protection and strength to the vessel.
2. Tunica Media:
This is the middle, muscular and elastic coat. It consists of un-striated muscle fibers with some yellow elastic fibers. The muscle fibers are arranged in a circular manner. The middle layer is strong. It holds the blood vessel open. This layer determines the amount of supply of blood to an organ and it maintains the blood pressure.
3. Tunica Intima:
This is the inner endothelial coat. This layer is consisting of flattened endothelial cell which is very smooth.
The amount of elastic tissue fiber are in large quantities in aorta and other large arteries. They contain less muscular tissue. The smaller arteries and arterioles have more muscle tissue. The aorta is the main artery of the body. After leaving the left ventricle of the heart it is divided and subdivided into so many arteries and arterioles.
The main branches are thoracic aorta and abdominal aorta. Thoracic aorta is divided into carotid and subclavian arteries. Abdomind aorta gives off a number of branches.
1. Coliac axis beneath the diaphragm and divides into three branches:
(a) Hepatic,
(b) Gastric and
(c) Splenic to supply liver, stomach, pancreas and spleen.
2. Mesenteric arteris: Supply the mesentery and intestines.
3. Renal arteries: in kindneys.
4. Testicular arteries: in the male and ovarian in the females.
5. Right and left common iliac arteries.
Type # 2. Veins:
Venules and veins carry blood towards the heart. Small venules unite to form veins. They carry deoxygenated blood from the lungs to the left auricle of the heart. Veins are consisting of three layers like arteries. The walls of the veins are much thinner than the arteries.
The three layers are:
(1) Tunica adventitia,
(2) Tunica media, and
(3) Tunica intima.
Middle muscular layer is thinner, less firm, more collapsible and much less elastic than the arteries. Though the walls of the veins are very thin, yet the vessels are very strong, due to the presence of connective tissues. In tunica intima layer of the vein, the endothelial cells are less elongated. Structure of all the veins are not the same. Some veins do not possess smooth muscles.
Most of the veins of the limbs and abdomen are provided with pocket shaped valves in their interior. They are so arranged that, blood can only pass towards the heart. They prevent the flow of blood in the opposite direction. These valves are semilunar pocket like folds which are formed by the folding of the intima layer of the veins.
These folds are opposite one another and their free edge is towards the direction of the flowing of the blood. When the veins swell out with blood, they give a” knotted appearance. Veins carry blood towards the heart. They begin as small vessels formed by the union of capillaries. These small veins unite and become larger veins, eventually forming the venous trunks which increase in size as they near the heart. Veins are more numerous and larger than arteries.
The main veins in the body are:
1. Superior Vena Cava which is about 7-5 C.M. (3″) long. It receives the blood from the head, neck, both upper limbs and the anterior walls of the thorax and empties its contents into the upper part of the right auricle of the heart.
2. The Inferior Vena Cava which receives blood as it passes up through the abdomen, the diaphragm to the heart. There are a number of smaller veins which open into these two large veins.
Type # 3. Capillaries:
The capillaries are the smallest blood vessels in which the arterioles terminate and the venules begin. Generally, these are arranged to form a network. Due to the continuous division of the artery, all the three coats disappear and a fine fiber like capillary vessels are formed. So their walls are becoming very thin.
As a result of this, there is exchange of gases, nutrients, lymph, fluid etc. are passed through this wall to the tissues. Capillaries therefore perform a very important function of distributing the materials to the tissues and cells by which all the processes of the body continue.
The composition of the blood varies in the arteries and veins. Arterial blood contains oxygen and is bright scarlet in colour, because the haemoglobin is combined with oxygen. If an artery is cut across, the bright red blood will be seen. Venous blood is darker and purplish in colour, as carbon dioxide is more and oxygen has been given up to the tissues.
If a vein is cut across, the blood flows out in an even stream. Blood in the capillaries is continuously changing its composition and colour due to the interchange of gases. The blood is oozing smartly on to the surface from the capillary bleeding.
