Main Blood Vessels
Mammals have a closed blood vascular system. It has this name because blood is transported from the heart to all different parts of the body and back in a set of closed tubes. We have already made a study of the heart as part of the blood vascular system. We are now going to make a stydy of the blood vessels.
Blood vessels differ in structure and function. Blood leaves the heart through the arteries, which conduct the oxygenated blood (except in the case of the pulmonary artery) to the various tissues and organs. Deoxygenated blood returns from the tissues and organs to the heart via a set of vessels, called veins (except the pulmonary vein). The actual exchange of oxygen, carbon dioxide, foodstuffs and waste matter between the blood and the tissue fluid occurs in microscopically small vessels, called capillaries.
We shall now look at the structure and function of arteries, veins and capillaries as well as some of the main blood vessels.
Blood Vessels of the body.
Arteries are vessels which transports blood away from the heart. They branch repeatedly to form arterioles (small arteries), the smallest and thinnest branches of arteries.
Arteries receive blood under high pressure from the ventricles of the heart. They must therefore
be able to stretch each time the heart beats, without collapsing under the increased
pressure. The walls of arteries consist of three layers, namely an outer layer, a thick middle
layer and an inner layer. The outer layer consist of white fibrous connective tissue
which merges to the outside with the loose connective tissue in which artery is found. This helps
to anchor the arteries because the heart pumps the blood through the arteries at a great
pressure. The thick middle layer consist of elastic connective tissue and involuntary muscle
tissue. This layer is supplied with two sets of nerves, one stimulating the muscles to relax
so that the artery is allowed to widen, and the other one causing the circular muscles to
contract, making the artery become narrower. The inner layer of endothelium consists of flat
epithelial cells which are packed closely together and which is continous with the
endocardium of the heart. The flat cells make the inside lining of the arteries smooth to limit
friction between the blood and the lining to a minimum.
The arteries carry blood from the heart
Veins are vessels which transports blood to the heart. From the capillaries the blood drains into very small veins called venules, which unite into larger veins along which the blood returns to the heart.
Since veins conduct blood back to the heart,the pressure exerted by the heartbeat on them
is much less than in the arteries. The middle muscular wall of a vein is therefore much
thinner than that of an artery. Veins differ from arteries also in that they have semi-
lunar valves, which prevent the blood from flowing backwards.
Veins transport blood to the heart
Arteries and veins are connected by blood capillaries, microscopically small tubes which form a network, the capillary bed in different tissues. The blood capillaries are so small that red blood corpuscles can only move in a single file through it. The exchange of all substances takes place in these capillaries.
When arteries reach the organs they serve and divide into small vessels, called arterioles
. These in turn divide again into minute capillaries which average 0.01 mm in
diameter. Their walls are only one cell thick - only the layer of endothelial cells
Capillaries are important for gaseous exchange.
The main blood vessels are the following:
The aorta is the largest and principal artery in the body. From the aorta branches lead to
all the organs of the body, supplying them with oxygen and nutrients.
The coronary artery is also a branch of the aorta. It supplies the heart tissue with oxygen
The pulmonary artery arises from the right-hand upper corner of the roght ventricle. It branches
into the left and right pulmonary arteries which lead to the right and left lung respectively,
where the blood is oxygenated.
This vein returns deoxygenated blood from the head, arms and thorax to the right atrium.
This vein returns deoxygenated blood from the abdomen and legs to the right atrium.
Deoxygenated blood containing carbon dioxide and waste products is returned from the heart
muscle to the right atrium through this vessel.
The four pulmonary veins, two from each lung, carry oxygenated blood from the lungs to the
The hepatic portal vein leads from the internal organs, especially the small intestine, to the liver. Its main purpose is to carry blood that contains digested food such as glucose, from the liver small intestine to the liver, where some of the food is deposited and stored such as glucose that is stored as glycogen in the liver.