After studying this section about Blood vessel , you should be able to:
- Describe the structures and functions of arteries, veins and capillaries
- explain the relationship between the different types of Blood vessel
- indicate the main factors controlling Blood vessel diameter.
1. What are Blood vessel
A tube through with the blood circulates in the body. The Blood vessel vary in structure, size and function, and there are Several types: arteries, capillaries, venules and vein, arterioles.
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2. Arteries and arterioles
These Blood vessel transport blood away from the heart. They vary considerably in size and your walls consist of three layers of tissue :
- tunica adventitia or outer layer of fibrous tissue
- tunica media or middle layer of smooth muscle and elastic tissue
- tunica intima or inner lining of squamous epithelium called endothelium
The amount of muscular and elastic tissue varies in the arteries depending upon their size of function. In the large arteries , including the aorta, sometimes called elastic arteries, the tunica media contains more elastic tissue and less smooth muscle. This allows the vessel wall to stretch, absorbing the pressure wave generated by the heart as it beats. These proportions gradually change as The arteries branch many times and become smaller until the arteries branch many times and become smaller until in the arterioles( the smallest arteries) the tunica media consists almost entirely of smooth muscle.
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3. Anastomoses and end-arteries
Anastomoses are arteries that form a link between main arteries supplying an area,e.g. the arterial supply to the palms of the hand and soles of the feet, the brain,the joints and, to a limited extent, the heart muscle. If one artery supplying the area is occluded, anastomotic arteries provide a collateral circulation. This is most likely to provide an adequate blood supply when the occlusion occurs gradually, giving the anastomotic arteries time to dilate.
4. Capillaries and sinusoids
The smallest arterioles break up into a number of minute vessels called capillaries. Capillary walls consist of a single layer of endothelial cells sitting on a very thin basement membrane, through which water and other small molecules can pass. Blood cells and large molecules such as plasma proteins do not normally pass through capillary walls. The capillaries form a vast network of tiny vessels that link the smallest arterioles to the smallest venules.
5. Veins and venules
veins return blood at low pressure to the heart. The walls of the veins are thinner than arteries but have the same three layers of tissue . They are thinner because there is less muscle and elastic tissue in the tunica media, as veins carry blood at a lower pressure than arteries. When cut, the veins collapse while the thicker-walled arteries remain open. When an artery is cut blood spurts at high pressure while a slower, steady flow of blood escapes from a vein. some veins possess valves, which prevent backflow of blood, ensuring that it flows towards the heart. They are formed by a fold of tunica intima and strengthened by connective tissue.
6. Blood supply
The outer layers of tissue of thick-walled Blood vessel receive their blood supply via a network of Blood vessel called the vasa vasorum. Thin-walled vessels and the endothelium of the others receive oxygen and nutrients by diffusion from the blood passing through them.
7. Control of blood vesseldiameter
The smooth muscle in the tunica media of veins and arteries is supplied by nerves of the autonomic nervous system. These nerves arise from the vasomotor centre in the medulla oblongata and they change diameter of Blood vessel, controlling the volume of blood they contain.
The Blood vessel most closely regulated by this nervous mechanism are the arterioles, since they contain proportionately more smooth muscle in their walls than any other blood vessels. The walls of large arteries such as the aorta contain mainly elastic tissue and so they tend to passively expand and recoil , depending on how much blood is passing through them. Veins also respond to nerve stimulation, although they have only a little smooth muscle in their tunica media,