The Blood Vessels

Blood vessels are named according to their structure and function. 


Vessels carrying blood away from the heart are' called arteries. These branches into smaller arteries called arterioles. The arterioles divide many times into microscopic capillaries, which are 'located among the cells of nearly all the body tissues. Within the organ or tissue the capillaries reunite formingVenules. The venulesjoin toformveins. 



Arteries: Arteries oxygenated blood except the pulmonary arteries which carry deoxygenated blood. Arteries are pink in colour and are situated within the muscles. Arteries vary in size. Aorta is 23 mm and arterioles are about 0.2 mm in diameter. Each artery consists of three layers: 

1) Tunica externa: It is the external layer of fibrous connective tissue  having collagen (a protein) fibres. 
Tunica media is the middle layer of smooth muscles and elastic fibers. Tunica intima is the inner layer of squamous endothelium. 
         
2) The middle layer is important to withstand higher pressure during ventricular systole. In the arterioles there are more circular muscles than the elastic tissue. The contraction of the circular smooth muscles of the arteries is under the control of nervous and endocrine system. When stimulated the muscles contracts, constricting the arteriole i.e, vasoconstriction and reducing the flow of blood in them. 



When the muscles are relaxed and there is vasodilatation of the arterioles more blood flows in the arterioles. 




Capillaries are microscopic one celled thick blood vessels. A capillary consists of a single layer of endothelial cells. Their number is highest in the regions where 'most of the metabolic activities are taking place. The average diameter of capillaries is 7 to 10 microns, just about that of single red blood cell. Hence blood cells move through the capillaries in a single row. 

Exchange of gasses, nutrients, wastes and hormones between the blood of the capillaries and various cells and tissues occur by diffusion and active transport. The number of capillaries which arise from a single arteriole is sufficiently great and the total cross sectional area available for the flow of the blood is increased. Veins: The blood vessels that bring blood back to the heart are called veins. A vein also consists of thee layers: tunica externa - the outer layer, Tunica meida - the middle layer, tunica intima - the inner layer, which are less developed and have less elastic fibers as compared to an artery. The lumen ofthe vein is large. Semilunar valves are present. Valves are formed from the folds of the iriner layer of the veins. Valves are present only in the lower part of the body specially in the abdomen and hind limbs. In the upper region above the heart there is no valve.