Unit I

9. Blood VesselsHistologically blood vessels show three basic layers tunica intima, media and adventitia. In the artery themedia is thicker than other two coats, while in the vein the adventitia is thicker than the other two coats.Depending upon the amount of elastic fibres or smooth muscles,the arteries are either elastic or musculartype.Elastic artery: Compared with the intima in other vessels elastic artery shows very thick intima, one fifthof total thickness of the wall. It is pale, showing incomplete laminae of elastic fibres embedded along withcells in an amorphous intercellular substance. The luminal surface is lined by the simple squamous endothelium.Internal elastic lamina is not easy to distinguish. Nuclei of fibroblast and macrophages are seen in theintima. The bulk of media is formed by concentrically arranged fenestrated laminae of elastic fibres 40- 70in number. Nuclei of smooth muscle cells are seen in the adjacent laminae. The media is limited externallyby distinct external lamina. Adventitia shows, thin irregularly arranged connective tissue containing bothcollagen and elastic fibres and vasa vasorum. Blood vessels and lymphatics are seen up to outer part of themedia. Unmyelinated sympathetic fibres supply the muscles in the arterial wall. During systole the elasticarterial wall is stretched and during diastole because of elastic recoil the artery contracts passively andmaintains the blood pressure during diastole e.g. Ascending aorta, arch of aorta, thoracic aorta, pulmonaryartery etc.Muscular artery: The size of lumen of this type of artery is regulated by the muscles, so it is also called asa distributing artery. Blood supply is under sympathetic control. Tunica intima shows simple squamousepithelium (endothelium) with a small amount of subendothelial connective tissue. Internal elastic lamina isseen as a wavy bundle at the junction of intima and media. It is usually in contracted condition giving awavy appearance to the intima and is easily seen as bright pink line. Split internal elastic lamina may beseen. The media shows spirally disposed smooth muscle cells with intercellular substance secreted by themuscle cells containing chiefly elastin. External elastic lamina made up of elastin is a substantial lamina at thejunction of the media and adventitia. The adventitia is half or two third the thickness of media and showscollagen and elastic fibres. Vasa vasorum and lymphatics are also seen e.g. Subclavian, axillary, femoral,popliteal artery etc.Arteriole: Smaller branches of an artery with less than 100µ diameter are called as arterioles.Tunicaintima shows simple squamous endothelial layer lying on base of intervening tissue. Internal elastic laminamarks the end of intima. Media shows circularly arranged smooth muscle fibres and is limited on its outersurface by external elastic lamina. Adventitia shows the mixture of collagen and elastic fibres. In smallerbranches of arterioles internal elastic lamina is not seen, while only one or two smooth muscle cells are seenin the media. If the diameter is less than 50µ then it is terminal arteriole and lateral branches of terminalarterioles are called metaarterioles, which supply a capillary bed. Arterioles are best seen in the slide ofspleen.Notes:43