Physiology- The cardiovascular system

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Discuss the structure of veins:

Veins are capacitance vessels. Hence, they have a large lumen and relatively thin tunica media when compared to arteries of the same size. The diameter of veins can vary quite drastically - 200 micrometers - 3cm. This is due to the abillity of veins to collapse in on themselves and expand out again without loss of elasticity. Veins are composed of 3 main layers. The tunica intima, media and adventitia. The epithelium lines the lumen of the vein. It is composed of simple squamous cells. It functions as a selectively permeable barrier. It is resposnisble for leukocyte production which is important for inflammatoroy responses to damaged tissues. The endothelium is also necessary for haemostasis and smooth muscle cell modulation. Haemostasis requires NO, prostacyclin and endothelin. Smooth muscle modulation needs NO, prostacyclin and von Willlebrand's factor. Underneath the epithelium( the subendothelial layer) loose connective tissue can be found along with fibroblasts and myointimal cells which are important in atherosclerosis. The tunica intima also has a basal membrane.

The tunica media is made up of smooth muscle vascular cells which regulate the lumen diameter, fibroblasts which produce connective tissues, collagen fibers to provide mechanical support while elastin is responsible for the elasticity of the vessels. The elastin is found as fenestrated sheets or membranes in the wall.

The adventitia has elastin and collegen also but it holds vasa vasorum which the media does not and automatic nerves as well.

The formula for determining the flow of blood in the veins is:

F= P1-P2

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R

R is proportional to 1/r^4

The mean arterial pressure of a system (MAP)

MAP= CO (Cardiac Output) x TPR (Total Peripheral Resistance)

TPR usually determined by resistance arteries

CO= HR x SV

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