Components of the circulation and their specific functions.doc - 6 Components of the circulation and their specific functions Components of the

Components of the circulation and their specific functions.doc

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6. Components of the circulation and their specific functions Components of the circulation A. Structure of vessel walls relates to their function B. Function 1. Arteries 2. Arterioles 3. Capillaries 4. Veins Aorta goes to large arteries which leads to small arteries. Blood vessels enter tissue and are in form of arterioles, then capillaries, venules, veins and vena cava. Specialised function of vessel types: Function depends upon: - Thickness of the vessel wall - Radius of the lumen - Wall composition can comprise of smooth muscle. Everything except capillaries have smooth muscles in their walls - Smooth muscle - Connective tissue with collagen and elastin - Endothelial cells (found in every vessel) line the whole cardiovascular system, secrete vasoactive substances nitric oxide (NO), endothelin, prostacyclin), express enzymes on their luminal membranes angiotensin converting enzyme (ACE, carbonic anhydrase). 1
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1. Components of the (systemic) circulation The basic components of the circulation are the arteries, arterioles, capillaries and veins . Each class of vessels is connected in series (i.e., arteries are connected in series to arterioles) while within each class (e.g., capillaries) branching means that the individual elements are connected in parallel. As well as their obvious function as conduits for blood, each of the components have specialized functions that are reflected by their structure, particularly the composition and thickness of vessel walls and the diameter of the lumen of the vessels. Diameter of the lumen determines the resistance to flow. Very small diameters are associated with resistance or exchange vessels. Composition and structure of the vessel walls reflect i) different functions of the various components. ii) constraints applied by the Laplace equation The Laplace equation states that T = Pr where T is wall tension, P is the transmural pressure and r the vessel radius. Thus, the aorta with a large radius requires a thick, strong wall to provide the wall tension necessary to prevent tearing as a result of the blood pressure. In capillaries, the blood pressure is only 4 × lower than in the aorta but the capillary wall is only one endothelial cell thick. The thin capillary wall can contain the blood because its very small diameter means that the wall tension required to oppose the stretching of the wall by the blood pressure is miniscule compared to that in the artery. Laplace’s equation explains why vessels with large diameters need strong, thick walls to contain the pressure exerted by the blood without stretching or even tearing. And why capillaries have such thin walls. Blood pressure on inside. And pressure on outside is virtually 0. We have the wall tension on the outside and has to be sufficient so that the pressure inside the blood vessel doesn’t stretch and ultimately tear the vessel. The relationship between the wall tension and the blood pressure and the radius gives the Laplace equation. Wall tension = - pressure x radius. T = wall tension P = pressure r = radius of vessel.
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