Blood Vessels And Their Function

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Arteries carry blood away from the heart and into arterioles. Arterioles are smaller arteries that control the blood flow from the arteries to capillaries. Capillaries are tiny vessels that link arterioles to veins. Veins carry blood from capillaries back to the heart. Arteries, arterioles and veins all have the same basic layered structure. From the outside inwards, these layers are: Tough outer layer that resists pressure changes from both within and outside. Muscle layer that can contract and so control the flow of blood. Elastic Layer that helps to maintain blood pressure by stretching and springing back. Thin Inner layer (endothelium) that is smooth to prevent friction and thin to allow diffusion. Lumen not actually a layer but the central cavity of the blood vessel through which blood flows. Artery Structure related to function: The function of the arteries is to transport blood rapidly under high pressure from the heart to the tissues. The muscle layer is thick compared to veins. This means smaller arteries can be constricted and dilated in order to control the volume of blood passing through them. The elastic layer is relatively thick compared to veins because it is important that blood pressure in arteries is kept high if the blood is to reach the extremities of the body. (Stretched at systole, and springs back during diastole. Stretching helps to maintain the high pressure and smooth pressure created by the beating of the heart. The overall thickness if the wall is large. This also resists the vessel bursting under pressure. There are no valves because blood is under constant high pressure and therefore does not tend to flow backwards. Arteriole Structure related to function: Arterioles carry blood, under lower pressure than arteries, from arteries to capillaries. They control the flow of blood between the two. The muscle layer is relatively thicker than in arteries. The contraction of this muscle allows the lumen of the arteriole to constrict; this restricts the flow of blood. The elastic layer is relatively thinner than in arteries because blood pressure is lower. Vein Structure related to function: Veins transport blood under low pressure, from the tissues to the heart. The muscle layer is relatively thin compared to the arteries due to carrying blood away from the tissues and therefore constriction/dilation cannot control flow of blood to the tissues. The elastic layer is relatively thin¸ the blood has a low pressure so the veins will not burst. The overall thickness of the wall is small as the pressure within the veins is too low to cause them to burst. Allows them to be flattened easily. There are valves throughout to ensure that blood does not flow backwards, which it may otherwise do due


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