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SBI3U BLOOD VESSELS Total length of human blood vessels = ~ 160 000 km ARTERIES FUNCTION: Rapid transport of high pressure blood from the heart to the capillary blood. STRUCTURE: Thick layer of muscle and elastic tissue walls stretch when blood pumped from the heart walls recoil when heart relaxes Elasticity helps maintain the blood pressure NOTE: arterioles are smaller and less elastic VEINS FUNCTION: Carry blood from capillary beds to the heart STRUCTURE: Thinner walls due to thinner layer of muscle walls stretch more easily blood under low pressure Contains valves maintain flow towards the heart (prevents back flow) NOTE: Contraction of skeletal muscles squeezes the veins blood is moved through the veins CAPILLARIES FUNCTION: Link arterial and venous systems Exchange of material between blood and cells of the body Forms dense capillary beds STRUCTURE: Smallest blood vessels large enough to let a single red blood cell pass Walls consist of only 1 layer of cells allows for rapid exchange of materials between blood and interstitial fluid (fluid that lies outside of circulatory system) NOTE: Amount of blood flowing in a particular capillary bed is regulated corresponds to the activity of the particular tissue/organ When skeletal muscles are at rest only 5-10% of the capillary beds will be open at any one time.