BLG 144 : Bio unit 4 lesson 2-3.docx
1 May 2012
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Arteries carry blood away from the heart towards the rest of the body. Pulmonary artery carries deoxygenated blood from the heart to the lungs. Pulmonary vein carries blood from the heart back to the lungs. Vein has thinner wall with a larger circumference and lacks elasticity. Capillaries are the smallest blood vessels, typically less than 1 mm long. Capillaries have a diameter that is so small that red blood cells travel in single file through them. The thin walls allow exchange of materials between blood and cells. Even though capillaries are the smallest blood vessels, they have the largest total surface area. In a closed circulatory system, blood always remains inside the capillaries. It never flows out to bathe the body"s cells directly. Since the capillaries have very thin walls, all exchange takes place inside the capillaries. Capillaries regulate the movement of fluids and other materials into and out of the blood stream.
