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blood vascular.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

The Cardiovascular System: Blood Vessels Blood Vessels • Blood is carried in a closed system of vessels that begins and ends at the heart • The three major types of vessels are arteries, capillaries, and veins • Arteries carry blood away from the heart, veins carry blood toward the heart • Capillaries contact tissue cells and directly serve cellular needs Continuous Capillary Structure Fenestrated Capillary Structure Discontinuous Sinusoidal Capillary Structure Generalized Structure of Blood Vessels • Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa • Capillaries are composed of endothelium with sparse basal lamina • Lumen – central blood-containing space surrounded by tunics Tunics • Tunica interna (tunica intima) • Endothelial layer that lines the lumen of all vessels • In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present • Tunica media • Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system • Controls vasoconstriction/vasodilation of vessels • Tunica externa (tunica adventitia) • Collagen fibers that protect and reinforce vessels • Larger vessels contain vasa vasorum Elastic (Conducting) Arteries • Thick-walled arteries near the heart; the aorta and its major branches • Large lumen allow low-resistance conduction of blood • Contain elastin in all three tunics • Withstand and smooth out large blood pressure fluctuations • Allow blood to flow fairly continuously through the body Muscular Arteries and Arterioles • Muscular arteries – distal to elastic arteries; deliver blood to body organs • Have thick tunica media with more smooth muscle and less elastic tissue • Active in vasoconstriction • Arterioles – smallest arteries; lead to capillary bed
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