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Blood Vessels and Routes Functions of Blood Vessels • Arteries carry blood AWAY from the heart • Veins carry blood TOWARD the heart • Vasa vasorum are blood vessel that supply blood to the walls of blood vessels Route of Blood Vessels • After leaving the heart the flow of blood passes through the following vessels in this order: - Elastic arteries - Muscular (or distributing) arteries - Arterioles - Capillaries - Venules - Veins - Vena cavae 1 Histology of Blood Vessels • Arteries have tree coats (tunics) and a lumen (space where the blood flows) (1) Tunica Interna (intima) – simple squamous epithelium (2) Tunica Media – smooth muscle (3) Tunica Externa (adventicia) – elastic fibers Properties of Arteries • Arteries have two properties (1) Elasticity (2) Contractility - vasoconstiction - vasodilation 2 Types of Blood Vessels • Elastic (conducting) arteries • Recoil to force blood through the capillaries after contraction of the heart • These are the large arteries that take blood to the head (carotid), arms (brachial), and legs (femoral) Types of Blood Vessels • Muscular (distributing) arteries • Can vasoconstrict and vasodilate to control the flow of blood to various parts of the body • These are the smaller (than elastic) arteries that supply blood to organs such as the kidney (renal), stomach (gastric), and liver (hepatic) Types of Blood Vessels • Arterioles • These are small branches of arteries that empty into blood capillaries • Consists of a single layer of simple squamous endothelial cell with a few smooth muscle fibers 3 Types of Blood Vessels • Capillaries • These are microscopic vessels that connect arterioles to venules • Their function is the exchange of gases, nutrients, and wastes between the blood and the tissues • Consists of a single layer of simple squamous endothelial cells Types of Blood Vessels • Venules • These are small veins that drain blood from several blood capillaries Types of Blood Vessels • • Veins These are large vessels that have the same three coats as arteries, but in different proportions (1) Tunica Interna – is extremely thin (2) Tunica Media – thinner than arteries (3) Tunical Externa – thicker than arteries Note: veins have valves that prevent the blood in them from flowing backwards 4 Capillary Exchange • The velocity of blood slows down as it passes through the capillaries because the total cross-sectional area of the capillaries is much larger than any other component of the circulatory system • This allows time for the exchange of gases and nutrients Capillary Exchange (continued) • Blood does not flow each capillary all the time – rather the capillary undergoes vasomotion (opening and closing) 5 to 10 times a minute • This is necessary because there is not enough blood in the body to fill every capillary at the same time 5 Starlings Law of the Capillaries • The amount of blood forced out of the arteriole end of the blood capillary by Blood Hydrostatic Pressure (BHP) is almost equal to the amount of blood drawn back into the venule end of the blood capillary by Blood Osmotic Pressure (BOP) • The fluid left over is picked up by a lymphatic capillary and directed back into the general circulation 6 Velocity of Blood Flow • The velocity of the blood flow is related to the total crosssectional area of the type of vessel the blood is in. For example: Cross-sectional area Velocity Aorta 2.5 cm2 40 cm/sec 10 – 40 cm/sec Arteries 30 cm2 Arterioles 40 cm2 0.1 cm/sec Less than 0.1 cm/sec Capillaries 2500 cm2 Venules 250 cm2 0.3 cm/sec 0.3 to 5 cm/sec Veins 80 cm2 5 to 20 cm/sec Vena Cavae 8 cm2 Skeletal Muscle Contractions • Contraction of skeletal muscles MILKS the veins pushing blood toward the heart • The valves in the veins prevent the blood from flowing backwards Breathing • A decrease in interthoracic pressure (during an exhalation) allows venous blood to flow upward toward the heart 7 Summary of Venous blood flow • The three factors listed above aid in venous blood flow back to the heart (1) The velocity of blood flow in the veins increases due to the greater total crosssectional area of veins over capillaries. (2) Skeletal muscle contractions force the venous blood upward towards the heart. (3) Breathing lowers pressure in the thorax. 8