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Vessels and Circulation.pdf

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BIOL 271

Vessels and Circulation Anatomy of Blood Vessels closed system of tubes arteries carry blood away from the heart (to tissues) elastic arteries muscular arteries arterioles capillaries are thin enough to allow exchange venules merge to from veins veins carry blood toward the heart Arteries 3 different layers (tunics) tunica interna (intima) simple squamous epithelium (endothelium) basement membrane internal elastic lamina tunica media circular smooth muscle elastic fibers (external elastic lamina) tunica externa (adventitia) elastic & collagen fibres vasoconstriction SNS innervates vascular smooth muscle (stimulation >> vasoconstriction) injury to artery or arteriole >> vasospasm decreases blood loss vasodilation decreased SNS stimulation >> vasodilation NO, K+, H+, lactic acid and chemicals of inflammation also >> vasodilation elastic arteries largest-diameter arteries (e.g.: aorta) abundance of elastic fibers in tunica media stretch during ventricular systole recoil on ventricular diastole, propelling blood aorta also propels blood into coronary aa muscular arteries medium sized aa called distributing arteries because they distribute blood to body (constrict and dilate to adjust flow) more smooth muscle in tunica media than elastic aa Arterioles very small aa deliver blood to capillaries vasoconstrictor and vasodilator to regulate BP and blood flow to capillaries Capillaries microscopic vessels usually connect arterioles to venules found near almost every cell in the body (none in epithelium, cartilage, cornea, and lens) most concentrated in tissues with high metabolic rates (muscle, liver, kidneys, brain) site of exchange of gases, nutrients, wastes between blood and interstitial fluid materials cross capillary wall… through endothelial cell embraces within pinocytic vesicles through intercellular clefts channels between tightly packed endothelial cells through fenestrations holes that go all the way through endothelial cells types: continuous continuous endothelium interrupted only by intercellular clefts found in skeletal and smooth muscle, connective tissue and lungs fenestrated many small holes/pores (fenestrations) in endothelial cell membranes found in kidneys, choroid and ciliary plexuses, villi of small intestine and in endocrine glands gets rid of larger waste products and absorbs larger nutrients sinusoids large lumen very large fenestrations in endothelial cell membranes, large intercellular clefts basement membrane incomplete or absent allow protein and blood cells to pass from tissue to bloodstream found in red bone marrow, liver, spleen, anterior pituitary, thyroid and parathyroid gland how RBCs enter from RBM Blood Flow through Capillaries flow regulated by contraction of metarterioles and pre capillary sphincters metarterioles divert blood to different parts of the body at different times (switches destination back and forth) branch from arterioles supplies bed of 10-100 capillaries pre capillary sphincters found at origin of true capillaries when open, blood flows through capillary bed when closed, blood flows through thoroughfare channel open/close 5-10x/min >> vasomotor (>> intermittent blood flow through capillary bed) Venules small veins that drain blood from several capillaries to vv thin tunica media, very porous endothelium allow phagocytic WBCs to leave blood during inflammation Veins same three coats as arteries: tunica interna thinner than aa thin folds form valves/cusps and prevent backflow tunica media thinner than aa little muscle, few elastic fibres no elastic lamina tunica extern thickest layer collagen, elastic fibres supports vein wall thinner wall, larger lumen than same diameter artery distensible, adapt to variations in blood volume and pressure venous sinuses (e.g.: dural, coronary sinuses) have no smooth muscle in wall, can't alter diameter Blood Distribution at rest, 64% of BV is in systemic veins and venules capacitance vessels (function as blood reservoir) blood is diverted from skin and abdominal organs when needed (e.g.: hemorrhage, increased muscle activity >> venoconstriction >> increased ABP) 13% of BV is in systemic aa. and arterioles 9% in pulmonary vessels 7% in heart 7% in capillaries Capillary Exchange movement of materials into and out of a capillary Exchange Processes diffusion all plasma solutes except large proteins pass freely lipid soluble substance (O2, CO 2 steroids) pass through endothelial cell membranes water-soluble substances (glucose, amino acids) pass through fenestrations or intercellular clefts blood brain barrier does not allow diffusion of water- soluble materials (consists of non-fenestrated epithelium with tight junctions) no intercellular cleft: anchored tightly together, so no diffusion glial cells wrap around capillary, making distance larger for substances to diffuse into brain transcytosis material carried across endothelium / through cell in tiny vesicles by endocytosis and exocytosis e.g.: passage of large, lipid-insoluble molecules (insulin or maternal antibodies through placental circulation to fetus) major in blood-brain barrier to transport glucose, etc to brain Bulk Flow filtration and reabsorption movement of large amount of dissolved or suspended material in the same direction driven by hydrostatic and osmotic pressures faster than diffusion or osmosis filtration moves fluid and dissolved substances from blood to interstitial space promoted by blood hydrostatic pressure and interstitial fluid osmotic pressure reabsorption is movement of fluid and dissolved substances from interstitial space to blood promoted by blood colloid (protein) osmotic and interstitial fluid (tissue) hydrostatic pressures change in volume of interstitial fluid Starling's Law of the Capillary arterial end: filtration: fluids move from blood to tissues mainly by blood hydrostatic pressure venous end: osmosis: fluids return to blood mainly by blood osmotic pressure created by albumin usually not a change in volume in interstitial space (fluid coming in and out regularly) net filtration pressure whether fluids leave or enter capillaries depends on balance of pressures net outward pressure = 10mmHg at arterial end of a capillary bed net inward pressure = 9mmHg at venous end of a capillary bed difference is settled by lymphatic system (drains extra volume) about 85% of filtered fluid returns to the capillary by osmosis the remaining 15% (about 3 litres of fluid per day) is absorbed and returned to the blood by lymphatic capillaries Hemodynamics blood flow is due to an interplay of forces determined by: pressure differences resistance to flow Velocity (Speed) of Blood Flow speed of flow is inversely related to cross-sectional area of blood vessel the larger the cross-sectional area, the lower the velocity each time an artery branches, the total cross-sectional area of all its branches is greater than that of the original vessel, so blood flow is slower blood in aorta flows 40 cm/sec; blood in capillaries flows 0.1 cm/sec slow rate in capillaries allows for exchange flow rate increases when vessels merge to form veins Volume of Blood Flow cardiac output = heart rate x stroke volume influenced by: blood pressure peripheral resistance (friction between blood cells and blood vessel walls) Blood Pressure force of blood pressing against the walls of blood vessels caused by contraction of the ventricles blood flows from areas of higher pressure to areas of lower pressure falls steadily in systemic circuit with distance from L ventricle (see diagram) the further you get away from the L ventricle, the lower the blood pressure gets highest pressure is in aorta and large systemic arteries systole: 110mmHg diastole: 70mmHg ~35mmHg entering capillaries (arteriole end of Starling's Law!) ~16mmHg leaving capillaries (venous end of Starling's Law!) 0mmHg entering the R atrium types of BP: arterial blood pressure (ABP) - measured with arm cuff capillary pressure (hydrostatic pressure) venous blood pressure arterial: measured as systolic/diastolic pulse pressure = systolic - diastolic (110-70 = 40mmHg) mean arterial pressure (MAP) = diastolic pressure + ⅓ pulse pressure = 70 + ⅓(40) = 70 + 13 = 83mmHg because cardiac output is influenced by blood pressure and resistance: cardiac output = MAP / R increased CO = increased BP if R remains constant decreased CO = decreased BP if R remains constant Peripheral Resistance opposition to blood flow result of friction between blood and blood vessel walls depends on: average blood vessel radius blood viscosity total blood vessel length average vessel radius narrower vessels offer resistance to blood flow inverse reaction: when vessel radius decreases to ½, resis
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