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The Cardiovascular System: Blood Vessels and Hemodynamics A. Anatomy of blood vessels 1. Arteries a. Elastic (conducting) arteries b. Muscular (distributing) arteries c. Anastomoses 2. Arterioles 3. Capillaries 4. Venules 5. Veins 6. Blood distribution B. Capillary exchange a. Diffusion b. Vesicular transport c. Bulk flow (filtration and reabsorption) C. Hemodynamics: physiology of circulation 1. Velocity of blood flow 2. Volume of blood flow a. Blood pressure b. Peripheral resistance 3. Venous return D. Control of blood pressure and blood flow 1. Cardiovascular center a. Input to cardiovascular center b. Output from cardiovascular center 2. Neural regulation a. Baroreceptors b. Chemoreceptors 3. Hormonal regulation 4. Autoregulation (local control) E. Blood vessel routes Arteries 1. lumen 2. tunica intima a. endothelium b. internal elastic lamina 3. tunica media 4. tunica adventitia Arterial Properties 1. elasticity 2. contractility a. vasoconstriction b. vasodilation Arterial Types 1. elastic (conducting) a. elastin b. pressure reservoirs 2. muscular (distributing) a. great contractility b. blood shunting c. anastomoses d. collateral circulation Elastic arteries Muscular Arteries Muscular Arteries Arterioles 1. highest contractility 2. blood shunting Routing of Blood Flow Capillaries 1. 2. 3. 4. 5. 6. microscopic distribution exchange simple squamous precapillary sphincters vasomotion Endothelium basement membrane Capillary Types 1. continuous 2. fenestrated 3. sinusoids Venules and Veins 1. 2. 3. 4. 5. same basic tunics larger lumen thinner tunica media very distensible valves Valve Blood Reservoirs Skin, liver, and spleen Capillary Exchange 1. diffusion 2. vesicular transport 3. bulk flow a. filtration b. reabsorption c. Starling's law of the capillaries What is vasomotion? Bulk Flow is Dependent On Four Pressures 1. blood hydrostatic pressure (BHP = 30 mm Hg-arterial) (outward force) = 10 mm Hg-venous 2. interstitial fluid hydrostatic pressure (IFHP = 0 to -3(suction) mm Hg) (inward force) 3. blood colloid osmotic pressure (BCOP = 28 mm Hg) (inward force) 4. interstitital fluid osmotic pressure (IFOP = 8 mm Hg) (outward force) Net Filtration Pressure (Arterial End) NFP = outward forces - inward forces = (BHP + IFOP) - (BCOP + IFHP) = (30 + 8) - (28 + 0) = (38) - (28) = +10 mm Hg If IFHP was (-3) then NFP = 13 mm Hg net flow of fluid is? out of the capillary (filtration) Net Filtration Pressure- Venous End NFP = outward forces - inward forces = (BHP + IFOP) - (BCOP + IFHP) = (10 + 8) - (28 + 0) = (18) - (28) = -10 mm Hg If IFHP was (-3) then NFP = -7 mm Hg net flow of fluid is? into of the capillary (reabsorption) The Forces of Capillary Filtration and Absorption Capillary Exchange Hemodynamics veins (80 cm2) velocity Relationship between blood flow velocity and total cross-sectional area of the vascular tree 80 mm/sec (IVC) 5 mm/sec 0.4 mm/sec arterioles (40 cm2) 15 mm/sec 1200 mm/sec total cross sectional area venae cavae (8 cm2) If total cross-sectional area then velocity (capillaries to venae cavae) arteries (20 cm2) If total cross-sectional area then velocity (aorta to capillaries) capillaries (2500 cm2) aorta (2.5 cm2) Blood flow (ml/min) Perfusion (ml/min/g) Blood flow velocity (mm/sec) Cross-sectional area (cm2) Flow α ∆Pressure Resistance Blood Pressure 1. What is blood pressure? 2. direct determinants of BP a. cardiac output b. blood volume c. peripheral resistance Volume of Blood Flow CO = SV x HR = 5.25 L/min (the volume of blood circulating through systemic or pulmonary vessels each minute) Two other factors influence cardiac output: 1) blood pressure 2) resistance (opposition) CO = mean arterial blood pressure (MABP) resistance (R) Resistance (Opposition to Flow) 1. blood viscosity 2. total blood vessel length 3. blood vessel radius Systemic vascular resistance (SVR) (total peripheral resistance (TPR) 1. major function of arterioles 2. vasodilation vs vasocontriction a. Blood flow is proportional to the fourth power of vessel radius. r = 1 mm r4 = 14 = 1 flow = 1 mm/sec r = 2 mm r4 = 24 = 16 flow = 16 mm/sec r = 3 mm r4 = 34 = 81 flow = 81 mm/sec b. Therefore, a 3-fold change in resistance exerts an 81-fold change in velocity. Mechanisms of Venous Return 1. 2. 3. 4. decreasing x-sec area venous valves muscle pumps respiratory pump Control of Blood Pressure and Blood Flow The cardiovascular center (CVC) consists of three groups of neurons: 1. cardioacceleratory neurons 2. cardioinhibitory neurons 3. vasomotor neurons CVC Input 1. input a. higher brain centers b. baroreceptors c. chemoreceptors Nervous Input to Medullary Cardiovascular Center higher brain cerebral cortex limbic system hypothalamus baroreceptors (blood pressure) chemoreceptors (O2, H+, CO2) medulla oblongata CVC Output 2. output a. vagus (X) nerve b. sympathetic neurons (1) heart (2) arterioles Nervous Output From Medullary Cardiovascular Center Medulla oblongata vagus nerve (X) heart (decrease rate) cardiac accelerator nerves (sympathetic) heart (increase rate and contractility) vasomotor nerves (sympathetic) blood vessels (vasoconstriction and vasodilation) Neural Control of the Heart Reflexes Reflexes activate the Vasomotor center – results in vasoconstriction/vasodilation of arterioles Baroreflexes and chemoreflexes carotid sinus and aortic sinus reflexes right atrial (Bainbridge) reflex Medullary ischemic reflex sensory fibers in c.n. IX carotid sinus baroreceptors cerebrum aortic sinus baroreceptors hypothalamus cardiovascular center in medulla SA node AV node spinal cord parasympathetic fibers in c.n. X sympathetic fibers in spinal nerves Baroreceptor Reflex Chemoreceptor Reflex Negative Feedback of Neural Control CONTROLLED CONDITION a stimulus or stress disrupts homeostasis by causing a decrease in blood pressure RETURN TO HOMEOSTASIS increased blood pressure RECEPTOR baroreceptors in carotid and aortic sinuses are stretched less, resulting in decreased rate of nerve impulses to the cardiovascular center CONTROL CENTER 1. increased sympathetic output via cardioacceleratory center 2. decreased parasympathetic output from cardioinhibitory center EFFECTORS 1. 2. contractility = stroke volume heart rate therefore cardiac output 3. vasoconstriction = resistance Hormonal Regulation 1. epinephrine and norepinephrine potent vasoconstrictor increased resistance increased BP increased HR + increased SV = increased CO increased BP 2. antidiuretic hormone increases water retention by kidneys increased blood volume increased BP 3. angiotensin II potent vasoconstrictor increased resistance increased BP 4. aldosterone promotes sodium retention by kidneys increased water retention increased blood volume increased BP 5. atrial natriuretic peptide decreases sodium retention by kidneys decreased water retention decreased blood volume decreased BP Autoregulation Ability of tissues to regulate their own blood supply Metabolic theory of autoregulation Inadequate perfusion = A. Decreased O2 vasodilation B. Increased wastes (CO2, H+, K+, adenosine) vasodilation Adequate perfusion again = vasoconstriction Vasoactive Chemicals - secreted by platelets, endothelial cells, perivascular tissues with trauma - Examples include Histamine, prostaglandins, bradykinin stimulate vasodilation Reactive hyperemia Summary of Blood Pressure Control decreased blood pressure leads to decreased activity of baroreceptors in carotid and aortic sinuses increased activity leads to decreased nervous input to cardioinhibitory center increased activity leads to increased activity of the: 1.cardioacceleratory center 2.vasomotor center decreased activity leads to increased sympathetic output from spinal cord norepinephrine secretion causes increased heart rate increased stroke volume leads to leads to increased cardiac output leads to negative feedback increased blood pressure increased vasoconstriction leads to increased resistance leads to Blood Vessel Routes (see Handout) Beginning of the Aorta Arterial Supply Head and Neck Arterial Circle of the Brain- Circle of Willis Arterial Supply to the Upper Arm Arterial Supply to the Thorax Arterial Supply to the Abdomen Arterial Supply to the Pelvic Region and Lower Limb