Lecture 8 - Circulatory Physiology and Smooth Muscle PDF

Circulatory Physiology and Smooth Muscle Sherwood: Chapter 10 Objectives Anatomy of Blood Vessels Systemic Circulatory Pressures Regulation of Cardiovascular Function 2 Anatomy of Blood Vessels Structure of Blood Vessel Wall All vessels consist of a lumen surroun...

Circulatory Physiology and Smooth Muscle Sherwood: Chapter 10 Objectives Anatomy of Blood Vessels Systemic Circulatory Pressures Regulation of Cardiovascular Function 2 Anatomy of Blood Vessels Structure of Blood Vessel Wall All vessels consist of a lumen surrounded by a wall Walls of all vessels, except capillaries, have three layers, or tunics: 1. Tunica intima 2. Tunica media 3. Tunica externa Capillaries Endothelium 4 Structure of Blood Vessel Wall Tunica intima Innermost layer that is in “intimate” contact with blood Endothelium: simple squamous epithelium that lines lumen of all vessels and sits on top of basement membrane Tunica media Middle layer composed mostly of smooth muscle (much thicker in arteries than veins) and elastin Sympathetic nerve fibers innervate this layer, controlling: Tunica externa (adventitia) Vasoconstriction: decreased lumen diameter Vasodilation: Outermost layer of increased wall lumen diameter Composed mostly of loose collagen fibers that protect and reinforce wall and anchor it to surrounding structures 5 The Blood Vessels The contribution of the different vessel tissue layers varies with vessel type Arteries are the thickest and are most involved in contraction and relaxation Capillaries are the thinest to permit nutrient exchange Smooth Muscle Spindle shaped fibers Found in walls of internal organs and blood vessels (except the heart) When fibers contract, “spindle” shape is pulled together, causing constriction (b) Thin and thick filaments are present Microscopic Anatomy of Smooth Muscle SR is less developed than in skeletal muscle and lacks a specific pattern T tubules are absent Plasma membranes have pouchlike infoldings called caveoli Ca2+ is sequestered in the extracellular space near the caveoli, allowing rapid influx when channels are opened There are no visible striations and no sarcomeres Thin and thick filaments are present Organization of Myofilaments in Smooth Muscle Thick filaments have heads along their entire length There is no troponin complex Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) at regular intervals Contraction of Smooth Muscle Whole sheets of smooth muscle exhibit slow, synchronized contraction They contract in unison, reflecting their electrical coupling with gap junctions Action potentials are transmitted from cell to cell Some smooth muscle cells: Act as pacemakers and set the contractile pace for whole sheets of muscle Are self-excitatory and depolarize without external stimuli Contraction Mechanism Special Features of Smooth Muscle Contraction Unique characteristics of smooth muscle include: Smooth muscle tone Slow, prolonged contractile activity Low energy requirements Response to stretch Regulation of Smooth Muscle Contraction Neural Sympathetic – norepinephrine Parasympathetic - acetylcholine Hormonal Epinephrine, histamines, prostaglandins, ANG Stress Stretch activated channels Other ATP, pH, CO2 Systemic Circulatory Pressures Blood Pressure The main force creating blood flow is ventricular contraction High pressure blood ejected from left ventricle to aorta and large arteries Because of elastic tissue, they recoil, propelling blood forward Pressure highest in arteries Continuously decreases through the circulatory system Systemic Circulation Pressures Pressure in heart and aorta Aortic pressure = 120 mmHg during ventricular systole:  systolic blood pressure Ventricular pressure falls significantly during ventricular relaxation (diastole) but aortic pressure stays high at 80 mmHg:  diastolic blood pressure Pressure in circulation Pressure wave decreases over distance because of friction Wave disappears at capillaries and beyond No pressure wave in venules or veins Implies blood flow through arteries is pulsatile and through veins is steady flow Measuring Blood Pressure with a sphygmomanomet er. Arterial Blood Pressure Arterial BP reflects two factors of the arteries close to the heart Their elasticity (compliance or distensibility) The amount of blood forced into them at any given time Arterial Blood Pressure Pulse Pressure (PP) = SBP – DBP MAP =[(2xDBP) + SBP]/3 Capillary Blood Pressure Capillary BP ranges from 20 to 40 mm Hg Low capillary pressure is desirable because high BP would rupture fragile, thin- walled capillaries Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues Venous Blood Pressure Venous BP is steady and changes little during the cardiac cycle The pressure gradient in the venous system is only about 20 mm Hg A cut vein has even blood flow; a lacerated artery flows in spurts Factors Aiding Venous Return Venous BP alone is too low to promote adequate blood return and is aided by the: Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart Valves prevent backflow during venous return Ohm’s Law Applied to Blood Flow P F = ΔP R F R Where F = flow ΔP = inlet pressure – outlet pressure Inlet Press. Outlet Press R = resistance to flow Flow Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period: Is measured in ml per min. Is equivalent to cardiac output (CO), considering the entire vascular system Is relatively constant when at rest Varies widely through individual organs, according to immediate needs Pressure Force per unit area exerted on the wall of a blood vessel by its contained blood Expressed in millimeters of mercury (mm Hg) Measured in reference to systemic arterial BP in large arteries near the heart The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas Maintaining Blood Pressure Maintaining blood pressure requires: Cooperation of the heart, blood vessels, and kidneys Supervision of the brain The main factors influencing blood pressure are: Cardiac output (CO) Total peripheral resistance (TPR) Blood volume Mean arterial pressure = CO x TPR Blood pressure varies directly with CO, TPR, and blood volume Resistance Resistance – opposition to flow Measure of the amount of friction blood encounters as it passes through vessels Generally encountered in the systemic circulation Referred to as peripheral resistance (PR) The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameter R = 8ηL Where: η = coefficient of friction π r4 L = vessel length r = vessel radius Resistance Factors: Blood Vessel Diameter Small-diameter arterioles are the major determinants of peripheral resistance Fatty plaques from atherosclerosis: Cause turbulent blood flow Dramatically increase resistance due to turbulence  TPR Arteriolar radius Blood viscosity Local (intrinsic) Extrinsic myogenic vasopressin heat/cold angiotensin paracrine epinephrine metabolites sympathetic nerves Local Controls (Autoregulation) Myogenic Increased stretch of vessel  vasoconstriction Decreased stretch of vessel  vasodilation Paracrine Nitric oxide  vasodilation Prostaglandins, histamines Metabolites potassium and hydrogen ions, adenosine, lactic acid Extrinsic Controls Baroreceptors pressure sensors in carotid sinus Extrinsic Controls Chemoreceptors O2, CO2 and H+ sensitive CO2 H+ Cardioaccelatory inc. CO Chemoreceptors centre O2 Vasomotor inc. BP centre Extrinsic Controls Other Hormones Epinephrine Long-Term Renal Regulation Direct regulation by BP Atrial natriuretic peptide Renin-angiotensin sytem  Kidneys respond to low BP by releasing renin Renin promotes the formation of angiotensin Angiotensin signals Angiotensin causes the release of blood vessel aldosterone constriction Reabsorption of water and sodium in the kidney Regulation of Mean Arterial Pressure (+) (+) Application questions: 1) What happens to MAP if during a marathon on a hot day, blood volume decreases due to high sweat loss? 2) What happens to MAP if you have a severed artery and lose a lot of blood? 3) Why does heart rate increase when you become dehydrated?