Chapter 9 Circulatory Responses to Exercise PDF

Chapter 9: Circulatory Adaptations to Exercise © 2007 McGraw-Hill Higher Education. All rights reserved.  Introduction  One major challenge to homeostasis posed by exercise is the increased muscular demand for oxygen  During heavy exercise, ox...

Chapter 9: Circulatory Adaptations to Exercise © 2007 McGraw-Hill Higher Education. All rights reserved.  Introduction  One major challenge to homeostasis posed by exercise is the increased muscular demand for oxygen  During heavy exercise, oxygen demands may  by 15 to 25 times  Two major adjustments of blood flow are;   cardiac output  Redistribution of blood flow  A thorough understanding of the cardiovascular system is essential to exercise physiology © 2007 McGraw-Hill Higher Education. All rights reserved.  Objectives  Give an overview of the design and function of the circulatory system  Describe cardiac cycle & associated electrical activity recorded via electrocardiogram  Discuss the pattern of redistribution of blood flow during exercise  Outline the circulatory responses to various types of exercise © 2007 McGraw-Hill Higher Education. All rights reserved.  Objectives  Identify the factors that regulate local blood flow during exercise  List & discuss those factors responsible for regulation of stroke volume during exercise  Discuss the regulation of cardiac output during exercise © 2007 McGraw-Hill Higher Education. All rights reserved.  The Cardiovascular System  Works with the pulmonary system – Cardiopulmonary or cardiorespiratory system  Purposes of the cardiorespiratory system – Transport O2 and nutrients to tissues – Removal of CO2 wastes from tissues – Regulation of body temperature  Two major adjustments of blood flow during exercise – Increased cardiac output – Redistribution of blood flow © 2007 McGraw-Hill Higher Education. All rights reserved.  The Circulatory System  Heart  – Creates pressure to pump blood  Arteries and arterioles  – Carry blood away from the heart  Capillaries  – Exchange of O2, CO2, and nutrients with tissues  Veins and venules  – Carry blood toward the heart © 2007 McGraw-Hill Higher Education. All rights reserved.  Structure of the Heart Fig 9.1 © 2007 McGraw-Hill Higher Education. All rights reserved.  Pulmonary and Systemic Circuits Pulmonary circuit – Right side of the heart – Pumps deoxygenated blood to the lungs via pulmonary arteries – Returns oxygenated blood to the left side of the heart via pulmonary veins Systemic circuit – Left side of the heart – Pumps oxygenated blood to the whole body via arteries – Returns deoxygenated blood to the right side of the heart via veins © 2007 McGraw-Hill Higher Education. All rights reserved.  Myocardium  The heart wall – Epicardium – Myocardium – Endocardium  Receives blood supply via coronary arteries – High demand for oxygen and nutrients  Myocardial infarction (MI) – Blockage in coronary blood flow results in cell damage – Exercise training protects against heart damage during MI © 2007 McGraw-Hill Higher Education. All rights reserved.  The Myocardium Fig 9.3 © 2007 McGraw-Hill Higher Education. All rights reserved.  Differences and Similarities between Heart muscles and Skeletal Muscles?? © 2007 McGraw-Hill Higher Education. All rights reserved.  Exercise Training Protects the Heart Regular exercise is cardioprotective – Reduce incidence of heart attacks – Improves survival from heart attack Exercise reduces the amount of myocardial damage from heart attack – Improvements in heart’s antioxidant capacity – Improved function of ATP-sensitive potassium channels © 2007 McGraw-Hill Higher Education. All rights reserved.  Endurance Exercise Protects Against Cardiac Injury During Heart Attack © 2007 McGraw-Hill Higher Education. All rights reserved.  The Cardiac Cycle  Systole – Contraction phase – Ejection of blood ~2/3 blood is ejected from ventricles per beat  Diastole – Relaxation phase – Filling with blood  At rest, diastole longer than systole  During exercise, both systole and diastole are shorter © 2007 McGraw-Hill Higher Education. All rights reserved.  Pressure Changes During the Cardiac Cycle  Diastole – Pressure in ventricles is low – Filling with blood from atria AV valves open when ventricular P < atrial P  Systole – Pressure in ventricles rises – Blood ejected in pulmonary and systemic circulation Semilunar valves open when ventricular P > aortic P  Heart sounds – First: closing of AV valves – Second: closing of aortic and pulmonary valves © 2007 McGraw-Hill Higher Education. All rights reserved.  The Cardiac Cycle Systole Diastole  ___________ phase  _________ phase Fig 9.5 © 2007 McGraw-Hill Higher Education. All rights reserved.  Arterial Blood Pressure  Expressed as systolic/diastolic – Normal is 120/80 mmHg  Systolic pressure – Pressure generated during ventricular contraction  Diastolic pressure – Pressure in the arteries during cardiac relaxation © 2007 McGraw-Hill Higher Education. All rights reserved.  Blood Pressure  Difference between systolic and diastolic Pulse Pressure = Systolic - Diastolic  Average pressure in the arteries MAP = DBP + 0.33(SBP – DBP) © 2007 McGraw-Hill Higher Education. All rights reserved.  Mean Arterial Pressure Blood pressure of 120/80 mm Hg MAP = 93mmHg © 2007 McGraw-Hill Higher Education. All rights reserved.  Hypertension:  Blood pressure above 140/90 mmHg  Primary (essential) hypertension – Cause unknown – 90% cases of hypertension  Secondary hypertension – result of some other disease process  Risk factor for: – Left ventricular hypertrophy – Atherosclerosis and heart attack – Kidney damage – Stroke © 2007 McGraw-Hill Higher Education. All rights reserved. Measurement of Blood Pressure © 2007 McGraw-Hill Higher Education. All rights reserved. Fig 9.7  How is Blood Pressure Regulated?  Determinants of mean arterial pressure – Cardiac output – Total vascular resistance MAP = cardiac output x total vascular resistance  Short-term regulation – Sympathetic nervous system – Baroreceptors in aorta and carotid arteries Increase in BP = decreased SNS activity Decrease in BP = increased SNS activity  Long-term regulation – Kidneys Via control of blood volume © 2007 McGraw-Hill Higher Education. All rights reserved.  Factors That Influence Arterial Blood Pressure Fig 9.8 © 2007 McGraw-Hill Higher Education. All rights reserved.  Electrical Activity of the Heart  Contraction of the heart depends on electrical stimulation of the myocardium  Conduction system – Sinoatrial node (SA node) Pacemaker, initiates depolarization – Atrioventricular node (AV node) Passes depolarization to ventricles Brief delay to allow for ventricular filling (0.10 sec) – Bundle Branches To left and right ventricle – Purkinje fibers Throughout ventricles © 2007 McGraw-Hill Higher Education. All rights reserved. Conduction System of the Heart ___________________ Fig 9.9 © 2007 McGraw-Hill Higher Education. All rights reserved.  Cardiac Output The amount of blood pumped by the heart each minute Product of heart rate and stroke volume – Heart rate Number of beats per minute – Stroke volume Amount of blood ejected in each beat Q = HR x SV Depends on training state and gender © 2007 McGraw-Hill Higher Education. All rights reserved. © 2007 McGraw-Hill Higher Education. All rights reserved.  Regulation of Heart Rate Parasympathetic nervous system – Via vagus nerve – Slows HR by inhibiting SA and AV node initial increase in heart rate during exercise, up to approximately 100 beats per minute, is due to a withdrawal of parasympathetic tone Sympathetic nervous system – Via cardiac accelerator nerves – Increases HR by stimulating SA and AV node Low resting HR due to parasympathetic tone Increase in HR at onset of exercise – Initial increase due to parasympathetic withdrawal Up to ~100 beats/min – Later increase due to increased SNS stimulation © 2007 McGraw-Hill Higher Education. All rights reserved. © 2007 McGraw-Hill Higher Education. All rights reserved.   Hypertension (parasympathetic)  Increased atrial pressure (sympathetic)  Body temperature © 2007 McGraw-Hill Higher Education. All rights reserved.   HRV  Depression  Age  Hypertension  Myocardial infarction  Physical inactivity  SYMPATHOVAGAL BALANCE © 2007 McGraw-Hill Higher Education. All rights reserved.  Beta-Blockade and Heart Rate Beta-adrenergic blocking drugs (beta-blockers) – Compete with epinephrine and norepinephrine for beta adrenergic receptors in the heart – Reduce heart rate and contractility Lower the myocardial oxygen demand Prescribed for patients with coronary artery disease and hypertension Will lower heart rate during submaximal and maximal exercise – Important for exercise prescription © 2007 McGraw-Hill Higher Education. All rights reserved.  Regulation of Stroke Volume End-diastolic volume (EDV) – Volume of blood in the ventricles at the end of diastole (“preload”) Average aortic blood pressure – Pressure the heart must pump against to eject blood (“afterload”) Mean arterial pressure Strength of the ventricular contraction (contractility) – Enhanced by: Circulating epinephrine and norepinephrine Direct sympathetic stimulation of heart © 2007 McGraw-Hill Higher Education. All rights reserved.  End-Diastolic Volume Frank-Starling mechanism – Greater EDV results in a more forceful contraction Due to stretch of ventricles Dependent on venous return Venous return increased by: – Venoconstriction Via SNS – Skeletal muscle pump Rhythmic skeletal muscle contractions force blood in the extremities toward the heart One-way valves in veins prevent backflow of blood – Respiratory pump Changes in thoracic pressure pull blood toward heart © 2007 McGraw-Hill Higher Education. All rights reserved.  The Skeletal Muscle Pump  Rhythmic skeletal muscle contractions force blood in the extremities toward the heart  One-way valves in veins prevent backflow of blood Fig 9.16 © 2007 McGraw-Hill Higher Education. All rights reserved. © 2007 McGraw-Hill Higher Education. All rights reserved. © 2007 McGraw-Hill Higher Education. All rights reserved.  Ventricular Contractility  Increased contractility results in increase stroke volume  Circulating epinephrine and norepinephrine  Direct sympathetic stimulation of heart  Increases the amount of calcium available for the cells. © 2007 McGraw-Hill Higher Education. All rights reserved.  Factors that Regulate Cardiac Output Parasympathetic Mean arterial nerves pressure Cardiac = Cardiac Rate x Stroke Volume Output Contraction EDV Sympathetic strength nerves Stretch Fig 9.18 Frank- Starling © 2007 McGraw-Hill Higher Education. All rights reserved.  Hemodynamics The study of the physical principles of blood flow © 2007 McGraw-Hill Higher Education. All rights reserved.  Physical Characteristics of Blood  Physical characteristics of blood –Plasma Liquid portion of blood Contains ions, proteins, hormones – Cells Red blood cells – Contain hemoglobin to carry oxygen White blood cells – Important in preventing infection Platelets – Important in blood clotting  Hematocrit – Percentage of blood composed of cells © 2007 McGraw-Hill Higher Education. All rights reserved.  ____________ Percent of blood composed of cells Fig 9.19 © 2007 McGraw-Hill Higher Education. All rights reserved.  Hemodynamics Relationships Among Pressure, Resistance, and Flow Blood flow – Directly proportional to the pressure difference between the two ends of the system – Inversely proportional to resistance blood flow =Pressure/Resistance Pressure – Proportional to the difference between MAP and right atrial pressure (Pressure) © 2007 McGraw-Hill Higher Education. All rights reserved.  Blood Flow Through the Systemic Circuit Fig 9.20 © 2007 McGraw-Hill Higher Education. All rights reserved.  Hemodynamics: Resistance  Resistance – Depends upon: Length of the vessel Viscosity of the blood Radius of the vessel Length x viscosity Resistance = Radius 4 © 2007 McGraw-Hill Higher Education. All rights reserved.  Sources of Vascular Resistance  MAP decreases throughout the systemic circulation  Largest drop occurs across the arterioles  Arterioles are called “resistance vessels” © 2007 McGraw-Hill Higher Education. All rights reserved.  Pressure Changes Across the Systemic Circulation Fig 9.21 © 2007 McGraw-Hill Higher Education. All rights reserved.  Oxygen Delivery During Exercise Oxygen demand by muscles during exercise is 15– 25x greater than at rest  Increased O2 delivery accomplished by: – Increased cardiac output – Redistribution of blood flow From inactive organs to working skeletal muscle © 2007 McGraw-Hill Higher Education. All rights reserved.  Changes in Cardiac Output  Cardiac output increases due to:  Increased heart rate  Linear increase to max  CO decreases with age. Max HR = 220 - Age (years)  Increased stroke volume  Plateau at ~40% VO2max  No plateau in highly trained subject  Oxygen uptake by the muscle also increased  Higher arteriovenous difference © 2007 McGraw-Hill Higher Education. All rights reserved. Changes in Cardiovascular Variables During Exercise Fig 9.22 © 2007 McGraw-Hill Higher Education. All rights reserved.  Redistribution of Blood Flow  Muscle blood flow to working skeletal muscle  Splanchnic blood flow  to less active organs  Liver, kidneys, GI tract © 2007 McGraw-Hill Higher Education. All rights reserved. Changes in Muscle and Splanchnic Blood Flow During Exercise Fig 9.23 © 2007 McGraw-Hill Higher Education. All rights reserved.  Redistribution of Blood Flow During Exercise Fig 9.24 © 2007 McGraw-Hill Higher Education. All rights reserved.  Increased Blood Flow to Skeletal Muscle During Exercise  What regulates blood flow to various organs during exercise? 1. Blood flow increased to meet metabolic demands of tissue This happens because of muscle vasodilation 2.O2 tension, CO2 tension, pH, nitric oxide 3. Vasoconstriction to visceral organs and inactive tissues – SNS vasoconstriction © 2007 McGraw-Hill Higher Education. All rights reserved.  Nitric Oxide Is an Important Vasodilator Produced in the endothelium or arterioles Promotes smooth muscle relaxation – Results in vasodilation and increased blood flow Important in autoregulation – With other local factors One of several factors involved in blood flow regulation during exercise – Increases muscle blood flow © 2007 McGraw-Hill Higher Education. All rights reserved.  Circulatory Responses to Exercise  Heart rate and blood pressure  Depend on:  Type, intensity, and duration of exercise  Environmental condition  Emotional influence © 2007 McGraw-Hill Higher Education. All rights reserved.  Transition From Rest -> Exercise and Exercise -> Recovery  At the onset of exercise: – Rapid increase in HR, SV, cardiac output – Plateau in submaximal (below lactate threshold) exercise  During recovery – Decrease in HR, SV, and cardiac output toward resting – Depends on: Duration and intensity of exercise Training state of subject © 2007 McGraw-Hill Higher Education. All rights reserved.  Incremental Exercise  Heart rate and cardiac output – Increases linearly with increasing work rate – Reaches plateau at 100% VO2 max  Blood pressure – Mean arterial pressure increases linearly Systolic BP increases Diastolic BP remains fairly constant  Double product – Increases linearly with exercise intensity – Indicates the work of the heart  Double product = HR x systolic BP © 2007 McGraw-Hill Higher Education. All rights reserved.  Arm vs. Leg Exercise  At the same oxygen uptake, arm work results in higher: – Heart rate Due to higher sympathetic stimulation – Blood pressure Due to vasoconstriction of large inactive muscle mass. © 2007 McGraw-Hill Higher Education. All rights reserved.  Heart Rate and Blood Pressure During Arm and Leg Exercise If these 2 factors increase so will _______________ __________ Fig 9.26 © 2007 McGraw-Hill Higher Education. All rights reserved.  Intermittent Exercise  Recovery of heart rate and blood pressure between bouts depend on: – Fitness level – Temperature and humidity – Duration and intensity of exercise © 2007 McGraw-Hill Higher Education. All rights reserved.  Prolonged Exercise  Cardiac output is maintained Gradual decrease in stroke volume – Due to dehydration and reduced plasma volume Gradual increase in heart rate – Cardiovascular drift. © 2007 McGraw-Hill Higher Education. All rights reserved.  HR, SV, and Q During Prolonged Exercise Fig 9.27 © 2007 McGraw-Hill Higher Education. All rights reserved.  Cardiovascular Adjustments to Exercise Fig 9.23 © 2007 McGraw-Hill Higher Education. All rights reserved.