C H A P T E R 7

1. C H A P T E R 7 CARDIOVASCULAR CONTROL DURING EXERCISE

2. Learning Objectives w Review the structure and function of the heart, vascular system, and blood. w Find out how the cardiovascular system responds to increased demands during exercise. w Explore the role of the cardiovascular system in delivering oxygen and nutrients to active body tissues.

3. Major Cardiovascular Functions w Delivery (e.g., oxygen and nutrients like glucose and FFA) w Removal (e.g., carbon dioxide and other waste products like lactate) w Transportation (e.g., hormones) w Maintenance of homeostasis (e.g., body temperature, pH) w Immunity (e.g., white blood cells, antibodies)

4. Cardiovascular System w A pump (the heart) w A system of conduit pipes (the blood vessels) w A fluid medium (blood)

5. HEART Paleolithic drawing of a mammoth in the El Pindal Cave in Spain – if the picture depicts the heart, it is the oldest known anatomical drawing. ?

6. HEART

7. Myocardium—Cardiac Muscle w Muscle thickness varies directly with stress placed on chamber walls. w Left ventricle has the larger muscle mass and can develop the higher pressures w With vigorous training, the left ventricle size increases. w Due to intercalated disks impulses travel from muscle fiber to muscle fiber in the heart (unlike skeletal muscle fibers)

8. Myocardium—The Cardiac Muscle w Intercalated disks allow the myocardium to act as a syncytium, i.e., it acts as “one large muscle fiber” so all fibers contract in a coordinated manner. Heart Muscle Skeletal Muscle Intercalated disk

9. CORONARY CIRCULATION

10. INTRINSIC CONDUCTION SYSTEM

11. Cardiac Conduction System w Sinoatrial (SA) node—pacemaker w Atrioventricular (AV) node w AV bundle (bundle of His) wPurkinje fibers—6 times faster transmission than conduction from heart muscle cell to heart muscle cell

12. Extrinsic Control of the Heart w Parasympathetic nervous system acts through the vagus nerve to decrease heart rate and, to a lesser extent, the force of contraction (PNS predominates at rest and during exercise up to a heart rate of about 100 b/min —vagal tone); ACh is the parasympathetic neurotransmitter. w Sympathetic nervous system is stimulated by stress to increase heart rate (chronotropic effect) and force of contraction (inotropic effect); norepinephrine (NE) is the sympathetic neurotransmitter. w Epinephrine and norepinephrine—released from adrenal medulla due to sympathetic nervous system activity

13. Autonomic Innervation of the Heart Powers and Howley, Exercise Physiology, 2004

14. Training Effects on Resting Heart Rate Resting heart rates in adults tend to be between 60 and 85 beats/min. However, extended endurance training can lower resting heart rate to 35 beats/min or less. This lower heart rate is thought to be due to decreased intrinsic heart rate and increased parasympathetic stimulation.

15. Cardiac Arrhythmias Bradycardia—resting heart rate below 60 beats/min; normal in trained persons Tachycardia—resting heart rate above 100 beats/min Premature ventricular contractions (PVCs)—feel like skipped or extra beats; ectopic origin, i.e., they originate outside the SA node – all heart tissue has an intrinsic rhythm; that generated by the SA node is usually fastest Ventricular tachycardia—three or more consecutive PVCs that can lead to ventricular fibrillation in which contraction of the ventricular tissue is uncoordinated

16. Bradycardia The decrease in resting heart rate that occurs as an adaptation to endurance training is different from pathological bradycardia, an abnormal disturbance in the resting heart rate.

17. Electrocardiogram (ECG or EKG) w Printout shows the heart's electrical activity and can be used to monitor cardiac changes and pathologies w The P wave—atrial depolarization w The QRS complex—ventricular depolarization and atrial repolarization w The T wave—ventricular repolarization

18. PHASES OF A RESTING ECG

19. TAKING AN EXERCISE ECG

20. S-T Segment Depression Powers and Howley, Exercise Physiology, 2004

21. Cardiac Cycle w Events that occur between two consecutive heartbeats (systole to systole) w Diastole—relaxation phase during which the ventricles fill with blood (T wave to QRS)—62% of cycle duration w Systole—contraction phase during which the ventricles expel blood (QRS to T wave)—38% of cycle duration

22. Wiggers Diagram – Cardiac Cycle

23. Thought Question What is the physical principle that dictates the opening and closing of the valves in the heart? Why would mitral valve regurgitation reduce exercise performance?

24. Stroke Volume (SV) w Volume of blood pumped by each ventricle per contraction w End-diastolic volume (EDV)—volume of blood in ventricle before contraction w End-systolic volume (ESV)—volume of blood in ventricle after contraction w SV = EDV – ESV, e.g., SV = 125 ml – 50 ml = 75 ml . Cardiac Output (Q) w Total volume of blood pumped by each ventricle per minute . w Q = HR ´ SV, e.g., Q = 60 beats/min x 75 ml/beat = 4,500 ml/min, or 4.5 l/min Stroke Volume and Cardiac Output

25. Ejection Fraction (EF) w Proportion of blood pumped out of the left ventricle each beat w EF = SV/EDV x 100 • Averages 60% at rest, e.g., EF = 75 ml/125 ml x 100 = 60%

26. Vascular System: Closed System w Arteries w Arterioles: control blood flow through tissue by vasoconstriction and vasodilation w Capillaries: where exchange between blood and tissues occurs, e.g., oxygen, carbon dioxide, glucose, FFA, etc. w Venules w Veins: possess valves to assist unidirectional flow of the blood

27. Vascular System: Closed System Plate from Andreas Vesalius’s De Humani Corporis Fabrica (1543) showing the vascular system.

28. Vascular System: Closed System • Blood flows because of • A pressure difference, i.e., • a ΔP • Flow is unidirectional • because of valves in the • heart and the veins Ruch and Patton, Physiology and Biophysics, 1974

29. Historical Note on the Circulation The first correct understanding of the circulation: “It has been shown by reason and experiment that blood by the beat of the ventricles flows through the lungs and heart and is pumped to the whole body. It must therefore be concluded that the blood in the animal body moves around in a circle continuously, and that the action or function of the heart is to accomplish this by pumping. This is the only reason for the motion and beat of the heart.” William Harvey, Exercitatio Anatomica De Motu Cordis et Sanguinis in Animalibus, 1628.

30. Distribution of Cardiac Output (Q) w During steady state conditions, tissue blood flows are matched to the metabolic demands of the tissues wExtrinsic neural control — sympathetic nerves innervating the smooth muscles within walls of vessels release NE, which generally causes the arterioles to constrict (alpha-adrenergic effect) wAutoregulation — with increased levels of metabolism, local vasodilator substances (e.g., K+) are released from the cells, overcoming the sympathetic vasoconstrictor effects, and causing the arterioles to dilate w Blood flow through a muscle is determined by the arterio-venous ΔP across the muscle and the radius of the arterioles

31. Blood Volume Distribution at Rest Note most of the blood volume is in the veins at rest, particularly in the viscera

32. Blood Pressure w Systolic blood pressure (SBP) is the highest arterial pressure and diastolic blood pressure (DBP) is the lowest arterial pressure in the cardiac cycle w Mean arterial pressure (MAP)—average pressure exerted by the blood as it travels through arteries – usually what is considered in physiological studies w Estimated MAP = DBP + [0.333 ´ (SBP – DBP)]

33. Blood Pressure Average systemic arteriolar constriction increases blood pressure; dilation reduces blood pressure ► Recall Ohm’s law: V = IR, or voltage (electromotive force) = current (flow) x resistance ► Similarly, in the cardiovascular system, P = Q x TPR, orpressure = cardiac output x total peripheral resistance, or P = Q/compliance ►Mean arterial blood pressure is the primary regulated variable in the cardiovascular system, assuring sufficient blood flow to the brain in an upright bipedal human ► Monitored by baroreceptors located in the aorta and carotid arteries: increased mean pressure → decreased sympathetic vasoconstriction and decreased inotropic effects on the heart; decreased pressure → increased sympathetic vasoconstriction and increased inotropic effects on the heart

34. Thought Question Remember, arterial pressure is the “regulated variable” in the cardiovascular system during exercise. As one exercises at increasing intensities, TPR decreases because of increased vasodilation in the active skeletal muscles due to local autoregulation overriding sympathetic vasoconstriction. What would you predict would happen once the person reaches maximal cardiac output? (Hint: look again at the equation.)

36. Functions of the Blood w Transports gas, nutrients, wastes, and hormones w Regulates temperature w Buffers and balances acid-base

37. COMPOSITION OF TOTAL BLOOD VOLUME

38. Blood Formed Elements and Hematocrit Blood formed elements w White blood cells—protect body from disease organisms w Blood platelets—contribute to blood coagulation w Red blood cells – >99% of the total blood cells — contain hemoglobin, which binds and carries oxygen to tissues, and to a lesser extent, carbon dioxide to the lungs from the tissues Hematocrit • Ratio of formed elements to the total blood volume • Hematocrit—Formed elements/total blood volume

39. Blood Viscosity w Thickness of the blood w The more viscous, the more resistant to flow w Higher hematocrits result in higher blood viscosity; the higher the hematocrit, the greater the oxygen carrying capacity; however, the greater viscosity requires greater cardiac work to pump the blood

40. . • Stroke volume (SV) may increase up to 40% to 60% VO2max in untrained individuals and up to maximal levels in trained individuals, but the SV response is highly variable. • Increases in HR and SV during exercise allow cardiac output (Q) to increase. . Cardiovascular Response to Acute Exercise • Heart rate (HR) increases as exercise intensity increases up to maximal heart rate. w Blood flow increases to the active muscles, and decreases to the inactive tissue, e.g., visceral organs w Mean arterial pressure increases, but not to near the same extent as Q; thus, TPR decreases nearly proportionally to the increase in Q to maintain a constant MAP

41. Resting Heart Rate w Averages 60 to 80 beats/min; can range from 28 to above 100 beats/min w Tends to decrease with age and with increased cardiovascular fitness w Is affected by environmental conditions such as altitude and temperature

42. Steady-State Exercise Heart Rate w Heart rate plateau reached during constant rate of submaximal work w Optimal heart rate for meeting circulatory demands at that rate of work w The lower the steady-state heart rate at a given exercise intensity (and the higher the stroke volume), the more efficient the heart

43. Maximum Heart Rate w The highest heart rate value one can achieve in an all-out effort to the point of exhaustion w Remains relatively constant day-to-day but decreases with aging • Can be estimated: HRmax = 220 – age in years or

44. Stroke Volume w Determinant of cardiorespiratory endurance capacity at maximal rates of work because maximal HRs don’t vary much in persons of the same age w May increase with increasing rates of work up to intensities of 40% to 60% of max or higher when starting from upright position w May continue to increase up through maximal exercise intensity, generally in highly trained athletes • Magnitude of changes in SV depends on position of body before and during exercise

45. Stroke Volume - Sometimes

46. STROKE VOLUME INCREASES IN CYCLISTS

47. Stroke Volume Increases During Exercise wFrank Starling mechanism — increase in venous return (i.e., increased pre-load) from muscle and respiratory pumps results in more blood in the ventricle causing it to stretch more and contract with more force according to the length-tension relationship. wIncreased ventricular contractility (without end-diastolic volume increases) from sympathetic stimulation. wDecreased total peripheral resistance due to increased vasodilation of arterioles in active muscles.

48. THE MUSCLE PUMP During exercise the muscle pump functions to return blood to the heart, or increase venous return; the thoracic pump serves the same function, i.e., to compress veins in the chest and abdomen to increase venous return to the heart

49. . CHANGES IN Q AND SV WITH INCREASING RATES OF WORK

50. w When exercise intensity exceeds 40% to 60%, further increases in Q may be more a result of increases in HR than SV since SV tends to plateau at higher work rates. . Cardiac Output w Resting value is approximately 5.0 L/min (70 kg male). w Increases linearly with increasing exercise intensity to maximal values of between 20 to 40 L/min. w The magnitude of cardiac output varies with body size.