Cardiovascular Dynamics During Exercise Chapters 15 & 16 Introduction At rest: O2 supply = O2 demand Exercise: O2 demand increases To the muscles To the heart To the skin Maintain flow to the brain How does the heart increase O2 supply to meet the O2 demand? Cardiac Output Q = heart rate times stroke volume Cardiac Output Blood flow per minute. At rest Q = 5-6 liters/min Q increases linearly with the demand for more O2 Indicator of oxygen supply How does cardiac output increase? Increase heart rate Increase stroke volume Heart Rate Resting heart rate Anxiety Dehydration Temperature Digestion Over-training The most important factor for increasing Q during acute exercise. Heart Rate What causes HR to increase during exercise? Decrease parasympathetic (vagal) stimulation Increase sympathetic stimulation Heart Rate Steady state exercise Why does heart rate level off during steady state exercise? Heart Rate Increases with intensity and levels off at maximal effort. – HRmax = 220 – age – (± 12) Stroke Volume Volume pumped per beat of the heart Influenced by preload and afterload Stroke Volume Increases until about 25-50% of maximum After that it may plateau (untrained) or continue to increase (trained) Decrease at maximum effort? Stroke Volume How does stroke volume increase during exercise? Increase preload (EDV) – Increase venous return • Muscle pump, etc. Decrease afterload – Vasodilation • Metabolic control and sympathetic stimulation Increase contractility (ESV) – Increase sympathetic stimulation Frank-Starling Mechanism Frank-Starling mechanism: the ability of the heart to alter the force of contraction is dependent on changes in preload. As the myocardial fibers are stretched, the force of contraction is increased. Because the length of the fiber is determined primarily by the volume of blood in the ventricle, EDV is the primary determinant of preload This graph depicts the Frank-Starling mechanism of compensation in CHF. The black curves represent ventricular function in a normal subject and the colored curve is with left ventricular dysfunction. Line N to A represents the initial reduction in cardiac output due to CHF. Line A to B represents the Frank-Starling mechanism of compensation; an increase in left ventricular end-diastolic pressure needed to maintain cardiac output. Stroke Volume Stroke Volume Increased sympathetic stimulation Vasodilation from ‘autoregulation’ Cardiovascular drift Caused by a decrease in venous return Cardiac output is maintained by…..? Cardiovascul ar Drift Stroke Volume SV greater in trained Most significant effect of training Result • • An increase in cardiac output… • Increase HR • Increase SV …results in an increase in O2 supply Hemodynamics Blood Vessels Arteries Arterioles Capillaries Venules Veins Physical Characteristics of • Plasma Blood Liquid portion of blood Contains ions, proteins, hormones • Cells Red blood cells Contain hemoglobin to carry oxygen White blood cells Platelets The Blood Arterial blood carries 20 ml of oxygen per 100 ml of blood Hematocrit Percent of blood composed of cells The Blood • Arterial blood: 97-98% saturated with O2 • Venous blood – Rest – 75% – Exercise – 25% Blood Pressure Expressed as systolic/diastolic Normal is 120/80 mmHg High is 140/90 mmHg Systolic pressure (top number) Pressure generated during ventricular contraction (systole) Diastolic pressure Pressure in the arteries during cardiac relaxation Blood Pressure • Pulse pressure Difference between systolic and diastolic Pulse Pressure = Systolic - Diastolic • Mean arterial pressure (MAP) Average pressure in the arteries MAP = Diastolic + 1/3(pulse pressure) Mean Arterial Pressure • Blood pressure of 120/80 mm Hg • MAP = 80 mm Hg + .33(120-80) • = 80 mm Hg + 13 • = 93 mm Hg Hemodynamics • Based on interrelationships between: – Pressure – Resistance Hemodynamics: Pressure Blood flows from high low pressure Proportional to the difference between MAP and right atrial pressure (P) Blood Flow Through the Systemic Circuit Hemodynamics: Resistance Resistance depends upon: Length of the vessel Viscosity of the blood Radius of the vessel A small change in vessel diameter can have a dramatic impact on resistance! Length x viscosity Resistance = Radius4 Hemodynamics: Blood Flow Directly proportional to the pressure difference between the two ends of the system Inversely proportional to resistance Pressure Flow = Resistance Sources of Vascular Resistance MAP decreases throughout the systemic circulation Largest drop occurs across the arterioles Arterioles are called “resistance vessels” Pressure Changes Across the Systemic Circulation Pressure Changes During the Cardiac Cycle Factors That Influence Arterial Blood Pressure Cardiovascular Control How can the blood vessels increase blood flow? Vasodilation to increase blood flow to muscles and skin Waste products (metabolic or local control) Sympathetic stimulation (cholinergic) Vasoconstriction to maintain blood pressure Sympathetic stimulation (adrenergic) Maximum muscle blood flow is limited by the ability to maintain blood pressure Vasodilation Vasoconstriction Blood Vessels Oxygen Extraction Measured as a-v O2 difference • a = O2 in arteries (20 ml/100 ml of blood) • v = O2 in veins (15 ml/100 ml of blood) • (a-v)O2 = 5 ml/100 ml of blood a-v O2 difference No change in O2 content in the blood Remains at 20 ml/100 ml of blood Decrease in O2 inside the muscle High pressure to a Low pressure Greater pressure difference between the blood and the muscles Oxygen moves from a HIGH pressure area (blood) to a LOW pressure area (muscle) Therefore, more O2 is extracted from the blood High pressure to a Lower pressure RESTING 20 ml or P02 98 EXERCISE 20 ml or P02 98 5 ml extracted PO2 = 40 15 ml extracted PO2 = 20 Lower PO2 due to an increase in O2 consumption (VO2) during exercise Oxygen Consumption VO2 liters per minute milliliters per kilogram per minute VO2 = oxygen supply x oxygen extraction VO2 = Q x a-v O2 difference VO2 = HR x SV x a-v O2 difference Oxygen Consumption An increase in oxygen supply leads to an increase in oxygen consumption Increase in cardiac output With help from HR and SV Increase in (a-v)O2 More O2 is supplied and extracted Therefore, more O2 can be used by the muscle fibers (mito) Oxygen Consumption Q and a-v O2 difference each account for 50% of the increase in VO2 during exercise Near maximal exercise, Q accounts for 75% of the increase in VO2 Oxygen Consumption VO2 increases with intensity VO2 = rate of blood flow times the O2 extracted from a given amount of blood VO2 = cardiac output x a-vO2 difference VO2 can increase by A greater blood flow Taking more oxygen out of every 100 ml of blood What limits aerobic exercise? Lack of oxygen supply? If so, wouldn’t the muscles be more anaerobic? And, wouldn’t the heart also be more anaerobic? But an anaerobic heart produces angina Maybe the central nervous system protects the heart from ischemia by causing muscle fatigue before the heart becomes