Cardiovascular system 2

1. Cardiovascular system 2 Cardiovascular responses to exercise

2. Blood supply during rest and exercise • Blood pressure must be maintained at the correct level so that there is sufficient blood flow around the body. Coordinated relaxation and constriction of some blood vessels maintains pressure, but also redistributes blood flow to the active muscles during exercise. • Cardiac output provides the most important indicator of the circulatory system’s functional capacity to meet the demands for physical activity. As with any pump, the rate of pumping (heart rate) and quantity of blood ejected with every stroke (stroke volume) • Cardiac output (L./min-1) = (heart rate x stroke volume) ÷ 1000 • Cardiac output increases in proportion to the intensity of the exercise up to a maximum attainable value. Increased cardiac output is a product of increased stroke volume (more venous back flow to heart and increased contraction strength) and increased heart rate. Stroke volume and heart rate increase during exercise

3. Cardiac output in different populations • With the ventilatory system providing sufficient gas exchange in healthy individuals, the cardiovascular system is crucial to maintain function and to maintain homeostasis during exercise, or to meet the demands for physical activity. • Factors such as gender, age and fitness status influence the total cardiac output during rest and exercise. Cardiac Output (CO) during rest Why would stroke volume increase when HR is low?

4. Cardiac output in different populations • Differences in stroke volume between men and women (with similar training) are mostly due to smaller body size. Stroke volume for women usually averages 25% below men’s values. • The difference in size and age influences the cardiac output of a child compared to an adult. A child has a 20% smaller stroke volume but a higher Maximum HR. • The difference in cardiac output between trained and untrained individuals relies solely on the stroke volume. Two factors probably interact as aerobic fitness improves • Increased vagal tone slows the heart, allowing more time for ventricular filling ( Vagal tone) • Enlarged ventricular volume and a more powerful myocardium eject a larger volume of blood with each systole

5. Average adult cardiac output: ± 5 L. (5000ml) • Average adult

6. Systolic and diastolic pressure during exercise Systolic pressure the force exerted by blood on arterial walls during ventricular contraction. In resting healthy adult the pressure is 120mmHg, this is the ideal balance for emptying and filling the heart chambers • During exercise the blood pressure changes and is dependent upon intensity and type of exercise • During high intensity isometric and anaerobic exercise, both systolic and diastolic pressure rise significantly due to increased resistance of the blood vessels. This is a result of muscles squeezing veins, increasing peripheral resistance and an increase in intra-thoracic pressure due to the contracting of the abdomen. Diastolic pressure the force exerted by blood on the arterial walls during ventricular relaxation. • In resting adult the pressure is 80 mmHg. The pressure lessens as the blood goes from arteries to arterioles to capillaries. The pressure in the venules and veins is low and consistent. • During steady aerobic exercise involving large muscle groups, the systolic pressure increases as a result of an increased cardiac output, while the diastolic pressure remains constant (or in well trained athletes may even drop) • High blood pressure can cause serious complications to the heart, brain and kidneys, whereas low pressure can result in insufficient oxygen and other nutrients reaching the muscle cells. • The blood pressure is regulated by the vasomotor control center. The redistribution is controlled primarily by vasoconstriction and vasodilatation

7. VO2Max, Fick equation The most commonly used marker of an individual’s aerobic fitness is the relationship between cardiac output, oxygen uptake, and difference between the oxygen content of arterial and mixed-venous blood (a-vO2 difference). This principle was discovered by German physiologist Adolf Fick in 1870 Cardiac output (mL./min-1) = [VO2(mL/min-1) / a-vO2(ml/dl blood-1)] x 100 Stroke volume = 71 ml/b HR = 70 b Cardiac Output = 5000 ml aVO2 rest = 20ml/dl mixed venous vO2 rest = 12-15 ml/dl mixed vO2 max = 2 – 4 ml/dl a-vO2 diff. at rest = 5 ml/dl => 75% O2 is “unused” bound to Hb In reality, this method is rarely used due to the difficulty of collecting and analyzing the gas concentrations. However, by using an assumed value for oxygen consumption, cardiac output can be closely approximated without the cumbersome and time-consuming oxygen consumption measurement

8. VO2Maximal aerobic and anaerobic capacities • Oxygen consumption (uptake) is the amount of oxygen a person consumes per unit of time (usually a minute). • The oxidation (burning) of fuel foods requires a definite amount of oxygen per unit mass of fuel. This amount can be measured indirectly by collecting expired air and comparing it with the composition of inspired air (how much oxygen has been used and CO2 produced). • At rest the oxygen uptake varies between 0.2 and 0.3 L./min-1 • Based on that 1 L. of oxygen liberates 22kJ of energy from glycogen, the BMR (basal metabolic rate) converts to between 4.4 and 6.6 kJ. For a 60kg person the BMR would be between 0.073 and 0.110 kJ per minute per kg of body mass. • During exercise the total body oxygen uptake increases proportionally to the intensity of the exercise, until a maximal work rate is reached. • Highest VO2 achieved is VO2max, this maximum oxygen uptake is known also as aerobic power • VO2max can therefore be quantitatively represented as the maximum amount of oxygen that a person can consume per minute during a progressive exercise test to exhaustion. The highest value represents the individual’s maximal physiological capacity to transport and use oxygen • A mean value of VO2max for male students is ±3.5L/min and females ±2.7L./min • Endurance athletes may reach between 4 and 6L./min VO2max depends on body mass as well as physical fitness, so often expressed in milliliters per kilogram of body mass per minute (mL kg-1 min-1) so that comparisons can be made

9. Factors affecting maximum aerobic power: • Chemical ability of the muscular tissues to use oxygen in breaking down fuels • Combined ability of the cardiovascular and the pulmonary systems to transport oxygen to the muscular system • VO2max decreases by about 10% per decade with ageing, starting in the late teens for women and mid-20’s for men. Oxygen consumption as an indirect way of measuring energy costs: a hypothetical example Net oxygen cost (oxygen consumed during exercise above which is needed during rest) 0.3 L/min at rest 2.275 L/min (65% of VO2max in males @ 3.5L/min) 20 min Jog x net oxygen cost = total net oxygen cost 20 x 1.975 = 39.5 L. 1 liter of oxygen produces 22kJ of heat energy in combination with food fuel: 22 x 39.5 = 869 kJ

10. Exercising in a hot environmentCardiovascular drift When exercising in a warm or neutral environment for 15 min or more your HR increases. This increase in HR is to compensate for the decrease in pulmonary arterial pressure and reduced stroke volume. To maintain cardiac output at reduced pressure the heart rate must be increased. Under these circumstances, a person usually must exercise at a lower intensity than if cardiovascular drift did not occur.

11. Cardiovascular drift • The drop in venous return is a result of the muscles demand for more blood, however, in warm environments the blood is also shunted to the skin for themoregulation (cooling) causing a shift in fluids to skin tissue. • The redistribution of blood plasma and water loss through sweating causes a reduced pulmonary arterial pressure and decreased stroke volume • Prevention / minimization • Consistently replace fluids and electrolyte imbalance • Acclimatize to environment • Weight training to supplement cardiovascular efforts