SECTION 111 THE CARDIONASCULAR SYSTEM Exercise Tolerance Evaluation A well-trained.20-year-old track star is in excellent health and has often run a mile in under 4 nimutes.As part of a large group study of young athletes engaged in endurance sports.he enters the exercise laboratory for evaluation of his physical condition during treadmill exercise. 1.What are the key cardiovascular changes that occur at the starting block during an actual race？ 2.As the treadmill is speeded up,what happens to heart rate,peripheral resistance,skin blood flow,cardiac output,and blood flow distribution? 3.Is an increase in PaCD,responsible for the increase in respiratioe,heart rate,and cardiac output? 4.Why does blood flow to the leg muscles increase? 5.Is there a change in leg muscle interstitial pressure during the exercise? 6.What happens to arterial and central venous oxygen levels? 7.What influences vemous return during the exercise? 8.What happens to stroke volume and blood pressure? 9.What factors affect skin blood flow and body terperatures during the exercise? 10.What linits the runner's performance,and how does training inprove ANSVER 1.In anticipation of a race.the runner's sympathetic nervous systen is activated (central command),and catecholamines are released from the adrenal medulla.This results in cardiac acceleration.increased mocardial contractility, increased cardiac output.peripheral vasoconstriction,and an increase in blood pressure
SECTION III THE CARDIOVASCULAR SYSTEM Exercise Tolerance Evaluation A well-trained, 20-year-old track star is in excellent health and has often run a mile in under 4 minutes. As part of a large group study of young athletes engaged in endurance sports, he enters the exercise laboratory for evaluation of his physical condition during treadmill exercise. 1. What are the key cardiovascular changes that occur at the starting block during an actual race? 2. As the treadmill is speeded up, what happens to heart rate, peripheral resistance, skin blood flow, cardiac output, and blood flow distribution? 3. Is an increase in PaCO2 responsible for the increase in respiration, heart rate, and cardiac output? 4. Why does blood flow to the leg muscles increase? 5. Is there a change in leg muscle interstitial pressure during the exercise? 6. What happens to arterial and central venous oxygen levels? 7. What influences venous return during the exercise? 8. What happens to stroke volume and blood pressure? 9. What factors affect skin blood flow and body temperatures during the exercise? 10. What limits the runner's performance, and how does training improve ANSWER 1. In anticipation of a race, the runner's sympathetic nervous system is activated (central command), and catecholamines are released from the adrenal medulla. This results in cardiac acceleration, increased myocardial contractility, increased cardiac output, peripheral vasoconstriction, and an increase in blood pressure
2.Heart rate continues to increase during the exercise,until the rate reaches a plateau of about 180 beats/nin at maxim effort.Peripheral resistance decreases because of relaxation of arterioles in active muscle and skin (as body temperature rises).Skin blood flow increases,which aids in heat loss.Blood flow to inactive muscle,kidsey,and gastrointestinal tract is reduced. 3.The inereased production of CD by the active museles is removed in the lungs by the greater depth and rate of respiration,and therefore arterial PCD:(Pac0) re图ins normal. 4.Blood flow to the leg mseles increases because of local factors.which include the release of vasodilator metabolites (e.g.adenosine).the increase in muscle temperature,and the local decrease in pl (increased CD,and lactic acid). The arterioles dilate and more capillaries open (capillary recruitment). 5.Interstitial pressure in the active muscles increases because of movenent of fluid from the vascular compartnent to the interstitial space.The greater hydrostatic pressure (increase in blood pressure plus arteriolar dilation)enhances capillary filtration,until the increase in tissue pressure counterbalances the elevated intracapillary pressure. 6.Oxygen extraction fron the blood perfusing the active museles is increased, and therefore venous blood oxygen content is reduced.However,the enhanced respiratory activity results in normal oxygenation of the blood in the lungs and hence a normal arterial oxygen content. 7.Venous return is facilitated in rumning by the following:(1) sympathetic-mediated comtraction of capacitance vessels,(2)muscle compression of the leg veins with one-way valves,and (3)the greater negative pressure in the thorax caused by deeper and more rapid respiration. 8.In the trained athlete.stroke volune is large at rest and increases further with exercise,whereas in the untrained individual,stroke volume shows little change and heart rate increases greatly.The actual cause of the increase in cardiac output is the decrease in peripheral resistance.Mean arterial blood pressure
2. Heart rate continues to increase during the exercise, until the rate reaches a plateau of about 180 beats/min at maximum effort. Peripheral resistance decreases because of relaxation of arterioles in active muscle and skin (as body temperature rises). Skin blood flow increases, which aids in heat loss. Blood flow to inactive muscle, kidney, and gastrointestinal tract is reduced. 3. The increased production of CO2 by the active muscles is removed in the lungs by the greater depth and rate of respiration, and therefore arterial PCO2 (PaCO2) remains normal. 4. Blood flow to the leg muscles increases because of local factors, which include the release of vasodilator metabolites (e.g., adenosine), the increase in muscle temperature, and the local decrease in pH (increased CO2 and lactic acid). The arterioles dilate and more capillaries open (capillary recruitment). 5. Interstitial pressure in the active muscles increases because of movement of fluid from the vascular compartment to the interstitial space. The greater hydrostatic pressure (increase in blood pressure plus arteriolar dilation) enhances capillary filtration, until the increase in tissue pressure counterbalances the elevated intracapillary pressure. 6. Oxygen extraction from the blood perfusing the active muscles is increased, and therefore venous blood oxygen content is reduced. However, the enhanced respiratory activity results in normal oxygenation of the blood in the lungs and hence a normal arterial oxygen content. 7. Venous return is facilitated in running by the following: (1) sympathetic-mediated contraction of capacitance vessels, (2) muscle compression of the leg veins with one-way valves, and (3) the greater negative pressure in the thorax caused by deeper and more rapid respiration. 8. In the trained athlete, stroke volume is large at rest and increases further with exercise, whereas in the untrained individual, stroke volume shows little change and heart rate increases greatly. The actual cause of the increase in cardiac output is the decrease in peripheral resistance. Mean arterial blood pressure
increases slightly.as does pulse pressure (i.e..systolic pressure increases a little more than does diastolic pressure). 9.At the start of the exercise,activation of the sympathetic nervous system causes coastriction of the skin resistance vessels.As body temperature rises during the exercise,the skin vessels dilate,and therefore heat loss is facilitated.If the exercise is very severe and exhaustion occurs,the skin vessels constrict and permit more of the circulating blood to perfuse the active muscles.This skin vasoconstriction can lead to serious hyperthermia. 10.The linitation of performance in this runner is oxygen delivery to the active muscles.Bence.the pumping capacity of the heart is the main limiting factor
increases slightly, as does pulse pressure (i.e., systolic pressure increases a little more than does diastolic pressure). 9. At the start of the exercise, activation of the sympathetic nervous system causes constriction of the skin resistance vessels. As body temperature rises during the exercise, the skin vessels dilate, and therefore heat loss is facilitated. If the exercise is very severe and exhaustion occurs, the skin vessels constrict and permit more of the circulating blood to perfuse the active muscles. This skin vasoconstriction can lead to serious hyperthermia. 10. The limitation of performance in this runner is oxygen delivery to the active muscles. Hence, the pumping capacity of the heart is the main limiting factor