This video will continue with the second part of the Cardiovascular System's Responses to Exercise. I will examine the main ways in which blood flow is increased to the working muscles, as well as address a number of key training adaptations associated with endurance training, related to the cardiovascular system. There are three primary ways in which we can increase blood flow to the working muscles. The first way, is for the heart to pump more blood per minute. This increase in cardiac output was discussed in the previous video, and as such will not be addressed here. The second method to enhance muscle blood flow is known as vasodilation. Here, the smooth muscle surrounding blood vessels relaxes. In doing so, the blood vessels can now expand and open up, allowing for greater blood flow to the working skeletal muscles. Finally, the third method for increasing muscle blood flow is known as shunting. Basically, blood vessels and other tissues such as the kidneys, liver, and stomach, undergo vasoconstriction. In this case, the smooth muscle surrounding these blood vessels contract, thereby, narrowing the blood vessels resulting in a significant reduction in flow. The purpose of this response is to redirect or shunt blood flow away from these organs to the active muscles, where it is needed. This figure indicates just how powerful smooth muscle relaxation or constriction in a blood vessel can be in increasing or decreasing blood flow. In this example, if the radius of the vessel is reduced a mere 0.8 millimeters, blood flow through that vessel will be cut in half. This is exactly what happens during exercise in blood vessels of less active organs, thereby redirecting blood flow to the working muscles. On the other hand, the blood vessels to the working muscles relax, thus increasing their radius, allowing for greater blood flow. Let's examine the main factors that cause smooth muscle in the blood vessels of our working muscles to relax. At the local level of the muscle, we have several metabolic factors that increase or decrease, which reflect the exercise intensity, and thus, the need to enhance blood flow. As we have covered previously, there is an exercise intensity dependent increase in the production of carbon dioxide and metabolic acids. The increase in carbon dioxide and hydrogen ions in muscle, will cause smooth muscle of the local blood vessels to relax, thereby increasing muscle blood flow. Additionally, a decrease in oxygen levels, as muscle oxygen consumption increases, will also cause relaxation of smooth muscle, enhancing blood flow. This is known as functional or active hyperemia shown here. Basically, this refers to the fact that when the metabolic rate of skeletal muscle increases, as it does during exercise, the resulting changes in the local environment of the cell, reflect greater functional or metabolic needs. Thus, the metabolic byproducts associated with an increase in metabolism can be sensed, resulting in greater blood flow. Shown here is the typical cardiovascular response to intense exercise. Cardiac output increases approximately fivefold compared to rest. Please notice that despite this large increase in the amount of blood being pumped by the heart, blood flow to the kidneys, gastrointestinal tract, and other tissues such as liver, is actually reduced below resting levels. This represents vasoconstriction in the blood vessels of these tissues, thereby redirecting blood flow to the exercising muscles. Also notice that the bulk of the cardiac output, in this example 88%, is going to the exercising muscles. This represents vasodilation in blood vessels of these muscles, greatly enhancing blood flow. Lastly, I wish to point out that vasoconstriction in blood vessels of tissues such as the kidneys and liver, is primarily regulated by the sympathetic nervous system. As shown here, the arteries are elevated by sympathetic nerve fibers. During exercise, sympathetic nerve activity to the blood vessels in these tissues increases, causing smooth muscle contraction and vasoconstriction. Now, let's examine the blood pressure response during a bout of exercise. As indicated here, systolic blood pressure increases steadily with an increase in exercise intensity. Diastolic pressure remains stable. These are normal responses in healthy individuals. However, please notice that when individuals are engaging in intense or heavy weight training, involving large muscle groups, systolic blood pressure can increase to well above 250 millimeters of mercury. For young healthy individuals, this transient increase in blood pressure, is generally not a problem. However, for individuals with cardiovascular disease, such an elevation in blood pressure could be dangerous placing a great strain on the heart. Now let's examine the key training adaptations in the cardiovascular system associated with endurance training. A hallmark adaptation is a lower heart rate both at rest and during submaximal exercise. You can afford to have a lower heart rate because training results in an increase in stroke volume, both at rest and during exercise. Thus, resting and submaximal exercise cardiac output can be maintained. There is also an increase in maximal cardiac output, which is entirely due to an increase in maximal stroke volume, as maximal heart rate does not change with training. Also, the arteriovenous oxygen difference is greater during exercise after training. Together, the increase in maximal cardiac output and arteriovenous oxygen difference result in an increase in maximal oxygen consumption or VO2max. Now, let's examine these training adaptations more closely. In previously untrained individuals, the increase in VO2max with training, is due to an equal increase in maximal cardiac output and maximal arteriovenous oxygen difference. A range of 15 to 40% increase in VO2max can be achieved with training in previously untrained individuals. For already trained individuals, increasing their training volume or intensity will produce only modest increases in their VO2max, as it is already high to begin with. Not surprisingly, cross-country skiers who recruit a large muscle mass while training, both lower and upper body, demonstrate the highest values for VO2max. Distance runners are a close second. These values are approximately twice as great as their younger sedentary counterparts. Shown here, are the typical training adaptations related to the cardiovascular system. Notice that resting heart rate is significantly lower in trained men and women. This athletic bradycardia can occur because of the increase in resting stroke volume, thereby maintaining resting cardiac output. Now notice that at maximal exercise, training has no effect on maximal heart rate, if anything it's a tad lower. Thus, the increase in maximal cardiac output with training is entirely due to the increase in maximal stroke volume. The heart becomes a more forceful pump after training. The other component to the increase in VO2max with training, is the arteriovenous oxygen difference. An increase in red blood cell number will improve oxygen transport to muscles. An increase in the number of capillaries per muscle fiber, will enhance oxygen diffusion into muscles. Finally, more mitochondria will allow for greater utilization of the oxygen delivered to muscles. Together, these training adaptations result in a greater arteriovenous oxygen difference, at any given workload. In review, this figure highlights the significant endurance training adaptations associated with the cardiovascular system, and VO2max in particular. In summary, blood flow to muscles can increase dramatically, dependent upon the exercise intensity. Blood vessels and working muscles, will dilate allowing for greater blood flow. Blood flow to other organs will decrease, thereby redirecting blood flow to working muscles. Endurance training results in improvements in all components of the Fick Equation.
