Cardiovascular Function During Exercise

Cardiovascular Function During Exercise

Key Points • The cardiovascular responses to exercise are coordinated by the brain, which regulates heart rate, blood pressure, and the distribution of blood flow throughout the body.

Key Points • Exercise with a large muscle mass, e.g., running, cycling, cross-country skiing, elicits different cardiovascular responses than does exercise with a small muscle mass, e.g., lifting weights with the arms.

Key Points • Despite the fact that various types of exercise can elicit the same heart rate response during exercise, they may produce cardiovascular adaptations that are quite different. • In other words, exercise should not be prescribed based upon heart rate alone.

Key Points • Competence in designing exercise training programs to improve various aspects of cardiovascular function requires an understanding of the basics of cardiovascular physiology.

Function • The CV system during exercise performs the important functions of: • 1. Delivering oxygen, some fuel (e.g., blood glucose and free fatty acids), and hormones to the contracting muscles.

Functions • 2. Removing metabolic by-products, such as carbon dioxide and lactic acid, from the muscles and transporting them to other organs for further metabolism.

Functions • 3. Transporting the heat generated during muscular contraction to the skin, where the heat is dissipated to the environment to minimize overheating.

CV Response • 1. Cardiac output increases by roughly 6 L/min for every 1 L/min increase in oxygen consumption (VO2).

CV Response • 2. The maximal rate of oxygen consumption (VO2max) is largely influenced by maximal cardiac output.

CV Response • This is why the measurement of VO2max provides the best non-invasive evaluation of overall cardiovascular function.

CV Response • Arterial blood usually contains 200mL of oxygen/L. • This value ordinarily remains constant, even during strenuous exercise.

CV Response • Mixed venous blood contains 30-100 mL of O2/L.

CV Response • As exercise intensity is increased, more arterial blood is directed to the contracting muscles, and those muscles extract more oxygen; therefore, less oxygen is left in venous blood, and the a-vO2 difference increases.

Muscle Mass • An important factor in determining how the cardiovascular system responds to exercise is the mass of the muscle involved in the exercise.

Heart Rate • The role of the nervous system is of primary importance in regulating heart rate. • The nerves that directly influence heart rate originate in the medulla oblongata at the base of the brain.

Heart Rate • The parasympathetic nerves to the heart release acetylcholine, which slows the heart beat; the sympathetic nerves release norepinephrine, which increases the heart rate and the force of contraction of the heart muscle.

Heart Rate • As a person moves from rest to mild exercise, the increase in heart rate up to roughly 100 bpm is predominately caused by a suppression of parasympathetic stimulation, i.e., the inhibition of the heart rate is removed.

Heart Rate • Heart rate increases beyond 100 bpm are brought about primarily by stimulation of the sympathetic nerves.

Heart Rate • How does the medulla know when and how much to increase the heart rate?

Heart Rate • It receives nerve input from many areas of the body and integrates those signals very precisely to determine the output it must send to the heart.

Heart Rate • The motor area of the cerebral cortex is an extremely important source of input to the cardiac control centers of the medulla.

Heart Rate • The motor cortex must recruit progressively more muscle fibers as exercise becomes more intense and as previously recruited fibers become fatigued.

Heart Rate • As more fibers are recruited, the motor cortex simultaneously stimulates the medulla so that it can quickly increase heart rate (and ventilation, blood pressure, etc.).

Heart Rate • This is termed a “feedforward” control system.

Heart Rate • The medulla also receives input (feedback) from nerves in the contracting muscles; these nerves sense changes in the chemical environment that signal a need for more blood.

Heart Rate • Many other types of input are delivered to the medulla that affect its regulation of the heart rate, but feedforward input from the motor cortex and feedback input from the contracting muscles are among the most crucial during exercise.

Heart Rate • When a large muscle mass is involved in exercise sufficiently intense to elicit VO2max, the medulla activates the sympathetic nerves to the heart to increase its rate to the maximum.

Heart Rate • The decrease in maximal heart rate with age is caused by decreased sympathetic drive from the medulla.

Heart Rate • During exercise involving a small muscle mass, it is not likely that heart rate will rise much above 150 bpm, despite maximal efforts that lead to muscle fatigue.

Heart Rate • This reduced maximal HR is a function of the reduced input to the medulla from the motor cortex and less feedback from the working muscles.

Heart Rate • In other words, fewer muscle fibers are being recruited, resulting in less feedforward stimulation being delivered to the medulla from the motor cortex and less feedback stimulation bring sent from the contracting muscles.

Heart Rate • To raise HR above 150 bpm, one ordinarily must recruit the large muscle mass groups of the legs.

Heart Rate • When cycling exercise with one leg produces the same O2 uptake as cycling with two legs, the HR will be higher for the exercise using the smaller muscle mass.

Heart Rate • Why is this so?

Heart Rate • When exercising at the same intensity with a larger muscle mass, there are more muscle fibers to “share” in the work compared to exercise with a small muscle mass.

Heart Rate • Furthermore, because the smaller muscle mass must produce more force per unit of mass, more metabolic by-products and other chemical changes will be produced in the muscles.

Heart Rate • These changes in the chemical environment will provide a large amount of feedback input to the medulla from the more intensely contracting muscles.

Heart Rate • Although HR may be similar during different types of exercise, the VO2 (energy expenditure) can vary substantially.

Heart Rate • Thus, performing pushups at a HR of 130 bpm requires much less O2 than does jogging at the same HR because a much smaller muscle mass is used to perform the pushups.

Heart Rate • Furthermore, different types of exercise at the same HR may produce quite different effects on ventricular filling and, consequently on stroke volume.

Heart Rate • Thus, when exercise training is prescribed only on the basis of HR, important exercise effects on the heart may be overlooked.

Arterial Blood Pressure • The aorta and the carotid arteries contain nerve endings sensitive to changes in blood pressure (baroreceptors); these receptors relay information about blood pressure status to the medulla by way of sensory nerves.

Arterial Blood Pressure • The medulla then makes the appropriate adjustments in cardiac output and blood vessel diameter to bring blood pressure to a suitable level.

Arterial Blood Pressure • Blood pressure is the product of cardiac output and total peripheral resistance.

Arterial Blood Pressure • Peripheral resistance is determined primarily by the extent to which small arterial vessels are constricted; when vessel constriction is more extensive, there is greater resistance to the flow of blood.

Arterial Blood Pressure • The pressure in the arteries is determined by the balance between cardiac output, i.e., the rate at which the heart is forcing blood into the aorta at one end of the arterial system,

Arterial Blood Pressure • and the total peripheral resistance, i.e., the extent to which small arteries at the other end of the system are open or closed.

Arterial Blood Pressure • Compare small muscle BP response v.s. large muscle BP response.

Cardiovascular Function During Exercise