Exercise and/or physical activity is beneficial for healthy individuals and for those at high risk for cardiovascular disease (CVD), as well as those with manifest CVD. This chapter addresses the hemodynamics and health benefits of physical activity and exercise conditioning programs, both in healthy individuals and those with or at risk for CVD.
During physical activity, energy expenditure increases, and the compensatory cardiovascular response represents an integration of neural, biochemical, and physiologic factors. The cardiovascular control center is believed to reside in the ventrolateral medulla of the brain and to respond to both central and peripheral inputs. Central impulses arise from somatomotor centers of the brain. Peripheral impulses are generated by mechanoreceptors, found in muscles, joints, and the vascular system; chemoreceptors, found in the muscles and the vascular system; and baroreceptors, found in the vascular system. These impulses are transmitted by autonomic afferent fibers. The central control center regulates cardiac output (CO) and its distribution to organs and tissues according to metabolic demand.
The feed-forward command system located in the motor cortex provides a coordinated and rapid cardiovascular response to optimize tissue perfusion and maintain central blood pressure. This central command provides the greatest control over heart rate (HR) during exercise1 and is also involved in the preexercise anticipatory response. Stimulation of the central control center by the higher command centers leads to alteration of autonomic tone. This may explain the influence of “emotions” on the cardiovascular response.
The cardiovascular control center also receives input from peripheral receptors. Stretch and tension of muscular and articular mechanoreceptors trigger afferent impulses that are important in the regulation of the circulatory response to dynamic exercise. Muscle chemoreceptors stimulated by products of metabolism influence the control center as well. This reflex neural input (termed the exercise pressor reflex) provides rapid feedback modifying the autonomic outflow in response to physical activity and exercise.2
Vascular baroreceptors are located in the aortic arch and carotid sinuses. They respond to changes in arterial blood pressure and regulate HR by eliciting reciprocal changes in both sympathetic and parasympathetic activity.3 The arterial baroreceptors protect the cardiovascular system from relatively short-term changes in blood pressure, as seen during physical exercise. Cardiopulmonary mechanoreceptors in the atria, ventricles, and pulmonary vessels aid in regulation of the circulatory response. An increase in blood pressure elicits reflex slowing of the heart, and the converse applies with hypotension. During physical activity, this feedback mechanism is altered so that blood pressure can rise. The aortic and carotid bodies contain chemoreceptors sensitive to arterial oxygen, carbon dioxide, and hydrogen ion concentrations. Whereas decreased arterial oxygen levels trigger an increase in arterial pressure, changes in carbon dioxide and hydrogen ion concentration have a relatively small effect.
CIRCULATORY ADJUSTMENTS WITH EXERCISE
The circulatory response to exercise involves a complex series of adjustments resulting ...