Pump up the volume: explaining and understanding the cardiovascular responses to exercise - includes continuing education test

Explaining and understanding the cardiovascular responses to exercise.

Most exercise professionals have little trouble describing the cardiovascular responses to acute exercise. We all know that when you exercise your heart beats faster and harder, resulting in an increase in heart rate, greater blood flow, and a rise in arterial blood pressure. The more difficult challenge is explaining these responses and understanding the intricacy with which they are controlled.

The factors regulating cardiovascular function during exercise are complex and diverse. Prolonged aerobic exercise results in an increased demand for oxygen and nutrients, an increased accumulation of metabolic by-products, the secretion of hormones from various endocrine glands, and a gradual rise in body temperature due to the increased metabolic rate. Only through an understanding of the interaction of these factors can the characteristic cardiovascular responses to a single bout of aerobic exercise be fully explained and understood.

It is important to know the physiological effects of exercise in order to understand the significance of warm-up prior to exercise and the benefits of cool-down afterward. Warm-up prior to exercise produces a gradual dilation of the blood vessels supplying the heart and skeletal muscles. This allows sufficient blood flow to the heart and skeletal muscles and reduces the risk of sudden increases in arterial blood pressure. A gradual cool-down following exercise results in a more appropriate drop in blood pressure due to a more gradual reduction in heart rate and blood flow. Adverse exercise reactions, such as muscle or chest pain during exercise or dizziness or light-headedness following exercise, can have a physiological basis. As a result, they may be minimized or avoided altogether with appropriate exercise instruction.

The Control of Heart Activity

Heart rate is affected during exercise by a combination of factors. Some of these factors arise from within the heart itself while others originate outside the heart. The heart possesses an internal pacemaker that can create electrical impulses automatically at an average rate of about 80-100 per minute. This impulse rate rises during exercise when the heart warms due to the increased metabolic rate. Heart rate also rises when the pacemaker physically stretches, as is the case during exercise when more blood returns from the body to the chamber that contains the pacemaker.

Heart rate is also significantly affected by the portion of the nervous system referred to as the autonomic nervous system (ANS). The ANS is responsible for the involuntary control of all basic body functions, including heart rate. It arises from within the medulla of the brain and physically connects to the heart by way of numerous peripheral nerves. The ANS is further subdivided into tWo divisions, the sympathetic and parasympathetic divisions. The parasympathetic nerves, when activated by impulses arising from the medulla, release a chemical neurotransmitter called acetylcholine into the heart that slows the activity of the pacemaker and subsequently slows heart rate. Upon activation, the sympathetic nerves release norepinephrine into the heart resulting in a rapid increase in the rate and force of contraction. With these two separate, opposing branches, the ANS provides better control over heart activity than either branch could independently.

For the ANS to provide control over the heart, it must receive input from multiple sources throughout the body to determine the demand for blood flow at any particular moment. The demand for blood increases when the body faces emotional or physical stress. Exercise, especially that associated with competition, is frequently accompanied by an increased level of anxiety and emotional stress.

An area of the brain referred to as the limbic system controls the body's physical and physiological responses to emotional stress. Upon experiencing anxiety, the limbic system signals the ANS to reduce the parasympathetic stimulation to the heart. At the same time, the ANS increases the sympathetic stimulation to the heart, which produces an increase in heart rate and blood flow. An increase in heart activity is frequently observed before exercise begins, and is considered an attempt by the body to prepare itself physically in anticipation of the physical task.

This reaction is essentially equivalent to the "fight or flight" reaction one experiences when startled. It is the body's way of defending itself from potential threats by furnishing skeletal muscle with increased amounts of blood, oxygen and fuel sources. These responses provide an enhanced opportunity for the body to fight or flee--or, in a more civilized world, compete.

During exercise, voluntary muscle contractions require many complex patterns of electrical impulses that arise from the cerebral cortex of the brain. The specific area of the brain that provides voluntary control over skeletal muscles is known as the motor cortex. This area provides not only the stimulation needed by the skeletal muscles, but also sends parallel signals of an equivalent magnitude to the medulla. Within the medulla, these signals give rise to changes in the ANS thee result in a rapid rise in heart rate. Exercise performed with greater intensity or with a greater amount of muscle mass requires even more stimulation from the motor cortex that subsequently, through its influence on the medulla, produces even higher heart rates and greater blood flow.