BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

14.7. The Heart

14.7.5. The Effects of Exercise on the Cardiovascular System

Short-term effects

During physical activity, working Muscles require a greater supply of oxygen and glucose for aerobic Respiration than at rest, and must more rapidly dispose of metabolic byproducts such as carbon dioxide and heat. This is achieved through a sharp increase in Blood supply, driven by several operating mechanisms.

During sustained heavy exercise, left ventricular output can rise from 4–6 l/min (at rest) to 15 l/min (in an untrained woman), 22 l/min (in an untrained man), and up to 30 l/min (in an athlete); in other words, at this rate of work, a medium-sized bathtub could be filled in about 2 minutes. This is accomplished not only by increasing The Heart rate, but also by increasing the stroke volume (i.e., a more complete emptying of the ventricles). Typically, this results in an approximate 30% increase in blood pressure. Roughly speaking, at maximum exertion, heart rate can triple while stroke volume doubles. In a trained athlete, this—combined with vasodilation induced by adrenaline and sympathetic nervous stimulation (see below)—can boost Muscle blood flow up to 25-fold, with roughly half of the effect resulting from vasodilation and the other half from elevated blood pressure. Vessel dilation occurs not only in skeletal muscles, but also in the Lungs and directly within the heart muscle itself.

What drives this increase in Cardiac Output? During the anticipation of exercise and in its initial phase, the sympathetic Nervous system and adrenaline stimulate enhanced heart activity. However, this pace is subsequently sustained by other neural and hormonal factors. For instance, the dilation of Veins in the muscles enhances venous return to the heart, which in turn increases cardiac output via the mechanism described in Section 14.7.4 (see also Fig. 14.24). In Organs with lower oxygen demands during exercise compared to muscles—particularly the gut, Liver, Kidneys, and Spleen—arterioles undergo vasoconstriction (Section 14.7.7). The effect of elevated blood carbon dioxide levels, caused by intensive muscular activity, is discussed in Section 14.7.7.

Ultimately, increased heat production by the body leads to the dilation of cutaneous Blood Vessels. Elevated blood Temperature stimulates the Hypothalamus (a region of the Brain), sending nerve impulses to the cardiovascular centers in the Medulla Oblongata, which stimulate the dilation of Skin arterioles and ensure enhanced heat dissipation into the environment.

Fig. 14.25 illustrates several Physiological effects of exercise in a marathon runner. An increase in stroke volume (by about 50%) and heart rate (by about 27%) drives their cardiac output up from 5 to 30 l/min. Note that stroke volume reaches its maximum peak much earlier than heart rate, although it rises much more rapidly at first. This is characteristic of a trained Organism. It is this elevated cardiac output (rather than respiratory traits) that constitutes the primary difference between a marathon runner and an untrained individual.

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Fig. 14.25. The Effect of exercise on a marathon runner, showing stroke volume and heart rate (HR) at various levels of cardiac output.

Long-term effects

Like any other muscle, the heart grows stronger with regular exercise. Consequently, long-term conditioning both strengthens The Heart and leads to increased cardiac output. As noted above, the typical difference in this indicator between a marathon runner and an untrained person is around 40%. The internal Chambers of the heart enlarge, and myocardial mass increases by 40% or more. Overall functional performance improves, driven by the expansion of the cardiac vascular network and an increase in both the number and size of Cell/35.html">Mitochondria within cardiomyocytes. It should be emphasized that this requires regular, prolonged aerobic exercise rather than brief bursts of intense physical activity that rely on anaerobic muscle respiration.