Adjustment of circulation during exercise
The response of circulatory system to exercise is an integration of nervous system, endocrine, musculoskeletal and cardiovascular system. During exercise the demands on the circulatory system greatly varies. In the most of the human beings there is an increase in oxygen consumption to five fold within in a few minutes of intense exercise. This increase in oxygen consumption is due to increased demand of oxygen by the working muscle.
During exercise the mechanoreceptors present in the muscles detect changes in the tension of the muscle and send afferent sensory information to the brain, thereby activating the cardiovascular control centre in the medulla.
The cardiovascular control centre reduces the activity of the parasympathetic nervous system and increases the activity of sympathetic nervous system, changing the efferent signals going to the heart and the arteriolar smooth muscle.
This change in parasympathetic and sympathetic activity has greater effect on cardiac output. Cardiac output can be increased 4-8 times the value at rest in humans where as in a trained thoroughbred horse a ten fold increase occurs during maximum exercise.
At the onset, parasympathetic activity decreases causing an increase in heart rate. At the same time, increase in muscular activity and breathing improves the function of respiratory and skeletal muscle pumps causing an increase in venous return to the heart, resulting in increased stroke volume. Therefore, during initial stages of exercise, the cardiac output increase is due to increase in heart rate and stroke volume.
Next, sympathetic stimulation of the heart increases, causing an increase in both heart rate and contractility. During the later stage of exercise, increase in heart rate, increases cardiac output rather than increase in stroke volume.
In addition to changes in cardiac output, blood flow pattern also changes during exercise. During rest, skeletal muscle receive only about 20% of cardiac output, whereas during exercise they receive 88% of total cardiac output i.e., blood flow to skeletal muscles is increased from 1.2 l/min at rest to 2.2 l/min during exercise. These changes in blood flow are as a result of vasodilation of the arterioles of skeletal muscle and heart. The increase in activity causes a generalised vasoconstriction of the arterioles of other organs.
In skeletal muscle, local release of paracrine factors causes vascular smooth muscle to release by opposing vasoconstrictive effects of ɑ-adrenergic receptor stimulation. These factors together cause an intense local vasodilatation thereby increasing the blood flow to the muscles.
Vasodilatation of the arterioles of skeletal muscles and vasoconstriction of arterioles of other organs causes a decrease in the total peripheral resistance.
As a result blood pressure increases slightly during exercise. This stimulates the baroreceptor reflex and brings back the BP to the normal by decreasing cardiac output or total peripheral resistance. The afferent signal from the muscle mechanoreceptors also changes the set point of the baroreceptor reflex allowing BP to increase slightly during exercise.