Adaptation of Heart Function to Physical Activity

Functioning tissue requires several times more oxygen and nutrients than the same tissue at rest, and the heart, along with the blood vessels, actively adapts to the emerging needs. During periods of intense muscle activity, the heart can pump 7 to 8 times more blood than normal, increasing the number of beats per minute and the volume of blood pumped out with each beat. Usually the heart pumps out about 75 ml of blood with each beat, but this value can increase to 200 ml.

The following stimuli can cause an increase in stroke volume:

1. Increased carbon dioxide content in the blood. During physical work, the formation of energy in a form convenient for use by organisms (macroergic phosphate bonds) increases; More carbon dioxide is formed in the tissues, and its increased entry into the blood prompts the heart to increase stroke volume.

2. Stretching of the heart muscle. During work, the pressure in the veins is higher and more blood has time to enter the chambers of the heart before they contract, which leads to stretching of their muscular walls. The force of muscle contraction increases within certain limits under the influence of the tensile force acting on the muscle at the beginning of its contraction; therefore, the greater the volume of blood in the heart at the beginning of systole, the more blood it will emit with each contraction. During physical activity, it is also possible for the heart rate to increase to 170-200 per minute.

Several factors may play a role here:

1. Fever. During muscle work, heat is generated sufficient to raise body temperature by several degrees. This affects the sinus node (in the same way as a fever) and the heart beats faster.

2. Hormones. The heart rhythm accelerates under the influence of both adrenaline, which under critical circumstances is produced by the adrenal glands in increased quantities, and thyroxine, secreted by the thyroid gland and accelerating metabolism throughout the body.

3. Nerves. Nervous regulation of heart rate is carried out by the “heart rate center” located in the medulla oblongata. From this center two groups of motor nerves go to the heart; one of them, passing through the sympathetic nerve trunk, speeds up the heart rate, and the other, passing through the vagus nerve, slows it down. Both groups of nerve fibers end in the sinus node and reduce or increase the frequency of impulses occurring in it.

This complex regulatory system quickly brings the heart rate into line with the body's metabolic needs and at the same time prevents overreaction, since the accelerated heart rate stimulates the stretch receptors of the aorta, which in turn slows the heart rate.