With short-term stress, for example, when we try to catch a bus, the body is able to quickly provide increased energy intake to the muscles. This is possible due to the availability of oxygen reserves, as well as through anaerobic reactions (energy production in the absence of oxygen). The need for energy increases significantly with prolonged physical activity. Muscles require more oxygen to provide aerobic reactions (energy production involving oxygen). Daily standards of physical activity: what are they?
Cardiac activity
The heart of a person at rest is reduced at a frequency of approximately 70-80 beats per minute. With physical activity, the frequency (up to 160 beats per minute) and the power of the heartbeats increase. At the same time cardiac ejection in a healthy person can increase more than fourfold, and for trained athletes - almost six times.
Vascular activity
At rest, blood is pumped by the heart at a rate of about 5 liters per minute. With physical activity, the speed rises to 25-30 liters per minute. The increase in blood flow is mainly observed in working muscles, which are most needed in it. This is achieved by lowering the blood supply of those areas that are less active at that time, and by expanding the blood vessels, which provides a greater flow of blood to the muscles that work.
Respiratory activity
Circulating blood should be sufficiently oxygenated (oxygenated), so the respiratory rate also increases. In this case, the lungs are better filled with oxygen, which then penetrates into the blood. With physical exertion, the rate of air intake into the lungs increases to 100 liters per minute. This is much more than at rest (6 liters per minute).
• The amount of cardiac output in a marathon runner can be 40% more than for an untrained person. Regular training increases the size of the heart and the volume of its cavities. During physical activity, the heart rate (the number of strokes per minute) and cardiac output (the volume of blood ejected by the heart in 1 minute) increase. This is due to increased nervous stimulation, which causes the heart to work hard.
Increased venous return
The volume of blood returning to the heart is enhanced by:
• reduction of vascular resistance in the muscle thickness due to vasodilation;
• Numerous studies have been carried out to study changes in the circulatory system during exercise. It was proved that they are directly proportional to the intensity of physical activity.
- work of muscles (their reduction and relaxation);
• movements of the chest with rapid breathing, which cause a "suction" effect;
• narrowing of the veins, which accelerates the movement of blood back to the heart. When the ventricles of the heart are filled with blood, its walls stretch and contract with greater force. Thus, the heart ejects an increased volume of blood.
During training, the flow of blood to the muscles increases. This ensures timely delivery of oxygen and other necessary nutrients to them. Even before the muscles begin to contract, the blood flow in them is enhanced by the signals coming from the brain.
Vascular expansion
Nervous impulses of the sympathetic nervous system cause dilatation (expansion) of the vessels in the muscle, allowing a larger volume of blood to flow to the muscle cells. However, in order to maintain the vessels in the dilated state after primary dilatation, local changes in tissues follow - a decrease in the level of oxygen, an increase in the level of carbon dioxide and other metabolic products accumulated as a result of biochemical processes in the muscle tissue. Local increase in temperature caused by additional heat production with muscle contraction also contributes to vasodilation.
Vascular narrowing
In addition to changes directly in the muscles, the blood filling of other tissues and organs decreases, which less need for increased energy intake during physical activity. In these areas, for example, in the intestine, narrowing of the blood vessels is observed. This leads to a redistribution of blood in those areas where it is most needed, providing increased blood supply to the muscles in the next cycle of blood circulation. With physical activity, the body consumes much more oxygen than at rest. Consequently, the respiratory system must respond to the increased need for oxygen by increasing ventilation. The frequency of breathing during training increases rapidly, but the exact mechanism of such a reaction is unknown. Increase in oxygen consumption and production of carbon dioxide causes irritation of receptors that detect changes in the gas composition of blood, which in turn leads to stimulation of respiration. However, the body's reaction to physical stress is observed much earlier than changes in the chemical composition of the blood will be recorded. This indicates that there are established feedback mechanisms that send a signal to the lungs at the beginning of physical exertion, thereby increasing the respiratory rate.
Receptors
Some experts suggest that a slight increase in temperature, which is observed, as soon as the muscles begin to work, provokes more frequent and deep breathing. However, control mechanisms that help us to correlate the characteristics of breathing with the amount of oxygen needed by our muscles are provided by chemical receptors that are located in the brain and large arteries. For thermoregulation with physical activity, the body uses mechanisms similar to those that are launched on a hot day to cool it, namely:
• expansion of skin vessels - to increase heat transfer to the external environment;
• increased sweating - sweat evaporates from the skin surface, which requires the cost of thermal energy;
• Increased ventilation of the lungs - heat is released through the exhalation of warm air.
Consumption of oxygen by the body in athletes can be increased 20 times, and the amount of heat released is almost directly proportional to the consumption of oxygen. If sweating on a hot and humid day is not enough to cool the body, a physical emergency can result in a life-threatening condition called heat stroke. In such conditions, first aid should be as soon as possible artificial lowering of body temperature. The body uses various mechanisms of self-cooling during physical activity. Increased sweating and pulmonary ventilation helps increase heat output.