Lung volume and vital capacity of the lungs are static characteristics measured in one respiratory cycle. But oxygen consumption and the formation of carbon dioxide occur in the body continuously.
Therefore, the constancy of the gas composition of arterial blood depends not on the characteristics of one respiratory cycle but on the rate of oxygen supply and removal of carbon dioxide over a long period of time. To a certain extent, the minute volume of breathing, or pulmonary ventilation, i.e. volume of air passing through the lungs, in 1 minute. The minute volume of breathing with uniform automatic (without consciousness) breathing is equal to the product of the respiratory volume by the number of respiratory cycles in 1 minute.
At rest, it is equal to an average of 8000 ml or 8 liters per 1 minute). It is believed that the minute volume of breathing provides information on ventilation of the lungs but in no way determines the effectiveness of breathing. With a lung volume of 500 ml, 150 ml of air in the airways first enters the alveoli during inspiration. When inhaling 500 ml of “fresh” air from the atmosphere, 350 ml of them enter the alveoli. The last 150 ml of inhaled “fresh” air fill the anatomical dead space and do not participate in gas exchange with blood. With a decrease in lung volume to 400 ml to maintain the previous value of the minute volume of breath, the respiratory rate should increase to 20 breaths per 1 minute. In this case, alveolar ventilation will be 5000 ml instead of 5600 ml, which are necessary to maintain the constancy of the arterial blood gas composition. To maintain gas arterial blood homeostasis, it is necessary to increase the respiratory rate to 22-23 breaths in 1 minute. This implies an increase in the minute volume of breathing to 8960 ml. With a tidal volume of 300 ml to maintain alveolar ventilation and, accordingly, gas blood homeostasis, the respiratory rate should increase to 37 breaths per minute. In this case, the minute volume of breathing will be 11100 ml, i.e. will increase by almost 1.5 times. Thus, the size of the minute volume of breathing per se does not yet determine the effectiveness of respiration.
A person can take control of the breath on him- or herself and, if he or she wishes, breathe with the stomach or chest, change the frequencies and the depth of breathing, the duration of inspiration and expiration, etc. However, no matter how the person changes his or her breathing, in a state of physical rest, the amount of atmospheric air entering the alveoli in 1 minute should remain approximately the same, namely, 5600 ml to ensure normal blood gas composition, cell needs and tissues in oxygen and in removing excess carbon dioxide. When deviating from this value in either direction, the gas composition of arterial blood changes. Immediately, the homeostatic mechanisms of its maintenance are triggered. They contradict the consciously formed overpriced or underestimated amount of alveolar ventilation. In this case, the feeling of comfortable breathing disappears, or there is either a feeling of lack of air, or a feeling of muscle tension. Thus, to maintain the normal gas composition of the blood while deepening breathing, i.e. with an increase in the lung volume, it is only possible to decrease the frequency of the respiratory cycles, and, conversely, with an increase in the respiratory rate, preservation of gas homeostasis is possible only with a simultaneous decrease in the tidal volume.
In addition to the minute volume of breathing, there is also the concept of maximum lung ventilation – the amount of air that can pass through the lungs in 1 minute with maximum ventilation. In an untrained adult male, maximum ventilation of the lungs during exercise can exceed 5 times the minute volume of breathing at rest. In trained people, the maximum ventilation of the lungs can reach 120 liters, i.e. minute volume of breathing can increase 15 times. With maximum ventilation, the ratio of tidal volume and respiratory rate is also essential. With the same value of maximum lung ventilation, alveolar ventilation will be higher with a lower respiratory rate and, accordingly, with a larger lung volume. As a result, more oxygen can enter the arterial blood at the same time and more carbon dioxide can escape from it.