People's breathing process includes three interrelated links: external breathing, including lung ventilation and lung ventilation; Transport of gas in blood; Internal respiration refers to the gas exchange between tissues and cells and blood.

When normal adults are quiet, it is best to breathe for 6.4 seconds at a time, and the amount of gas inhaled and exhaled each time is about 500 ml, which is called tidal volume. When people inhale hard until they can't breathe any more; Then exhale hard until you can't breathe any more. At this time, the amount of exhaled gas is called vital capacity. Normal adult male lung capacity is about 3500-4000 ml, and female lung capacity is about 2500-3500 ml.

Gas exchange in alveoli is carried out through alveoli and capillary walls. Gas exchange in tissues is also carried out through capillary walls. How are gases exchanged? Gas molecules, whether gaseous or dissolved in body fluids, are constantly moving and diffuse. A gas always diffuses from more places to less places, that is, it always diffuses from places with high concentration to places with low concentration until it reaches equilibrium. The concentration of gas is related to pressure, with high concentration and high pressure; Low concentration and low pressure. So it can also be said that gas diffuses from a place with high pressure to a place with low pressure. The gas exchange between alveoli and tissues is realized by this diffusion.

Air consists of oxygen, carbon dioxide, nitrogen and so on. All kinds of gases have a certain pressure. The total pressure of all kinds of gases in the air is atmospheric pressure, and the pressure of each gas is the partial pressure of the gas. Because gas always diffuses from a place with high pressure to a place with low pressure, a certain gas molecule always diffuses from a place with high partial pressure to a place with low partial pressure.

Specifically, when venous blood flows through pulmonary capillaries, because the partial pressure of oxygen in alveolar gas is higher than that in venous blood, and the partial pressure of carbon dioxide in venous blood is lower than that in venous blood, oxygen diffuses from alveoli to venous blood, while carbon dioxide diffuses from venous blood to alveoli. This is gas exchange in alveoli (Figure 1 1). After gas exchange, venous blood becomes arterial blood. Because the outside air continuously enters the lungs, the composition of alveolar gas remains relatively constant, so the partial pressure of oxygen in alveoli is always higher than that of venous blood, and the partial pressure of carbon dioxide is always lower than that of venous blood, so oxygen always diffuses from alveoli to blood, and carbon dioxide always diffuses from venous blood to alveoli. When arterial blood flows through the tissue, the partial pressure of oxygen in the tissue is lower than that in arterial blood, while the partial pressure of carbon dioxide in arterial blood is higher. So oxygen diffuses from arterial blood to tissue, and carbon dioxide diffuses from tissue to blood. This is gas exchange within the organization. After gas exchange, arterial blood becomes venous blood. Because tissue cells constantly consume oxygen and produce carbon dioxide during metabolism, the partial pressure of oxygen in tissue is always lower than arterial blood, while the partial pressure of carbon dioxide is always higher than arterial blood, so oxygen always diffuses from arterial blood to tissue, and carbon dioxide always diffuses from tissue to blood.

From the gas exchange process, we can know that the gas partial pressure difference is the driving force of gas exchange.

The volume of lung gas varies with the depth of breathing. When a normal person breathes calmly, the amount of gas inhaled or exhaled each time is about 500 mL, which is called tidal volume (figure 12). If you continue to inhale hard after inhaling calmly until you can't inhale any more, the increased inspiratory volume is called supplementary inspiratory volume, which is about 1 500 mL. If you continue to exhale forcefully after breathing calmly until you can't breathe any more, the extra expiratory volume, called expiratory volume, is about 1 500 mL. The sum of tidal volume, supplementary inspiratory volume and supplementary expiratory volume, that is, the amount of gas that can be exhaled after trying to inhale and exhale, is called vital capacity, which is about 3 500 mL. The vital capacity is not the maximum amount of gas that the lungs can hold, because even if we try our best to exhale gas, some gas remains in the lungs, which is called residual gas, about 1 500 mL. The sum of vital capacity and residual volume is called the total capacity of the lung, that is, the total capacity of gas in the lung when the maximum inspiratory volume is completed. Not only conjoined lungs, there is always a certain amount of gas in their alveoli. Even if the lungs collapse completely in vitro, there is still a small amount of gas in their alveoli, which will never be exhausted. This is because, after birth, the gas will remain in the lungs for the first time, and only a part of the gas can be updated in the lungs every time you breathe.

The total amount of gas inhaled or exhaled by the lungs per minute is called the minute ventilation of the lungs. The minute ventilation of the lung is equal to the product of tidal volume and respiratory rate. The tidal volume of an adult in a quiet state is 500 mL, and he breathes 16 ~ 18 times per minute, so the ventilation volume of an adult in a quiet state is 8 ~ 9 L. When engaged in strenuous exercise and manual labor, the tidal volume and breathing frequency will increase greatly. The maximum ventilation per minute can reach 100 ~ 1 10 L for men and 80 L for women. The maximum ventilation volume reflects the maximum ventilation function of the lungs and the outside world per unit time.

Not every breath of inhaled gas can enter the alveoli, and some gas stays in the nasal cavity, pharynx, larynx, trachea and bronchus. And cannot exchange gas with blood. The volume of this gas is generally about 150 ml. So when breathing quietly, the amount of gas inhaled into alveoli is only 350ml (500ml-150ml = 350ml). This is the primary ventilation of alveoli. The minute ventilation of alveoli is directly affected by respiratory frequency and tidal volume, among which tidal volume has a greater influence. For example, when the tidal volume is 500 mL and the respiratory rate is 16 times per minute, compared with the tidal volume is 250 mL and the respiratory rate is 32 times per minute, the alveolar ventilation per minute of the former is equal to (500 mL- 150 mL)× 16 times, that is, 5.6l；; ; The latter's alveolar ventilation per minute is equal to (250 mL- 150 mL)×32 times, that is, 3.2 L. The above examples show that from the perspective of gas exchange efficiency, deep and slow breathing is more effective than shallow and fast breathing. During exercise, high breathing frequency and shallow breathing depth table are often one of the important factors of insufficient oxygen supply.