Composition of alveolar gases: difference between alveoli and atmosphere

2021-05-17 12:20 AM

As soon as atmospheric air enters the respiratory tract, it is exposed to the fluids covering the respiratory surface. Even before the air enters the alveoli, it becomes almost completely moist.

After the alveoli are ventilated, the next step in respiration is the exchange (diffusion) of O2 from the alveoli into the pulmonary blood and the diffusion of CO2 in the reverse direction. from the blood to the alveoli. Diffusion is simply the random back-and-forth diffusion of gas molecules across the respiratory membrane and adjacent fluids. However, in respiratory physiology, we are concerned not only with the basic mechanism of diffusion but also with the rate at which it occurs, which requires a great understanding of the physics of diffusion and gas exchange.

Alveolar air does not have the same concentration of gases as atmospheric air.

They have distinct differences:

First, alveolar air is only partially replaced by atmospheric air with each breath.

Second, O2 is continuously absorbed into the pulmonary blood from the alveolar air.

Third, CO2 continuously diffuses from the pulmonary blood into the alveoli.

Fourth, dry atmospheric air entering the respiratory tract is humidified even before it reaches the alveoli.

Humidification of the air in the respiratory tract

The atmosphere is almost N2 and O2, it usually contains a small amount of water vapor and almost no CO2. However, as soon as atmospheric air enters the respiratory tract, it is exposed to the fluids covering the respiratory surface. Even before the air enters the alveoli, it becomes almost completely moist.

The normal partial pressure of water vapour at a body temperature of 37°C is 47 mm Hg, hence the partial pressure of water vapour in the air alveoli. Because the pressure in the alveoli cannot rise above atmospheric pressure (760mm Hg). This steam simply dilutes all other gases in the inhaled air.

Board. The partial pressure of respiratory gases (in mmHg) as they enter and leave the lungs (at sea level)

Alveolar gas is slowly being renewed by gases in the atmosphere

FRC (functional residual capacity) of men has an average value of 2.3 litres. However, only 350ml of new air is brought into the alveoli when we inhale normally, so there is also 350ml of alveolar gas that goes out when we exhale.

So the volume of alveolar gas replaced with atmospheric gas with each breath is only 1/7 of that of FRC.

The figure shows a slow renewal of alveolar gas, and after 16 breaths excess air has not been completely removed from the alveoli.

Figure. Air exhaled from the alveoli with successive breaths

 

Figure. The rate at which excess gas is removed from the alveoli

The figure shows the rate at which alveolar air is removed, showing half of the air removed in the first 17 seconds with pulmonary ventilation in a normal person, when the lung ventilation rate in person is half that of a normal person it takes 34 seconds to remove ½ of the alveolar gas, when the lung ventilation rate in a person is 2 times that of a normal person, it takes 8 seconds.

The importance of slow replacement of alveolar gas

The slow change in alveolar gas is important to prevent sudden changes in blood gas concentrations and also to make breathing more stable, preventing excessive increases or decreases in oxygen in the lungs. tissue, tissue CO2 concentration, tissue pH when respiration is interrupted.

O2 concentration and partial pressure in alveoli

O2 is continuously absorbed from the alveoli into the pulmonary blood through inhalation. Therefore, the concentration of O2, as well as its partial pressure, depends on:

(1) the rate at which O2 is absorbed into the blood.

(2) the rate of penetration of new O2 into the lungs by ventilation.

Figure. Effect of alveolar ventilation on the partial pressure of oxygen (PO2) in the alveoli at two rates of oxygen uptake from the alveoli - 250 ml/min and 1000 ml/min. Point A is the normal operating point.

The figure shows the effect of pulmonary ventilation and the rate of O2 absorption into alveolar blood and alveolar partial pressure of O2 (PO2). With 1 curve representing O2 absorption at the rate of 250ml O2/min (solid line) and 1000ml O2/min (dashed line). When ventilation is normal at 4.2 l/min and with O2 uptake rate at 250 mL/min, point A is the normal operating point with PO2 at point A being 104 mmHg but when O2 uptake rate is 1000 mL. O2/min, it is necessary to increase the rate of ventilation in the lungs to 4 times to maintain PO2 to normal. When a person breathes in a normal atmosphere at sea level pressure, the PO2 never exceeds 149 mmHg.

CO concentration and partial pressure in alveoli

CO2 is continuously formed in the body and then into the venous blood and carried to the alveoli, and then it is further pushed out of the alveoli through respiration.

 

Figure. Effect of alveolar ventilation on the alveolar partial pressure of carbon dioxide (PCO2) at two rates of carbon dioxide excretion from the blood - 800 ml/min and 200 ml/min. Point A is the normal operating point.

The figure shows the relationship between the alveolar partial pressure of CO2 (PCO2) and the pulmonary ventilation of CO2 with two CO2 excretion rates of 200ml CO2/min and 800ml. CO2/min. From the figure above, we see that at point A, where the respiratory activity is normal, with a CO2 excretion rate of 200ml CO2/min, the lung ventilation rate is 4.2l and PCO2 = 40mmHg.

First: PCO2 in the alveoli will increase directly proportional to the rate of CO2 excretion.

Second: The amount of CO2 in the alveoli will be inversely proportional to the ventilation rate in the lungs, ie, when the lung ventilation increases, CO2 will come out more and at this time, the alveolar CO2 will be less.

Exhaled air is a mixture of dead space and alveolar gas

Dead space is the volume of air that is inhaled without participating in gas exchange.

The components of exhaled air are determined by:

(1) the gas volume of dead space.

(2) the volume of gas exchanged in the alveoli.

Figure. The partial pressures of oxygen and carbon dioxide (PO2 and PCO2) in different parts of the normally expired air.

The figure shows the process of changing the partial pressures of CO2 and O2 during normal expiration. First, in this air is dead space, which is a typical humid gas. Then gradually the alveolar air mixes with the dead space air, and now the exhaled air is alveolar gases.

Thus, the normal exhaled air consists of alveolar gas and dead space gas, and at this time the gas concentration and partial pressure of each gas.