Lecture five
Composition of alveolar airits relation to atmospheric airAlveolar air does not have the same concentrations of gases as atmospheric air (see Table 1). There are several reasons for this difference: 1- The alveolar air is only partially replaced by atmospheric air with each breath. 2- Oxygen is constantly being absorbed into the pulmonary blood from the alveolar air. 3- Carbon dioxide is constantly diffusing from the pulmonary blood into the alveoli. 4- Dry atmospheric air that enters the respiratory passages is humidified even before it reaches the alveoli.
Partial pressures of respiratory gases as they enter and leave the lungs (at sea level)atmospheric air
(mm Hg) humidified air (mm Hg) alveolar air
(mm Hg) expired air
(mm Hg) N2 597.0 (78.62%) 563.4 (74.09%)569.0 (74.9%)566.0 (74.5%)O2 159.0 (20.84%) 149.3 (19.67%)104.0 (13.6%)120.0 (15.7%)CO20.3 (0.04%) 0.3 (0.04%)40.0 (5.3%)27.0 (3.6%)H2O3.7 (0.50%) 47.0 (6.20%)47.0 (6.2%)47.0 (6.2%)TOTAL760.0 (100.0%)760.0 (100.0%)760.0 (100.0%)760.0 (100.0%)
Table (1)
Humidification of the air in the respiratory passagesAs soon as the atmospheric air enters the respiratory passages, it is exposed to the fluids that cover the respiratory surfaces. Even before the air enters the alveoli, it becomes totally humidified and simply diluted all the other gases in the inspired air by the water vapor .
Rate at which alveolar air is renewed by atmospheric air: with normal alveolar ventilation, about one half of the alveolar gas is removed in 17 seconds. The slow replacement of alveolar air is important in preventing sudden changes in gas concentrations in the blood. This stabilizes the respiratory control mechanism and prevents excessive increases and decreases in tissue oxygenation, carbon dioxide concentration, and pH when respiration is temporarily interrupted.
Oxygen concentration and partial pressure in the alveoli
Oxygen concentration in the alveoli and its partial pressure as well, is controlled by(1) The rate of absorption of oxygen into the blood (-ve relation) and
(2) The rate of entry of new oxygen into the lungs by the ventilatory process (+ve relation).
The normal alveolar PO2 value is 104 mm Hg as oxygen absorption into the blood at a rate of 250 ml/min and normal ventilatory rate of 4.2 L/min. during moderate exercise, The rate of absorption of oxygen into the blood is increases to1000 ml/min and the rate of alveolar ventilation must increase fourfold to maintain the alveolar PO2 at the normal value of 104 mm Hg.
● a very increase in alveolar ventilation can never increase the alveolar PO2 above 149 mm Hg as long as the person is breathing normal atmospheric air at sea level pressure, because this is the maximum PO2 in humidified air at this pressure.
CO2 concentration and partial pressure in the alveoli
Carbon dioxide is formed in the body and carried into the blood to the alveoli; it is continually being removed from the alveoli by ventilation. Normally alveolar PCO2 is 40 mm Hg when rate of carbon dioxide excretion is normal and equal to 200 ml/min and the normal rate of alveolar ventilation of 4.2 L/min.
The alveolar partial pressure of carbon dioxide is affected by:
(1) The alveolar PCO2 increases directly in proportion to the rate of carbon dioxide excretion.
(2) The alveolar PCO2 decreases in inverse proportion to alveolar ventilation.
Expired Air
The first portion of the expired air is the dead space air from the respiratory passageways which is typical humidified air. Then, progressively more and more alveolar air becomes mixed with the dead space air until all the dead space air has finally been washed out and nothing but alveolar air is expired at the end of expiration. The expired air PO2= 120.0 mmHg and the expired air PCO2= 27.0 mmHg which is mixing of both dead space and alveolar air.Diffusion of gases through the respiratory membrane
The respiratory unit (fig-9) is composed of a respiratory bronchiole, alveolar ducts, atria, and alveoli. There are about 300 million alveoli in the two lungs, and each alveolus has an average diameter of about 0.2 mm. The alveolar walls are extremely thin, and between the alveoli is an almost solid network of interconnecting capillaries. Indeed, because of the extensiveness of the capillary plexus, the flow of blood in the alveolar wall has been described as a sheet of flowing blood. The alveolar gases are in very close to the blood of the pulmonary capillaries. Gas exchange not only occurs in the alveoli but in all the terminal portions of the lungs these membranes are collectively known as the respiratory membrane, also called the pulmonary membrane.Respiratory Membrane (fig-10)
Respiratory membrane consists of the following different layers:1. A layer of fluid lining the alveolus and containing surfactant.
2. The alveolar epithelium composed of thin epithelial cells.
3. An epithelial basement membrane.
4. A thin interstitial space between the alveolar epithelium and the capillary membrane.
5. A capillary basement membrane that in many places fuses with the alveolar epithelial basement membrane
6. The capillary endothelial membrane
Despite the large number of layers, the overall thickness of the respiratory membrane in some areas is as little as 0.2 micrometer (fig-11), and it averages about 0.6 micrometer, except where there are cell nuclei. It has been estimated that the total surface area of the respiratory membrane is about 70 m2 in the normal adult human male. This is equivalent to the floor area of a 25by-30foot room. The total quantity of blood in the capillaries of the lungs at any given instant is 60 to 140 ml. Now imagine this small amount of blood spread over the entire surface of a 25by-30foot floor, and it is easy to understand the rapidity of the respiratory exchange of oxygen and carbon dioxide. The average diameter of the pulmonary capillaries is only about 5 micrometers, which means that red blood cells must squeeze through them. The red blood cell membrane usually touches the capillary wall, so that oxygen and carbon dioxide need not pass through significant amounts of plasma as they diffuse between the alveolus and the red cell. This, too, increases the rapidity of diffusion.
Factors that affect the rate of gas diffusion through the respiratory membrane
The factors that determine gas diffusion through the membrane are:(1) The thickness of the membrane. The rate of diffusion through the membrane is inversely proportional to the thickness of the membrane (edema ( respiratory membrane thickness).
(2) The surface area of the membrane. The rate of diffusion through the membrane is decreased with decrease surface area (Removal of an entire lung, emphysema (the total surface area).
(3) The diffusion coefficient for transfer of each gas through the respiratory membrane depends on the gass solubility in the membrane and, inversely, on the square root of the gass molecular weight. The rate of diffusion in the respiratory membrane is almost exactly the same as that in water. Therefore, for a given pressure difference, carbon dioxide diffuses about 20 times as rapidly as oxygen. Oxygen diffuses about twice as rapidly as nitrogen.
(4) The partial pressure difference of the gas between the two sides of the membrane (from high to low partial pressure).
Fig-9 Respiratory unit
Fig-11 A, Surface view of capillaries in an alveolar wall.
B, Cross-sectional view of alveolar walls and their vascular supply