Gas exchange occurs across the membrane of the alveoli. This membrane is a composite of three layers, the squamous epithethial lining, the endothelial cells lining adjacent capillary beds and fused basal laminae between the alveolar and the endothelium. Within the membrane is the alveolar space. The cells of the membrane are designed to encourage gas exchange within the capillaries. The diffusion is rapid.
Efficiency of Gas Exchange
The efficiency factors of gas exchange include:
- The differences in partial pressure across the membrane are substantial (PO2 and PCO2).
- The distance for exchange travel is practically nothing across the thin membrane. The basal laminae reduces the distance to about 0.5 μm.
- The gases being exchanged are lipid soluble which is great as the surfactant on the membrane is a key factor in efficient alveolar function.
- The total area of the alveoli is huge, and in fact more than all the rest of the body’s surfaces combined. The controlled pulmonary ventilation and circulation allows the maximum speed of exchange to occur when the pressure levels of oxygen are high in the alveolar sac.
The temperature at the exchange point, the solubility of the gases into the bloodstream and the partial pressure of each gas all have an effect on the amount of gas (or oxygenation) that enters the oxygen poor blood arriving from the tissues. Scientific laws affect the direction and rate of diffusion across the respiratory membrane that separates the air within the alveoli from the blood in alveolar capillaries
Laws of Gas Exchange
The following laws are given to explain how partial pressure, diffusion and direction all play into the oxygenation of the blood:
Daltons’ law of partial pressure determines the rate (how much) of a gas will diffuse across the alveolar membrane. The symbol for this partial pressure is “P.” Boyle’s law determines the pressure gradient and the direction it will flow (toward lower concentrations). Henry law explains that equilibrium between the blood and the alveolar air is the goal (diffusion). In general, blood arrives at the alveoli with a PO2 of about 40 mmHg and a PCO2 of roughly 45 mmHg. Blood departs the alveoli with a PO2 of about 100 mmHg and a PCO2 of roughly 40 mmHg.
Of the above laws, the most important is Dalton's law, the difference in partial pressure of these gases across the membrane. This law is one of the two that determines the rate (how much of) a gas will diffuse across the alveolar membrane (the other is Henry’s law).
Dalton’s law explains as follows:
The air we breathe is a combination of gases. In the atmosphere, the different gases will bounce and collide as gases do. Because the mixture of gases is known, we have Dalton’s law which states that at any given time, the percentage of each separate gas colliding around in the air we breathe makes its contribution, in proportion to its abundance, to total atmospheric pressure.
Partial Pressure
Partial pressure builds on Dalton’s law and can be described as the pressure contributed by a single gas in a mixture of gases. Naturally, all the partial pressures added together equal the total pressure exerted by a gas mixture.
For the air we breathe:
Atmospheric pressure = 760 mmHg (mmHg means millimeters of mercury)
Inhaled air carries different percentages of:
- nitrogen (N2) – 78.6% = 597 mmHg
- oxygen (O2) – 20.9% = 159 mmHg
- carbon dioxide (CO2) – 0.04% = 0.3 mmHg
- water (H2O) – 0.5% = 3.7 mmHg
For the air that reaches the alveoli:
Alveolar air carries different percentages of:
- N2 75.4% = 573 mmHg
- O2 – 13.2% = 100 mmHg
- CO2 – 5.2% = 40 mmHg
- water – 6.2% = 47 mmHg
PN2 + PO2 + PH2O + PCO2 = 760 mmHg
The importance is in the percentage of each which we breathe and the percentage of each at the alveolar sac as a percentage of pressure (760 mmHg). So, for example, the partial pressure of O2 is 20.9% in the atmosphere and 13.2% in the alveolar sac, and so on.
Conclusion
The distribution of gases in the alveolar sac is sufficient to oxygenate blood. In gas exchange, the partial pressures of gases in the blood coming from the body arrive in different proportions from those of the alveolar air (the blood being 40 mmHg of O2 and 45 mmHg of CO2). As a result, diffusion toward equilibrium of the gases in the alveolar space and the tissue capillaries occurs (Henry’s law). Thus the blood leaves the exchange point with different percentages than when it arrived. The blood carries more oxygen and less carbon dioxide after the exchange because the concentration gradients of these two gases, created by the differences in partial pressure between the blood and the alveolar air, force exchange which oxygenates the blood.
Sources
- Martini, F. and Nath, J. L.(2009) Fundamentals of anatomy & physiology (8th ed.). San Francisco, CA: Pearson Education, Inc.
