AssessmentsGas exchange in the lungs, blood and tissues
Gas exchange in the lungs, blood and tissues
Content Reviewers:Rishi Desai, MD, MPH
It describes both the bulk flow of air into and out of the lungs and the transfer of oxygen and carbon dioxide into the bloodstream through diffusion.
So, if we were to draw a path for the oxygen molecules entering the body, it would start from the nose or mouth and end up in the lungs, where it reaches the alveoli which are wrapped in an intricate network of tiny blood vessels called pulmonary capillaries.
Now, the important role in this process belongs to the alveolo–capillary membrane where the layer of alveolar cells lining the alveoli meets the endothelial cells that make up the pulmonary capillary, and is where gas exchange happens.
With that in mind, let’s just say that when it comes to the surface area of the alveolo-capillary membrane, bigger is better because a respiratory membrane with a large surface area has more gas to diffuse across it in a given period of time leading to a more efficient gas exchange.
If there’s less surface area for gas exchange to occur, the rate of diffusion decreases.
So, in healthy lungs, respiratory membrane is 0.5–1 micrometer thick.
Fick’s law states that the net rate of diffusion - V of any particular gas across the alveolar-capillary membrane, is proportional to the driving force, which is the difference between the partial pressure of the gas in the alveolar sacs, or PA, and the partial pressure of the gas in the blood, or Pa, and also proportional to the surface area of the membrane, or A, but inversely proportional to the wall’s thickness - T.
Specifically, the driving force for diffusion is the partial pressure difference of the gas across the membrane, and NOT the concentration difference.
So, the diffusion of oxygen and carbon dioxide are driven across the respiratory membrane by their partial pressure gradients.
Therefore, if the oxygen partial pressure in alveolar air is 100mm Hg and the one of mixed venous blood entering the pulmonary capillary is 40mm Hg, then we have a driving force for oxygen across the alveolar-capillary barrier of 60mm Hg.
Basically, a steep oxygen partial pressure gradient occurs through the alveolo-capillary membrane because the partial pressure of oxygen in the alveolar air is greater than the partial pressure of oxygen in the pulmonary arteries, causing oxygen to rapidly cross the respiratory membrane from the alveoli into the blood.
However, the partial pressure difference is less than that of oxygen, about 5 mm Hg.
This brings us to what is called Dalton’s law, which states that the sum of partial pressures of all the gases in a mixture equals the total pressure of that mixture.
Thus, for dry gas, the partial pressure is the total pressure multiplied by the fractional concentration of dry gas, while the relationship for humidified gas is determined by correcting the barometric pressure for the water vapor pressure.
For dry gas -> Px=Pb x F For humidified gas -> Px = (Pb - PH2O) x F, where Px is the partial pressure of a gas, Pb is the barometric pressure, PH2O is the water vapor pressure at 37°C/98.6°F having a value of 47mmHg.
F is the fractional concentration of a gas.
To exemplify this, in dry inspired air, the PO2 is approximately 160 mm Hg, which is computed by multiplying the barometric pressure of oxygen, which is 760mmHg, by the fractional concentration of O2, 21% (760 mm Hg x 0.21 = 160 mm Hg).
For practical purposes, there is no CO2 in dry inspired air and PCO2 is zero.
In humidified tracheal air, it is assumed that the air becomes fully saturated with water vapor.
At 37°C, PH2O is 47 mm Hg.
Thus in comparison to dry inspired air, humidified tracheal air has a lower PO2 because the O2 is “diluted” by water vapor.
Again, recall that partial pressures in humidified air are calculated by correcting the barometric pressure for water vapor pressure, then multiplying by the fractional concentration of the gas.
Thus the PO2 of humidified tracheal air is 150 mm Hg ([760 mm Hg − 47 mm Hg] × 0.21 = 150 mm Hg).
PAO2 is 100 mm Hg, which is less than the PO2 in inspired air, which is 160mm Hg, and PACO2 is 40 mm Hg, which is greater than the PCO2 in inspired air, which is close to 0mm Hg.
So, according to Dalton’s law, these partial pressure values influence the moving of these gases, meaning they will move from an area of high concentration to an area of low concentration.
This movement is governed by Henry's Law which states that at a constant temperature, the amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
Basically, according to this law, gases can be forced to dissolve into a liquid, let’s say blood, if there is enough pressure applied and a controlled volume.
It also says that once that pressure is released, gases can come out of solution.
Thus for blood:
Cx = Px X Solubility, where Cx is the concentration of dissolved gas(mL gas/100mL blood), Px is the partial pressure of gas (mm Hg) and Solubility refers to the solubility of gas in blood (mL gas/100mL blood per mm Hg)
So, to test this relationship, let’s consider the PO2 of arterial blood is 100 mm Hg and given that the solubility of O2 is 0.003 mL O2/100 mL blood per mm Hg, we should be able to know what is the concentration of dissolved O2 in blood, right?
Well, yes, because O2 = PO2 X solubility, so the concentration of dissolved O2 is 0.3 mL/100mL blood.
Boyle’s Law states that for a fixed amount of a gas kept at a fixed temperature, pressure and volume are inversely proportional.
The primary purpose of gas exchange is to get rid of carbon dioxide and take up oxygen. Gas exchange takes place between blood and alveoli in the lungs, and then between blood and tissue cells all around the body through simple diffusion. Gasses cross the membranes at the alveolar-capillary membrane in the lungs, where oxygen enters and carbon dioxide exits the bloodstream. Oxygen then travels through the bloodstream to all body parts to be used in cellular respiration, where it is exchanged for carbon dioxide that's transported back into the lungs and then exhaled.