Gas exchange in the lungs, blood and tissues
Definitions & Key takeaways
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.
Gas exchange is the physical process by which gases move passively, meaning that no energy is required to power the transport, by diffusion across a surface.
External respiration is another term for gas exchange. 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.
Internal respiration, on the other hand, describes the capillary gas exchange in body tissues. While the flow of air from the external environment happens due to pressure changes in the lungs, the mechanisms of alveolar gas exchange are more complex.
The primary three components of gas exchange are the surface area of the alveolo-capillary membrane, the partial pressure gradients of the gasses, and the matching of ventilation and perfusion.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.
So, from the alveoli, the gas molecules will go into the blood in the capillaries. Carbon dioxide follows the same path, but in the opposite direction, moving from the blood in the capillaries to the air in the alveoli and then getting exhaled.
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.
Another aspect related to the alveolo-capillary membrane which influences gas exchange is its thickness. 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.
And this is all times the diffusion coefficient - D, which varies from gas to gas. Therefore,V=(PA-Pa)ADTSpecifically, 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.
The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary. However, the partial pressure difference is less than that of oxygen, about 5 mm Hg.
The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg.Now, the partial pressures of inhaled air and alveolar air determine why oxygen goes into the alveoli, and why carbon dioxide leaves the alveoli.
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.
Therefore: 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). Because there is no CO2 in inspired air, the PCO2 of humidified tracheal air also is zero.
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.
Now, within the lungs, oxygen and carbon dioxide diffuse between the air in the alveoli and the blood, that is between a gas and a liquid.
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. The concentration of a gas in solution is expressed as volume percent (%), or volume of gas per 100 mL of blood (mL gas/100 mL blood).
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.Actually, this law interconnects with Boyle’s law during breathing cycle and gas exchange.
Boyle’s Law states that for a fixed amount of a gas kept at a fixed temperature, pressure and volume are inversely proportional.
Basically, if volume increases, then pressure decreases and the other way around. This law has applicability in the respiratory system during inspiration and expiration.
In inspiration, lung volume increases, which decreases alveolar pressure, creating a negative pressure gradient that allows oxygen to flow into the lungs.
In expiration, the alveolar pressure increases and the lung volume decreases, making the air exit the lungs. Correlating this information with Henry’s law, we can conclude that in inspiration, carbon dioxide will return to its gaseous state because the alveolar pressure is decreased and it can not push it into blood.
Less carbon dioxide in the blood makes a smaller concentration gradient between blood and alveoli space, so it becomes harder for the gas exchange to happen.
On the other hand, during expiration, the volume is decreased and the alveolar pressure is increased. That will force oxygen into blood because pressure pushes it there.
More oxygen into blood creates a larger concentration gradient and oxygen exchange is more efficient. At rest, the time that it takes for the diffusion of carbon dioxide from the pulmonary capillary blood into the alveoli is about the same as that of oxygen because the solubility of carbon dioxide is much greater than that of oxygen—by a factor of about 20—in both blood and alveolar fluids.Alright, so in order to understand Henry’s law better, let’s consider emphysema again, where the alveoli or the air sacs of the lungs are damaged.
This damage usually means that air is trapped there. In terms of volume and pressure, trapped air means a constant high volume, and a constantly low pressure.
As a result, in emphysema, carbon dioxide will easily diffuse out of blood, but oxygen will not be forced into blood and that will lead to hypoxemia or low partial pressure of oxygen.An everyday example of Henry’s law is carbonated soft drinks.
Before the bottle or can is opened, the gas above the drink is almost pure carbon dioxide at a pressure slightly higher than atmospheric pressure.
The drink itself contains dissolved carbon dioxide. When the bottle or can is opened, some of this gas escapes, giving the characteristic hiss.
Because the pressure above the liquid is now lower, some of the dissolved carbon dioxide comes out of solution as bubbles.
If a glass of the drink is left in the open, the concentration of carbon dioxide in solution will come into equilibrium with the carbon dioxide in the air, and the drink will go flat.Now, the last puzzle piece for an optimal external respiration is to have a close match between ventilation aka the amount of gas reaching the alveoli, and perfusion which is the blood flow in pulmonary capillaries, creating a balance.
So when alveoli have high PO2 and low PCO2 the bronchioles serving the alveoli constrict because there isn’t much CO2 that needs to be removed, and the pulmonary capillaries dilate allowing the blood to pick up the O2.
On the other hand, when alveoli have low PO2 and high PCO2 the bronchioles serving the alveoli dilate because the CO2 needs to be removed, and the pulmonary capillaries constrict because there’s relatively little O2 to be picked up and that allows blood to go to other alveoli that may have more O2.
Therefore, PCO2 controls ventilation by changing bronchiolar diameter, and PO2 controls perfusion by changing arteriolar diameter.Now, since we discussed all the factors which influence external respiration, let’s check if we got it right as to what happens during this process.
So, the mixed venous blood returns from the tissues, via the veins, to the right heart. It is then pumped from the right ventricle into the pulmonary artery, which delivers it to the pulmonary capillaries.
The composition of this mixed venous blood at this point reflects metabolic activity of the tissues: the PCO2 level in the blood is greater than the PO2 level, at a value of 45 mm Hg, because the tissues have produced CO2 and added it to venous blood.
The PO2 in the blood is at 40 mm Hg, because the tissues have taken up and consumed O2.On the other hand, the oxygenated blood leaving the pulmonary capillary is delivered to the left heart and becomes systemic arterial blood.
The oxygenation is affected by the exchange of O2 and CO2 between alveolar air and pulmonary capillary blood. Because diffusion of gases across the alveolo-capillary membrane is fast, blood leaving the pulmonary capillaries normally has the same PO2 and PCO2 as alveolar air, meaning there is a complete equilibration.
Hence, PaO2 is 100 mm Hg and PaCO2 is 40 mm Hg, just as PAO2 is 100 mm Hg and PACO2 is 40 mm Hg. This arterialized blood will now be returned to the left heart, pumped out of the left ventricle into the aorta, and begin the cycle again.Now, there is a small discrepancy between alveolar air and systemic arterial blood: Systemic arterial blood has a slightly lower PO2 than alveolar air.
This discrepancy is the result of a physiologic shunt, which allows a small amount of pulmonary blood flow to bypass the alveoli and skip oxygenation.
The physiologic shunt has two sources: bronchial blood flow and a small portion of coronary venous blood that drains directly into the left ventricle rather than going to the lungs to be oxygenated.
When the physiologic shunt in pulmonary blood flow is increased, it is called a ventilation/perfusion defect. If the size of the shunt increases, equilibration between alveolar gas and pulmonary capillary blood cannot adequately occur and pulmonary capillary blood is not fully oxygenated.
What is known as the “A − a” difference expresses the difference in PO2 between alveolar gas (“A”) and systemic arterial blood (“a”).
If the shunt is small, which defines it as physiologic, then the “A − a” difference is small or negligible; if the shunt is larger than normal, then the “A − a” difference increases to the extent that O2 equilibration fails to occur.Now, let’s switch gears to internal respiration, where the partial pressures and diffusion gradients are reversed from the situation described in external respiration.
However, the factors that influence tissue gas exchange are similar to the factors of alveolar gas exchange, and include partial pressure gradients between the blood and tissues, the blood perfusion of those tissues, and the surface areas of those tissues.So, during internal respiration, the partial pressure of oxygen in tissues is low at about 40 mm Hg, because oxygen is continuously used for cellular respiration.
In contrast, the partial pressure of oxygen in the blood is about 100 mm Hg. This creates a pressure gradient that causes oxygen to dissociate from hemoglobin, diffuse out of the blood, cross the interstitial space, and enter the tissue.
Hemoglobin that now has little oxygen bound to it loses much of its brightness, so that blood returning to the heart is more burgundy in color, in contrast to external respiration where dark red deoxygenated blood from the pulmonary circuit becomes bright red oxygenated blood to enter systemic circulation.On the other hand, considering that cellular respiration continuously produces carbon dioxide, the partial pressure of carbon dioxide is lower in the blood than it is in the tissue, causing carbon dioxide to diffuse out of the tissue, cross the interstitial fluid, and enter the blood.
It is then carried back to the lungs either bound to hemoglobin, dissolved in plasma, or in a converted form. By the time blood returns to the heart, the partial pressure of oxygen has returned to about 40 mm Hg, and the partial pressure of carbon dioxide has returned to about 45 mm Hg.
The blood is then pumped back to the lungs to be oxygenated once again during external respiration.##SummaryOk, quick recap: Gas exchange between blood and alveoli, as well as between blood and tissue cells, take place by simple diffusion.
At the respiratory membrane, where the alveolar and capillary walls meet, gases move across the membranes, with oxygen entering the bloodstream and carbon dioxide exiting.
Besides the surface area of the alveolo-capillary membrane, the partial pressure gradients of the gasses and the matching of perfusion and ventilation are vital for the exchange to happen.
cellular respiration continuously produces carbon dioxide The partial pressure of carbon dioxide is lower in the blood than it is in the tissue causing carbon dioxide to diffuse out of the tissue cross the interstitial fluid and enter the blood It is then carried back into the lungs either bound to hemoglobin dissolved in plasma or in a converted form By the time blood returns to the heart the partial pressure of oxygen has returned to about 40 millimeters of mercury And the partial pressure of carbon dioxide has returned to about 45 millimeters of mercury The blood is then pumped back to the lungs to be oxygenated Once again during external respiration ok quick recap gas exchange between blood and alveoli as well as between blood and tissue cells takes place by simple diffusion at the respiratory membrane where the alveolar and capillary walls meet gasses move across the membrane with oxygen entering the bloodstream and carbon dioxide exiting Besides the surface area of the alveolar capillary membrane the partial pressure gradients of the gasses and the matching of perfusion and
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