Lets learn about gas exchange.
Oxygen enters our lungs when we inhale.
Carbon dioxide exits our lungs when we exhale.
Oxygen travels from the lungs to the blood and is carried to the tissues.
The tissues use oxygen for making energy.
This process produces carbon dioxide.
The carbon dioxide moves into the blood and is carried to the lungs to be eliminated when
we exhale.
Before we can learn more details about gas exchange we need to learn about Daltons law
of partial pressures.
Daltons law says that the total pressure exerted by a mixture of gases is equal to the sum
of the partial pressures of each gas in the mixture.
We can use a balloon to illustrate Daltons law.
The balloon is filled with air.
Air is a mixture of gases including about 79% nitrogen, 21 % oxygen and .04% carbon
dioxide along with trace amounts of other gases.
Lets say the total pressure inside of the balloon equals atmospheric pressure which
is about 760 mm Hg.
If air is about 79% nitrogen then the pressure exerted by nitrogen in the balloon is .786
times 760 which comes out to 596 mm Hg.
In other words the PN2 is 597 mm Hg.
Likewise if air is about 21 % oxygen then the partial pressure of oxygen is .209 x 760
or 159 mm Hg.
We say that the partial pressure of oxygen, or PO2 is 159 mm Hg.
It is important to know that gas moves from an area of higher partial pressure to an area
of lower partial pressure.
Lets see how this works in the respiratory and blood systems.
The PO2 inside of the alveolus of the lung is about 104 mm Hg.
The PCO2 is about 40 mm Hg.
The PO2 inside of the capillaries carrying deoxygenated blood surrounding the alveolus
is about 40 mm Hg.
Oxygen diffuses from the higher PO2 of 104 to the lower PO2 of 40.
So oxygen moves from the alveolus to the blood.
The movement of oxygen into the blood increases the PO2 of the blood to 104 mm Hg.
Oxygen is then carried to the tissues.
Along the way the oxygenated blood mixes with some deoxygenated blood causing the PO2 to
drop to about 95 mm Hg.
The tissue PO2 is about 40 mm Hg.
When the oxygenated blood PO2 of 95 meets the tissue PO2 of 40 the partial pressure
gradient allows for the movement of oxygen into the tissue by diffusion.
Oxygen travels from higher to lower partial pressure.
Since oxygen leaves the blood the PO2 drops to 40 mm Hg.
The tissues produce CO2 and the tissue PCO2 is 45 mm Hg.
. The PCO2 of oxygenated blood entering the
tissues is at 40 mm Hg which is lower than the tissue PCO2 of 45, carbon dioxide follows
its partial pressure gradient and diffuses into the blood.
This causes the blood PCO2 to rise to 45 mm Hg.
Carbon dioxide is carried to the lungs.
When carbon dioxide reaches the lungs it meets an alveolar PCO2 of 40 mm Hg.
Carbon dioxide follows its partial pressure gradient and diffuses into the alveoli.
This causes the blood PCO2 to drop back to 40 mm Hg.
Here is an overview of the entire process.
Oxygen enters the lungs�the alveolar PO2 = 104 and the deoxygenated blood PO2 = 40.
Oxygen diffuses into the blood raising the blood PO2 to 104.
The oxygenated blood PO2 drops to 95 and oxygen diffuses into the tissues that have a PO2
of 40.
This causes the deoxygenated blood PO2 to drop to 40.
Carbon dioxide produced by the tissues raises the tissue PCO2 to 45.
The PCO2 of oxygenated blood entering the tissues is 40.
This causes carbon dioxide to diffuse into the blood raising the blood POC2 in the deoxygenated
blood to 45.
Carbon dioxide travels to the lungs and diffuses into the alveoli which have a PCO2 of 40 and
carbon dioxide is expelled from the lungs during exhalation.
We hope you have learned something about gas exchange and see you next time.