Gas exchange Flashcards
Henrys law
The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid
Fick’s Law of Diffusion
The rate of gas transfer (V gas) is directly proportional to the tissue area, the diffusion coefficient of the gas, and the difference in the partial pressure of the gas on the two sides of the tissue, and inversely proportional to the thickness of the membrane.
PiO2
Partial pressure of inspired O2 ~ 150mmHg
PbO2
The partial pressure of oxygen in the atmosphere 760mmHg
FiO2
Fraction (percentage) of inspired oxygen from the air at sea level ~ 21% = PiO2
PAO2
Partial pressure of oxygen in the surrounding capillaries/the arterial blood ~ 100-105 mmHg
Why does the PiO2 change from 160 to 150mmHg by the time it reaches the alveoli?
Due to the air being humidified and moisturized. You must subtract the pressure that the water vapor exerts (PH20) from the PbO2.
What is the equation to find the final PiO2?
PbO2 - PH2O {(760mmHg - 47mmHg) x .21}
Equation for PAO2
PiO2 - (PaCO2/R) (R=0.8)
A-a gradient
The difference in PO2 between the PAO2 and PaO2
The formula for A-a gradient
PAO2 - PaO2
Normal range of A-a gradient
5 to 10mmHg
Factors that affect the A-a gradient
Anatomic shunts, V/Q mismatch
Formula for PAO2
(Pb - PH2O) x FiO2 - PaCO2/0.8
PaCO2 normal range
35-45 mm Hg
PaO2
Partial pressure of oxygen in the surrounding capillaries/the arterial blood ~ 75-100 mmHg
What is the V to Q pressure difference in Zone 1 of the lungs?
PA>Pa>Pv
What is the V to Q pressure difference in Zone 2 of the lungs?
Pa>PA>Pv
What is the V to Q pressure difference in Zone 3 of the lungs?
Pa>Pv>PA
What occurs in Zone 1 of the lungs?
In Zone 1 alveolar pressure exceeds that of the pulmonary arterial pressure. Consequently, the pulmonary arteries become compressed, this area is being ventilated but not profused. This is known as dead space and in this zone little to no gas exchange can occur.
What occurs in Zone 2 of the lungs?
In Zone 2 Pulmonary arterial pressure exceeds that of Alveolar pressure and Pulmonary venous pressure; however alveolar pressure is still greater than pulmonary venous pressure. The blood flow in this Zone is largely cardiac cycle-dependent.
What occurs in Zone 3 of the lungs?
In Zone 3 Pulmonary arterial pressure is greater than Pulmonary venous pressure and both are greater than Alveolar pressure. Typically located in the bottom portion of the lungs, in this Zone pulmonary blood flows past unventilated alveoli and no oxygenation takes place in a condition know as Shunt.
What are the four factors that affect diffusion according to Fick?
- Membrane surface area.
- The diffusion coefficient of that particular gas for that membrane.
- The difference in pressure between the gases across the membrane.
- The thickness of the membrane.