# Gas Transport in the Airways Flashcards

What is normal partial pressure of arterial O_{2}? (P_{aO2})

90-100 torr (at sea level)

What is partial pressure?

In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture.

For example, P_{barometric} = P_{O2} + P_{N2} + P_{CO2} + etc…

Are gases floating around in blood?

No. Oxygen is not in a gaseous state. It is talked about in terms of partial pressures but it is actually desolved and bound to hemoglobin. When talked about in partial pressures measured in Torr, it’s hypothetical.

Normal P_{aO2} is 90-100 Torr at sea level. What is it in Denver?

80-85 Torr because there is less oxygen in the air.

What is P_{IO2}? How is it calculated? ***must memorize this equation***

Partial pressure of O_{2} in inspired air.

It is calculated by subtracting the partial pressure of water (47 torr) from the barometric pressure and multiplying by the fraction of O2 (F_{O2}).

47 torr is used as the partial pressure of water because this is the estimated partial pressure of water in water-saturated air at 37 degrees C (how it would be once it enters the body)

***must memorize this equation***

What is F_{O2 }equal to? (give %)

21%

Use Dalton’s Law to calculate the P_{IO2} at sea-level given that P_{B} is 760 torr.

**150 torr**

760 torr - 47 torr = 713 torr

713 torr x 0.21 = 149.73 torr

When using Dalton’s Law to calculate partial pressure of inspired O_{2}, what is the assumed value of the partial pressure of water? (P_{H2O})

47 torr

If a patient is breathing 100% O_{2}, what is the partial pressure of inspired oxygen (P_{IO2}) at sea-level (P_{B} = 760 torr)?

**713 torr**

760 torr - 47 torr = 713 torr

713 torr x 1.0 = 713 torr

*note* F_{O2} = 1.0 because the patient is breathing 100% O_{2}

What is P_{AO2}? What is P_{aO2}?

P_{AO2} is the partial pressure of O_{2} in alveolar air.

P_{aO2} is the partial pressure of O_{2} in arterial blood.

Why is the partial pressure of oxygen less in the alveoli than in the airways?

This is because oxygen that goes into alveoli is used for metabolic reactions and CO_{2} is the biproduct. This is why CO_{2} displaces oxygen in the alveoli but does not displace nitrogen.

What is the respiratory exchange ratio? (R) What is the respiratory exchange ratio seen in a “normal” diet?

The respiratory exchange ratio = CO_{2} produced/O_{2} consumed (in the various metabolic reactions happening in tissues)

The respiratory exchange ratio seen in a “normal” diet is 0.8.

What is the alveolar gas equation? ***must memorize this equation***

P_{AO2} = P_{IO2} - (P_{ACO2}/R)

***must memorize this equation***

The partial pressure of oxygen in an alveolus is equal to the partial pressure of inspired oxygen minus the partial pressure of alveolar carbon dioxide divided by the respiratory exchange ratio.

In a normal diet, R = 0.8

What is a typical value for alveolar partial pressure of carbon dioxide? (P_{ACO2})

40 torr

Calculate the typical partial pressure of alveolar oxygen at sea level using the alveolar gas equation

**100 torr**

P_{AO2} = P_{IO2} - (P_{ACO2}/R)

P_{AO2} = 150 torr - (40 torr/0.8)

P_{AO2} = 150 torr - 50 torr = 100 torr