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

If a patient is breathing 100% O_{2}, what is the respiratory exchange ratio (R)?

R = 1

Remember that R = CO_{2} produced/O_{2} consumed

In a normal situation where R = 0.8, there is more O_{2} being consumed than CO_{2} being produced. So, if you look at an alveolus, there will be a greater amount of O_{2} entering the capillary than CO_{2} coming out of the capillary. This uneven exchange results in a deficit of pressure in the alveolus which is replaced mostly by N_{2}. However, when a patient is breathing 100% O_{2}, this deficit is replaced entirely by oxygen. This results in the hemoglobin in the perfusing capillaries to become completely saturated and O_{2} consumed goes down to balance the CO_{2} being produced (R = 1).

This makes sense because when using the alveolar gas equation with an R value of 1, P_{AO2} = P_{IO2} - P_{ACO2}. So, the partial pressure of alveolar oxygen is the partial pressure of inspired oxygen minus the partial pressure of alveolar carbon dioxide.

PAO2 = PIO2 - (PACO2/R)

Why is it important for P_{aCO2} to be close to 40 torr?

Because P_{aCO2} affects blood pH

True or False: The equilibration of CO_{2} between capillaries and alveoli is so fast that the partial pressure of CO_{2} of capillaries is considered to be equal to the partial pressure of CO_{2} of the alveoli.

True

What is the rate-limiting step for removal of CO_{2} from blood?

Ventilation. There are 2 steps to CO_{2} removal from blood:

1. diffusion

2. ventilation

Diffusion is super fast but ventilation is relatively slow. This is why there is a close relationship between CO_{2} removal from blood and ventilation.

What happens to CO_{2} in alveoli and blood with a high ventilation rate?

CO_{2} decreases. At high ventilation rate, CO_{2} is eliminated more quickly from the alveoli and, as we learned, the equilibration of CO_{2} between blood and alveoli is very fast so the CO_{2} is being eliminated quickly from the blood as well.

What happens to CO_{2} in alveoli and blood with a low ventilation rate?

CO_{2} increases. At low ventilation rate, CO_{2} is eliminated more slowly from the alveoli and, as we learned, the equilibration of CO_{2} between blood and alveoli is very fast so the CO_{2} is being eliminated more slowly from the blood as well. (CO_{2} levels in blood and alveoli are the same)

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

**P _{aCO2} = (V_{CO2} / V_{A}) k**

V_{CO2} = CO_{2} production in 1 minute

V_{A }= alveolar ventilation in 1 minute

In words, the alveolar partial pressure of CO_{2} is equal to the ratio CO_{2} production and elimination multiplied by a constant "k".

Also, notice that the arterial partial pressure of CO_{2} is inversely related to alveolar ventilation. The more ventilation (elimination) of CO_{2}, the lower your arterial partial pressure of CO_{2}.

P_{aCO2} is inversely related to ______

alveolar ventilation

What is P_{aCO2} if V_{A} decreasese by 50%

P_{aCO2} will double

Normal P_{aCO2} of 40 torr would double to become 80 torr

When a patient has a change in alveolar ventilation, what is a general equation that can be used to calculate the new P_{aCO2}?

True or False: P_{aCO2} = P_{ACO2}

True. The partial pressure of CO_{2} of the arterial blood is equal to the partial pressure of CO_{2} in an alveolus because the equilibration between the two is so fast.

True or False: on the left side of the alveolar ventilation equation, you can use either P_{aCO2} or P_{ACO2} .

True. The partial pressure of CO_{2} of the arterial blood is equal to the partial pressure of CO_{2} in an alveolus because the equilibration between the two is so fast.

What is P_{AO2} if V_{A} decreases by 50%?

Alveolar ventilation equation: P_{AO2} = P_{IO2} - (P_{ACO2}/R)

Alveolar gas equation: P_{ACO2} = (V_{CO2}/V_{A}) k

If V_{A} decreases by 50%, then P_{ACO2} doubles from 40 torr to 80 torr.

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

P_{AO2} = 150 Torr - (80 torr/0.8) = 150 torr - 100 torr = **50 torr**

Blood CO_{2} is directly regulated by _____

Alveolar ventilation

Blood O_{2} is _____ regulated by alveolar ventilation.

Indirectly. Alveolar ventilation has a direct effect on alveolar and blood CO_{2} which has an effect on Blood O_{2}.