Gas Transport in the Airways

Question 1

What is normal partial pressure of arterial O₂? (P_aO2)

Answer 1

90-100 torr (at sea level)

Question 2

What is partial pressure?

Answer 2

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…

Question 3

Are gases floating around in blood?

Answer 3

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.

Question 4

Normal P_aO2 is 90-100 Torr at sea level. What is it in Denver?

Answer 4

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

Question 5

What is P_IO2? How is it calculated? *must memorize this equation*

Answer 5

Partial pressure of O₂ 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*

Question 6

What is F_O2 equal to? (give %)

Answer 6

21%

Question 7

Use Dalton’s Law to calculate the P_IO2 at sea-level given that P_B is 760 torr.

Answer 7

150 torr

760 torr - 47 torr = 713 torr

713 torr x 0.21 = 149.73 torr

Question 8

When using Dalton’s Law to calculate partial pressure of inspired O₂, what is the assumed value of the partial pressure of water? (P_H2O)

Answer 8

47 torr

Question 9

If a patient is breathing 100% O₂, what is the partial pressure of inspired oxygen (P_IO2) at sea-level (P_B = 760 torr)?

Answer 9

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% O₂

Question 10

What is P_AO2? What is P_aO2?

Answer 10

P_AO2 is the partial pressure of O₂ in alveolar air.

P_aO2 is the partial pressure of O₂ in arterial blood.

Question 11

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

Answer 11

This is because oxygen that goes into alveoli is used for metabolic reactions and CO₂ is the biproduct. This is why CO₂ displaces oxygen in the alveoli but does not displace nitrogen.

Question 12

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

Answer 12

The respiratory exchange ratio = CO₂ produced/O₂ consumed (in the various metabolic reactions happening in tissues)

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

Question 13

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

Answer 13

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

Question 14

What is a typical value for alveolar partial pressure of carbon dioxide? (P_ACO2)

Answer 14

40 torr

Question 15

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

Answer 15

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

Question 16

If a patient is breathing 100% O₂, what is the respiratory exchange ratio (R)?

Answer 16

R = 1

Remember that R = CO₂ produced/O₂ consumed

In a normal situation where R = 0.8, there is more O₂ being consumed than CO₂ being produced. So, if you look at an alveolus, there will be a greater amount of O₂ entering the capillary than CO₂ coming out of the capillary. This uneven exchange results in a deficit of pressure in the alveolus which is replaced mostly by N₂. However, when a patient is breathing 100% O₂, this deficit is replaced entirely by oxygen. This results in the hemoglobin in the perfusing capillaries to become completely saturated and O₂ consumed goes down to balance the CO₂ 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)

Question 17

Why is it important for P_aCO2 to be close to 40 torr?

Answer 17

Because P_aCO2 affects blood pH

Question 18

True or False: The equilibration of CO₂ between capillaries and alveoli is so fast that the partial pressure of CO₂ of capillaries is considered to be equal to the partial pressure of CO₂ of the alveoli.

Answer 18

True

Question 19

What is the rate-limiting step for removal of CO₂ from blood?

Answer 19

Ventilation. There are 2 steps to CO₂ 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 CO₂ removal from blood and ventilation.

Question 20

What happens to CO₂ in alveoli and blood with a high ventilation rate?

Answer 20

CO₂ decreases. At high ventilation rate, CO₂ is eliminated more quickly from the alveoli and, as we learned, the equilibration of CO₂ between blood and alveoli is very fast so the CO₂ is being eliminated quickly from the blood as well.

Question 21

What happens to CO₂ in alveoli and blood with a low ventilation rate?

Answer 21

CO₂ increases. At low ventilation rate, CO₂ is eliminated more slowly from the alveoli and, as we learned, the equilibration of CO₂ between blood and alveoli is very fast so the CO₂ is being eliminated more slowly from the blood as well. (CO₂ levels in blood and alveoli are the same)

Question 22

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

Answer 22

P_aCO2 = (V_CO2 / V_A) k

V_CO2 = CO₂ production in 1 minute

V_A = alveolar ventilation in 1 minute

In words, the alveolar partial pressure of CO₂ is equal to the ratio CO₂ production and elimination multiplied by a constant “k”.

Also, notice that the arterial partial pressure of CO₂ is inversely related to alveolar ventilation. The more ventilation (elimination) of CO₂, the lower your arterial partial pressure of CO₂.

Question 23

P_aCO2 is inversely related to ______

Answer 23

alveolar ventilation

Question 24

What is P_aCO2 if V_A decreasese by 50%

Answer 24

P_aCO2 will double

Normal P_aCO2 of 40 torr would double to become 80 torr

Question 25

When a patient has a change in alveolar ventilation, what is a general equation that can be used to calculate the new P_aCO2?

Answer 25

Question 26

True or False: P_aCO2 = P_ACO2

Answer 26

True. The partial pressure of CO₂ of the arterial blood is equal to the partial pressure of CO₂ in an alveolus because the equilibration between the two is so fast.

Question 27

True or False: on the left side of the alveolar ventilation equation, you can use either P_aCO2 or P_ACO2 .

Answer 27

True. The partial pressure of CO₂ of the arterial blood is equal to the partial pressure of CO₂ in an alveolus because the equilibration between the two is so fast.

Question 28

What is P_AO2 if V_A decreases by 50%?

Answer 28

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

Question 29

Blood CO₂ is directly regulated by _____

Answer 29

Alveolar ventilation

Question 30

Blood O₂ is _____ regulated by alveolar ventilation.

Answer 30

Indirectly. Alveolar ventilation has a direct effect on alveolar and blood CO₂ which has an effect on Blood O₂.

Question 31

What is hypoventilation? What are its consequences? Give one example of when this could happen.

Answer 31

Hypoventilation occurs when ventilation is inadequate. V_a (alveolar ventilation) decreases, so P_aCO2 increases. This can happen in severe obstructive disease.

Question 32

What is hyperventilation? What are its consequences? Give one example of when this can happen.

Answer 32

Hyperventilation is an increased alveolar ventilation of carbon dioxide that exceeds the body’s production of carbon dioxide. This results in a high V_A (alveolar ventilation) and a low P_aCO2. This can happen due to high altitude.

Question 33

What is hyperpnia? What are the consequences? When does this happen?

Answer 33

Hyperpnia is when there is increased alveolar ventilation but also increased CO₂ production. This can happen during exercise and it results in a normal P_aCO2. If CO₂ production is held constant, an increase in alveolar ventilation (V_A) would result in a lower P_aCO2. However, during exercise, there is also a higher production of CO₂ so P_aCO2 stays normal.

Question 34

What is the difference between hyperventilation and hyperpnia?

Answer 34

There is an increased alveolar ventilation (V_A) in both hyperventilation and hyperpnia but in hyperventilation you have a low P_aCO2. In Hyperpnia, P_aCO2 is normal.

So, when you’re exercising, you can’t say that you’re hyperventilating because your P_aCO2 is normal.