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Flashcards in Gas Transport in the Airways Deck (34):
1

What is normal partial pressure of arterial O2? (PaO2)

90-100 torr (at sea level)

2

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, Pbarometric = PO2 + PN2 + PCO2 + etc...

3

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.

4

Normal PaO2 is 90-100 Torr at sea level. What is it in Denver?

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

5

What is PIO2? How is it calculated? *must memorize this equation*

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 (FO2).

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*

 

 

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6

What is FO2 equal to? (give %)

21%

7

Use Dalton's Law to calculate the PIO2 at sea-level given that PB is 760 torr.

150 torr

 

760 torr - 47 torr = 713 torr

713 torr x 0.21 = 149.73 torr

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8

When using Dalton's Law to calculate partial pressure of inspired O2, what is the assumed value of the partial pressure of water? (PH2O)

47 torr

9

If a patient is breathing 100% O2, what is the partial pressure of inspired oxygen (PIO2) at sea-level (PB = 760 torr)?

713 torr

760 torr - 47 torr = 713 torr

713 torr x 1.0 = 713 torr

*note* FO2 = 1.0 because the patient is breathing 100% O2

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10

What is PAO2? What is PaO2?

PAO2 is the partial pressure of O2 in alveolar air.

PaO2 is the partial pressure of O2 in arterial blood.

11

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 CO2 is the biproduct. This is why CO2 displaces oxygen in the alveoli but does not displace nitrogen.

12

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

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.

 

13

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

PAO2 = PIO2 - (PACO2/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

14

What is a typical value for alveolar partial pressure of carbon dioxide? (PACO2)

40 torr

15

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

100 torr

 

PAO2 = PIO2 - (PACO2/R)

PAO2 = 150 torr - (40 torr/0.8)

PAO2 = 150 torr - 50 torr = 100 torr

16

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

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, PAO2 = PIO2 - PACO2. 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)

17

Why is it important for PaCO2 to be close to 40 torr?

Because PaCO2 affects blood pH

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.

True

19

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

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.

20

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

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.

21

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

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)

22

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

PaCO2 = (VCO2 / VA) k

VCO2 = CO2 production in 1 minute

V= 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.

23

PaCO2 is inversely related to ______

alveolar ventilation

24

What is PaCO2 if VA decreasese by 50%

PaCO2 will double

Normal PaCO2 of 40 torr would double to become 80 torr

25

When a patient has a change in alveolar ventilation, what is a general equation that can be used to calculate the new PaCO2?

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26

True or False: PaCO2 = PACO2

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.

27

True or False: on the left side of the alveolar ventilation equation, you can use either  PaCO2 or PACO2 .

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.

28

What is PAO2 if VA decreases by 50%?

Alveolar ventilation equation: PAO2 = PIO2 - (PACO2/R)

Alveolar gas equation: PACO2 = (VCO2/VA) k

If VA decreases by 50%, then PACO2 doubles from 40 torr to 80 torr.

PAO2 = PIO2 - (PACO2/R)

PAO2 = 150 Torr - (80 torr/0.8) = 150 torr - 100 torr = 50 torr

 

29

Blood CO2 is directly regulated by _____

Alveolar ventilation

30

Blood O2 is _____ regulated by alveolar ventilation.

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

31

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

Hypoventilation occurs when ventilation is inadequate. Va (alveolar ventilation) decreases, so PaCO2 increases. This can happen in severe obstructive disease.

32

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

Hyperventilation is an increased alveolar ventilation of carbon dioxide that exceeds the body's production of carbon dioxide. This results in a high VA (alveolar ventilation) and a low PaCO2. This can happen due to high altitude.

33

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

Hyperpnia is when there is increased alveolar ventilation but also increased CO2 production. This can happen during exercise and it results in a normal PaCO2. If CO2 production is held constant, an increase in alveolar ventilation (VA) would result in a lower PaCO2. However, during exercise, there is also a higher production of CO2 so PaCO2 stays normal.

34

What is the difference between hyperventilation and hyperpnia?

There is an increased alveolar ventilation (VA) in both hyperventilation and hyperpnia but in hyperventilation you have a low PaCO2. In Hyperpnia, PaCO2 is normal.

 

So, when you're exercising, you can't say that you're hyperventilating because your PaCO2 is normal.