Module 3 Section 5 (Gas Exchange)

Question 1

Describe the importance of partial pressures for gas exchange.

Answer 1

Partial pressure is extremely important in predicting the movement of gases. Gases tend to equalize their pressure in 2 regions that are connected. A gas will move from an area where its partial pressure is higher to an area where its partial pressure is lower.

Question 2

Describe how arterial partial pressures of oxygen and carbon dioxide always return to the same values for circulation to the tissues.

Answer 2

Interestingly, PAˇO2and PAˇCO2 remain essentially constant despite inhalation and exhalation. This is mainly due to the low amount of fresh air that reaches the alveoli and the fast rate of diffusion of O2 from the alveoli into the blood.

• If there was an increase in PAˇO2, gas exchange increases to equalize it.
• The same is true for PAˇO2, the more CO2 that is expired just enhances the partial pressure gradient so more CO2 leaves the blood to the alveoli to keep it constant.

Question 3

Describe gas exchange across the capillary walls.

Answer 3

As blood passes through the lungs, carbon dioxide moves from the blood to the alveoli and oxygen moves from the alveoli to the blood.

• This movement of gases is by diffusion and is driven by partial pressure gradients.
• The process of ventilation is constantly replenishing alveolar oxygen and removing carbon dioxide.

Question 4

Describe the importance of capillary transit time in gas exchange.

Answer 4

Capillary Transit Time is the duration of exposure of capillary blood to alveolar gas.

Gases can only be exchanged between the blood and the alveoli when the blood is in the pulmonary capillaries. Because of this, the flow rate of the blood can influence gas exchange.

• At rest, blood remains in the pulmonary capillaries for about 0.75 seconds, which is about three times as long as necessary for exchange.
• Even during exercise, capillary transit time only decreases to 0.4 seconds
• Under physiological conditions, capillary transit time is not a limitation to gas exchange.
• If diffusion is impaired, equilibration still occurs before the blood reaches the end of the capillary, but only at rest.
• If the time available for diffusion decreases due to (i.e., an increase in cardiac output), the person with normal diffusion is unaffected, but the impaired person will experience a drop in arterial PˇO2.

Question 5

Define gas exchange.

Answer 5

The diffusion of oxygen from the alveoli into the blood, and carbon dioxide from the blood to the alveoli.

Question 6

True or false: even if a person has normal respiratory function, they could have impaired diffusion of gases.

Answer 6

True

Question 7

True or false: the factors that determine gas exchange are the same as those for convective flow through the airways (Flow = ∆P/R)

Answer 7

True

In this case though, the pressure gradient is based on the partial pressures of the gas in the alveoli (PA) and the pulmonary artery (PV), and the resistance to diffusion.

• Resistance to diffusion is dependent upon the surface area of the membrane (A), its thickness (T), and the diffusibility (D) of the gas (D is a constant, and as such, we can essentially ignore it).
• PV is used to signify the partial pressure of a substance within the pulmonary artery. This artery contains mixed venous blood, as it delivers blood from all systemic tissues to the lung.

Based on this, we can express the diffusion of - O2and C O2 as:
VCO2 = (PAˇCO2 - PVˇCO2) * A/T
- VO2 = (PAˇO2 - PVˇO2) * A/T

Question 8

Define partial pressure

Answer 8

The pressure that would be exerted by one of the gases in a mixture of gases, if it occupied the same volume on its own.

Question 9

True or false: air is actually a mixture of gases

Answer 9

True

And according to Dalton’s law, the pressure exerted by gas in a mixture is directly proportional to the percentage of that gas in the mixture.

• Ex: 79% of air is nitrogen so the partial pressure of nitrogen is 0.79 x 760 mmHg, which is 600 mmHg.
• Oxygen is 21% of air, so its partial pressure is 160 mmHg.
• The amount of carbon dioxide in air is so little that its partial pressure is only 0.23 mmHg.

Question 10

What are the 2 key factors that determine the amount of gas that can dissolve in a liquid?

Answer 10

1) Partial pressure in air: the greater its partial pressure, the more gas will be driven into the liquid.
2) Solubility in the liquid: the more soluble a gas is in a liquid, the more will dissolve.

Question 11

True or false: alveolar air does not have the same composition of inspired air.

Answer 11

True

When inhaling air into the airways, it immediately becomes saturated with water vapour.
- This water vapour, like any gas, contributes to the gas mixture and affects the partial pressures of the other gases.
- At body temperature, PˇH2O is 47 mmHg, this means the remaining gases account for 713 mmHg.
• This means the partial pressure of inspired nitrogen is 563 mmHg, since 79% of air is nitrogen, and the partial pressure of inspired oxygen is 150 mmHg, since 21% of inspired air is oxygen.

Question 12

With a tidal volume of 500 ml, really only about 350 ml is moving in or out with the remaining 150 ml being mixed inhalation and expiration gases. What occurs to this volume at the end of inspiration?

Answer 12

At the end of inspiration, only about 15% of alveolar gas is actually “fresh” air, so there is a further drop in the partial pressure of oxygen. The actual PAˇO2 is around 100 mmHg, as can be calculated using the alveolar gas equation

Question 13

What is the alveolar gas equation?

Answer 13

PIˇO2 - PAˇCO2/R = PAO2

PIˇO2 is the partial pressure of inspired oxygen (150 mmHg).

PAˇCO2 is the partial pressure of alveolar carbon dioxide (about 40 mmHg), and R is the respiratory quotient or ratio of metabolic carbon dioxide formation to oxygen consumption, roughly 0.8 in healthy people.

Question 14

Recall that the partial pressures of oxygen and carbon dioxide in the alveoli are 100 and 40 mmHg, respectively. The blood entering the lungs has travelled through the systemic circulation and therefore has a lower partial pressure of mixed venous oxygen (PVˇO2= 40 mmHg) and a higher partial pressure of mixed venous carbon dioxide (PVˇCO2= 46 mmHg).

Using what you have learned in previous Modules about diffusion, what do you think will occur to oxygen and carbon dioxide due to these gradients and why?

Answer 14

Because of these gradients, oxygen will move from the alveoli into the blood until the partial pressures are equalized. The same occurs for carbon dioxide leaving the blood.
- However, since ventilation keeps alveolar oxygen at 100 mmHg and carbon dioxide at 40 mmHg, blood leaving the lungs contains the gases in these partial pressures.

Question 15

How do the partial pressures of oxygen and carbon dioxide relate to the actual amount of dissolved gases in the blood?

Answer 15

This is affected by both the partial pressure and the solubility of a gas.
- In the case of oxygen and carbon dioxide, carbon dioxide is 20 times more soluble in the blood so even if their partial pressures were the same, the actual concentration of dissolved carbon dioxide would be greater.

Oxygen:

• Blood leaving the lungs has a carbon dioxide partial pressure of 40 mmHg, which is about 480 ml CO2/L.
• Blood returning to the lungs has a partial pressure of 46 mmHg, which is about 520 ml CO2/L.
• As you can see there isn’t a dramatic change in dissolved CO2. This is because of the essential role that CO2 (and carbonic acid) have in acid-base balance.

Carbon Dioxide:

• Blood leaving the lungs has an oxygen partial pressure of 100 mmHg, which is about 200 ml O2/L.
• Blood returning to the lungs has a partial pressure of 40 mmHg, which is about 150 ml O2/L.
• From this, you can calculate that at rest, 50 ml O2/L was removed by the tissues. The remaining 150 ml O2/L represents the functional reserve for when there is an increased tissue demand.

Question 16

What are some of the factors that affect gas exchange?

Answer 16

Surface Area:

• The greater the surface area, the greater amount of gas that can be exchanged.
• At rest, some pulmonary capillaries are closed. However, during exercise, the increase in pulmonary blood pressure will open these capillaries and this effectively increases the available surface area for gas exchange.
• Conversely, in lung diseases, (ex: emphysema), where there is alveolar damage, the surface area available for gas exchange is decreased.
• The loss of alveolar walls in the emphysematous tissue, results in larger but fewer alveoli. This reduces the surface area available for gas exchange.

Capillary Transit Time:
- Described in previous card.

Membrane Thickness:

• The thickness of the barrier separating the blood and the alveoli can be increased due to inflammation. This increased thickness results in a decrease in gas exchange.
• This is observed is mucous secreting lung disease such as asthma.
• If the membrane is thickened enough, the capillary transit time may not be sufficient for complete gas equalization.

Question 17

True or false: the fundamental principles of how gases are exchanged across the systemic capillaries are essentially identical to those in the pulmonary capillaries.

Answer 17

True

Cells are constantly undergoing oxidative metabolism in which they consume oxygen and produce carbon dioxide.

• The average cellular PˇO2is around 40 mmHg and the cellular PˇCO2 is around 46 mmHg.
• Since arterial PˇO2is around 100 mmHg and arterial PˇCO2is around 40 mmHg, it is clear to see that oxygen will move into the tissues and carbon dioxide will move into the blood.
• Analogous to what we see in the lungs, by the time the blood has left the systemic capillaries, its partial pressures for oxygen and carbon dioxide have equilibrated to the tissue values of 40 mmHg and 46 mmHg for oxygen and carbon dioxide, respectively.
• As tissue metabolism increases, such as during exercise, its PˇO2decreases and its PˇCO2increases, further increasing the pressure gradients driving exchange and allowing the tissues to get the necessary amount of oxygen while eliminating carbon dioxide.
• This would result in a decrease in the partial pressure of O2 returning to the lungs, which would result in more O2 moving from the alveoli to the blood.
• Tissue metabolism creates the driving forces for gas exchange in both the systemic and pulmonary capillaries.

Question 18

What are the steps of gas exchange? (7) **

Answer 18

** Slide 11 **

1) The venous blood entering the lungs is low in O2 (40 mmHg; this value decreases during exercise) and high CO2 (46 mmHg), due to the consumption of O2 and production of CO2 by the tissues.
2) Alveolar PˇO2 remains high (100 mmHg) and alveolar PˇCO2 low (40 mmHg) because only a portion of the alveolar air is replaced with fresh atmospheric air during each breath.

3) The partial pressure gradients for O2 (100 - 40 = 60 mmHg) and CO2 (46 - 40 = 6 mmHg) b/w the alveoli and pulmonary capillary blood cause O2 to diffuse into the blood and CO2 to diffuse into the alveoli.
- Diffusion continues into the blood and alveolar partial pressures become equal.

4) Blood leaving the lungs has, compared to the lungs, a high partial pressure and content of O2 and a low partial pressure and content of CO2. These partial pressures and contents are identical to those delivered to the tissues.
5) The partial pressures of O2and CO2 are, compared to those in arterial blood, lower and higher, respectively, in the O2-consuming, CO2-producing tissue cells.
6) O2 diffuses from the arterial blood into cells to support their metabolic requirements, and metabolically produced CO2 diffuses into the blood.
7) Having equilibrated with the tissue cells, the blood leaving the tissues is relatively low in O2 and high in CO2. The blood then returns to the lungs to once again replenish on O2 and release CO2.