L09 (pk4) - Alveolar gases and diffusion

Question 1

What is the approximate partial pressure of inspired oxygen in the mouth and why?

Answer 1

20kPa (150mmHg)

There is approximately 20% of oxygen in dry air

Question 2

What is the approximate partial pressure of inspired carbon dioxide in the mouth and why?

Answer 2

0kPa (0mmHg)

Since approximately 0% of carbon dioxide in dry air

Question 3

What is the approximate partial pressure of inspired oxygen in the conducting zone and why?

Answer 3

20kPa (150mmHg)

• When we breathe in, the conducting zone fills with fresh air
• Therefore, it has the same inspired partial pressure of gases (gas tensions) as in the mouth
• No gas exchange with the blood, therefore no impact on gas tensions

Question 4

What is the approximate partial pressure of inspired carbon dioxide in the conducting zone and why?

Answer 4

0kPa (0mmHg)

• When we breathe in, the conducting zone fills with fresh air
• Therefore, it has the same inspired partial pressure of gases (gas tensions) as in the mouth
• No gas exchange with the blood, therefore no impact on gas tensions

Question 5

What is the approximate partial pressure of expired carbon dioxide in the conducting zone and why?

Answer 5

5kPa

• When breathe out, the conducting zone fills with expired air coming from alveoli
• At end of expiration, the carbon dioxide has increased

Question 6

What is the approximate partial pressure of expired oxygen in the conducting zone and why?

Answer 6

13kPa

• When breathe out, the conducting zone fills with expired air coming from alveoli
• At end of expiration, the oxygen has decreased

Question 7

Why is the inspired and expired air have different partial pressures in the conducting zone?

Answer 7

Tidal nature of respiration

Question 8

Why do the gas tensions in the (larger) alveolar space stay relatively stable?

Answer 8

1. Movement via diffusion only due to the large surface area
2. Relatively little air moves into a large space (350ml air mixes into 2500ml, FRC, per inspiration)
   - So, alveolar space is not emptying and filling with each breath like in the conducting zone

Question 9

What is the approximate partial pressure of oxygen in the alveolar space?

Answer 9

PAO2 = 13kPa (100mmHg)

Question 10

What is the approximate partial pressure of carbon dioxide in the alveolar space?

Answer 10

PACO2 = 5kPa (40mmHg)

Question 11

What is the approximate partial pressure of venous oxygen in the pulmonary capillary?

Answer 11

5kPa (40mmHg)

Question 12

What is the approximate partial pressure of venous carbon dioxide in the pulmonary capillary?

Answer 12

6kPa (46mmHg)

Question 13

What happens to the partial pressure of the gases in the pulmonary capillary as it passes through the lungs

Answer 13

• As the blood passes through the lungs, it will equilibrate across the alveolar capillary membrane
• The blood leaving the lungs, will have the same blood gas tensions as the tensions in the alveolar space

Question 14

What sets the blood gas tensions in the pulmonary arteries leaving the lungs?

Answer 14

Alveolar gas tensions (partial pressures) set the arterial blood gas tensions
- Altering alveolar gas tensions will alter blood gas tensions

Question 15

What happens to PAO2 AND PaO2 if you don’t breathe enough?

Answer 15

Dec PAO2 –> Dec PaO2

opposite for others relating to CO2 and breathing too much

Question 16

What determines the alveolar PCO2 tensions?

Answer 16

1. Metabolism

2. Ventilation

Question 17

What is the effect on PACO2 if there is an increased metabolism?

Answer 17

Inc met => Inc V.CO2 => conc PACO2

Question 18

What is the effect on PACO2 if there is an increased ventilation:

Answer 18

Inc vent => Inc V.A => dilute PACO2

• Increasing ventilation will dec PACO2
• ‘Blow out CO2 faster than it arrives’

Question 19

What is the equation relating alveolar PCO2 and the factors that affect PACO2

Answer 19

1. PACO2 a V.CO2
2. PACO2 a 1/V.A
   Overall:
   PACO2 a (V.CO2)/V.A