Gas exchange in lungs

Question 1

What is the pO2 of inhaled air?

Answer 1

The same as the pO2 of the atmospheric air

Question 2

What is the calculation for the inhaled pO2?

Answer 2

21% x 101kPa

Question 3

How does the pO2 change in the upper respiratory tract? Why?

Answer 3

Decreases
as the inhaled air is humidified with water in the airways evaporating into water vapour
dilutes the oxygen in the air

Question 4

What is the actual vapour pressure in the upper respiratory tract?

Answer 4

6.28kPa

Question 5

What is the calculation for the pO2 in the upper respiratory tract?

Answer 5

21% x (101 - 6.28kPa)

Question 6

How does the pO2 change in the alveoli? Why?

Answer 6

Decreases

as oxygen diffuses from the alveoli into the pulmonary capillaries

Question 7

What is the pO2 in the alveoli?

Answer 7

13.3kPa

Question 8

What is the pO2 in the pulmonary capillaries? Why?

Answer 8

13.3kPa
because oxygen diffuses into the pulmonary capillaries until the pO2 in the pulmonary capillaries is equal to the pO2 in the alveoli

Question 9

How long does it take for the pO2 in the pulmonary capillaries to increase to 13.3kPa?

Answer 9

A third of the time that the blood is in the pulmonary capillaries

Question 10

What is the advantage of the pO2 in the pulmonary capillaries increasing to 13.3kPa so quickly?

Answer 10

So that during exercise
when there is increased blood flow through pulmonary capillaries
still enough time for pO2 in pulmonary capillaries to increase to 13.3kPa

Question 11

How does the pO2 of air in the alveoli compare with the pO2 of expired air?

Answer 11

Higher pO2 in expired air
because expired are includes air from anatomical dead space, conducting part of respiratory tract
which wasn’t involved in gas exchange
therefore contains more oxygen

Question 12

What is the pCO2 in the pulmonary capillaries?

Answer 12

5.3kPa

Question 13

What is the pCO2 in the alveoli? Why?

Answer 13

5.3kPa
because carbon dioxide diffuses from the pulmonary capillaries into alveoli until the pCO2 in the alveoli is the same as the pCO2 in the pulmonary capillaries

Question 14

How does the pCO2 change from the upper respiratory tract to the alveoli? Why?

Answer 14

Increases

as carbon dioxide diffuses from the pulmonary capillaries into the alveoli

Question 15

What is meant by diffusion barriers in the lungs?

Answer 15

Structures that oxygen and carbon dioxide have to pass through as they diffuse between the alveoli and the pulmonary capillaries

Question 15

What are the factors affecting gas exchange across the respiratory membrane?

Answer 15

Partial pressure difference of gas across membrane

Diffusion coefficient of gas

Surface area of membrane

Thickness of membrane

Question 16

What are the actual diffusion barriers in the lungs?

Answer 16

Fluid lining alveolus

Alveolar epithelium

Alveolar basement membrane

Tissue fluid

Pulmonary capillary basement membrane

Pulmonary capillary endothelium

Plasma

Red blood cell membrane

Question 19

How do the partial pressure gradients of oxygen and carbon dioxde in the alveoli and pulmonary capillaries compare to each other? Why?

Answer 19

Need a larger partial pressure gradient for same rate of diffusion of oxygen
because it has a lower solubility coefficient
so it has a lower diffusion coefficient

Question 20

What is the solubility coefficient of oxygen at 37C?

Answer 20

0.01

Question 21

What is the solubility coefficient of carbon dioxide at 37C?

Answer 21

0.2

Question 22

Does oxygen or carbon dioxide have a faster rate of diffusion between the alveoli and pulmonary capillaries? Why?

Answer 22

Carbon dioxide
because it has a higher solubility coefficient
so it has a higher diffusion coefficient

Question 23

How are the lungs adapated to increase the rates of diffusion of oxygen and carbon dioxide?

Answer 23

Thin membrane
shorter diffusion distance between alveoli and red blood cells in pulmonary capillaries

Large surface area of alveoli
Large surface area of pulmonary capillaries

Question 24

What is meant by the ventilation:perfusion ratio?

Answer 24

The amount of alveolar ventilation compared to blood flow into the lungs

Question 25

How does normal alveolar ventilation compare to normal cardiac output?

Answer 25

They are both approximately 5L/min

Question 26

What is the normal ventilation:perfusion ratio?

Answer 26

1

Question 26

What are the pO2 and pCO2 of the alveoli and pulmonary capillaries when the ventilation:perfusion ratio is 1?

Answer 26

pO2 - 13.3kPa

pCO2 - 5.3kPa