Gas exchange in the lung

Question 1

How does oxygen get from the atmosphere to cells?

Answer 1

1. O2 inhaled from atmosphere into alveoli within lungs
2. O2 diffuses from alveoli into blood within pulmonary capillaries
3. O2 transported in blood, predominantly bound to Hb.
4. O2 diffuses into cells/tissues for use in aerobic respiration
5. CO2 diffuses from respiring tissues to blood-exchanged at lungs

Question 2

What does gas exchange involve?

Answer 2

Gas exchange involves diffusion of blood gases through multiple structures and mediums

Question 3

Movement of O2 through different surfaces

Answer 3

1. O2 enters the alveolar airspace from the atmosphere
2. O2 dissolves in alveolar lining fluid(ALF)
3. O2 diffuses through alveolar epithelium, basement membrane and capillary endothelial cells
4. O2 dissolves in blood plasma
5. O2 binds Hb molecules

Question 4

When must the oxygenation of blood occur?

Answer 4

Oxygenation of blood must occur during brief time taken for RBC’s to flow through pulmonary capillaries

Question 5

What factors determine rate of diffusion?

Answer 5

• Surface area(Alveolar surface area)
• Distance^2(Epithelial and endothelial cell thickness+basement membrane thickness+fluid layer depth)
• Partial pressure gradient between alveolar air and capillary blood

Question 6

What will an increase in distance do to the rate of diffusion?

Answer 6

Decrease diffusion

Question 7

How do gases behave?

Answer 7

Gases behave in terms of partial pressure.

Question 8

How would you change certain factors for maximum diffusion?

Answer 8

• Increase partial pressure gradient
• Increase in surface area
• Decrease in distance

Question 9

What does fibrosis do and how does it affect rate of diffusion?

Answer 9

Fibrosis increases basement membrane thickness and causes rate of diffusion to decrease

Question 10

What does a pulmonary oedema do and how does it affect rate of diffusion?

Answer 10

Increases thickness of fluid layer and causes a decrease in rate of diffusion

Question 11

What is maintained between alveoli and blood and how?

Answer 11

Pressure gradients between alveoli and blood are maintained by adequate ventilation

Question 12

What happens as ventilation increases?

Answer 12

Partial pressure of O2 within alveoli increases as ventilation increases

Question 13

What does an increase in alveolar partial pressure of oxygen result in?

Answer 13

Results in very diminishing returns for arterial partial pressure of oxygen

Question 14

Affects of a decrease in ventilation to alveolar and arterial partial pressure of oxygen

Answer 14

A decrease in ventilation decreases both alveolar and arterial partial pressure of oxygen

Question 15

What happens as ventilation decreases?

Answer 15

Decrease in ventilation will increase both partial pressure of CO2 in alveoli and arterieals

Question 16

What happens to arterial partial pressure of CO2 in hypoventialtion and hyperventilation?

Answer 16

Hypoventilation-Increase in partial pressure of CO2

Hyperventialtion-Decrease in partial pressure of CO2

Question 17

What happens to arterial pressure of O2 in hypoventilation and hyperventilation?

Answer 17

Hypoventilation-Approx same partial pressure of O2 to normal value
Hyperventilation-Decrease in partial pressure of O2

Question 18

What other factor is required for maintaining pressure gradients for diffusion?

Answer 18

-Adequate perfusion

Question 19

What 2 factors need to be matched to enable efficient gas exchange?

Answer 19

Perfusion(Q) needs to be matched to alveolar ventilation(VA)

Question 20

Normal V/Q ratio

Answer 20

1

Question 21

Likely cause of a V/Q ratio >1

Answer 21

Hypoperfusion(“dead space effect”)

Question 22

Likely cause of a V/Q ratio <1

Answer 22

Hypoventilation (Shunt)

Question 23

What is ventilation-perfusion coupling maintained by?

Answer 23

Maintained by hypoxic vasoconstriction

Question 24

What is the homeostatic mechanism to reduce ventilation-perfusion mismatching?

Answer 24

1)Under normal conditions, blood flow and ventilation are matched
2)If ventilation of a specific alveoli decreases, PACO2 will rise and PAO2 will fall therefore causing a decreased oxygenation of blood flowing through innervating capillaries
3)A decrease in PAO2 induces vasoconstriction, reducing blood flow
Blood flow diverted to another alveoli with higher ventilation

Question 25

What happens in most cases of V-Q inequality?

Answer 25

Increase in PaCO2 induces a reflex hyperventilation that clears excess CO2, but doesn’t increase PaO2

Question 26

What does an embolism cause?

Answer 26

1. Embolism occludes pulmonary artery supplying a region of the lung
2. Results in the increase of the V/Q ratio of the unperfused alveoli and a decrease in V/Q ratio of the other alveoli
3. Perfusion of the other alveoli increases due to cardiac output being diverted to this aveoli because of the embolism
4. Unless ventilation of the other alveoli increases, to match perfusion, hypoxaemia and hypercapnia will occur

Question 27

What can causes an increase in V/Q ratio?

Answer 27

• Heart faliure
• Blocked vessels
• Loss/damage to capillaries

Question 28

What is physiologic dead space?

Answer 28

Ventilation without perfusion

Question 29

What is shunt?

Answer 29

Perfusion without ventilation

Question 30

What can cause a decrease in V/Q ratio ?

Answer 30

• Pneumonia

- Acute lung injury

Question 31

How is the respone of a pulmonary shunt to O2 therapy?

Answer 31

Poor

Question 32

What is the respiratory exchange ratio?

Answer 32

Relationship between CO2 elimination and O2 consumption

Question 33

What is the main determinant in of RER and approx value?

Answer 33

Main determinant is the particular metabolic substrate being used and value is approx 0.8 for the modern diet

Question 34

What is A-a O2 gradient normally?

Answer 34

<2kPa

Question 35

When do you know that hypoventilation is contributing to hypoxaemia?

Answer 35

when PaCO2>6kPa

Question 36

How do you know that the oxygen reaching the alveoli is diffusing into the blood?

Answer 36

Calculate A-a gradient and should be approx 2kPa or less