Gas exchange in the lungs

Question 1

How does oxygen get from the atmosphere to cells?

Answer 1

O2 inhaled from atmosphere into alveoli within lungs
↓
O2 diffuses from alveoli into blood within pulmonary capillaries
↓
O2 transported in blood predominantly bound to haemoglobin
↓
O2 diffuses into cells/tissues for use in aerobic respiration
↓
CO2 diffuses from respiring tissues to blood - exchanged at the lungs

Question 2

What are the structures that blood gases have to move through?

Answer 2

1) Airspace
2) Alveolar lining fluid
3) Alveolar epithelial layer
4) Basement membrane + interstitial fluid
5) Capillary endothelial layer
6) Blood plasma
7) Lung capillary

Question 3

What are the steps O2 has to take to bind to Hb?

Answer 3

O2 enters the alveolar airspace from the atmosphere 
↓
O2 dissolves in ALF 
↓
O2 diffuses through alveolar epithelium, basement membrane + capillary endothelial cells 
↓
O2 dissolves in blood plasma 
↓
O2 binds to Hb molecule

Question 4

What are oxygenation times like?

Answer 4

→Oxygenation of blood must be rapid
→In typical conditions it takes 0.75 seconds for a RBC to pass through a pulmonary capillary

→During intensive exercise it takes 0.25 seconds.

Question 5

What is the rate of diffusion determined by?

Answer 5

→Partial pressure gradient between two areas
→Size of the diffusion distance
→Surface area

Question 6

What is abnormal diffusion caused by?

Answer 6

→Thickening of blood-gas barrier

Question 7

What is the equation for rate of diffusion?

Answer 7

→Rate of diffusion ∝
(Surface area/Distance^2) x (Pa - Pc)

→SURFACE AREA : Alveolar surface area
→DISTANCE = Epithelial + endothelial cell thickness + basement membrane thickness + fluid layer depth

→(PA-PC) = Partial pressure gradient between alveolar air and capillary blood.

Question 8

What are criteria for maximum diffusion?

Answer 8

↑ Surface area
↑ partial pressure gradient
↓ Distance ( barrier thickness )

Question 9

What is a result of emphysema?

Answer 9

↓Surface area

Question 10

What is a result of fibrosis?

Answer 10

↑ Basement membrane thickness

Question 11

What is a result of pulmonary oedema?

Answer 11

↑ Increased thickness of fluid layer/oedema

Question 12

What is hypoventilation and what happens as a result of it?

Answer 12

→Don’t breathe at a sufficient rate so the partial pressure gradient decreases.

Question 13

What is the relationship between ventilation and partial pressure of O2?

Answer 13

→ Hypoventilation - Decrease in PaO2

→ Hyperventilation - Increase in PaO2

Question 14

What is the relationship between ventilation and partial pressure of CO2?

Answer 14

→ Hypoventilation - Increase in PaCo2

→ Hyperventilation - Decrease in PaCo2

Question 15

What needs to be matched for efficient gas exchange?

Answer 15

→blood flow through pulmonary capillaries needs to be matched to alveolar ventilation

Question 16

Why does perfusion need to be matched to ventilation?

Answer 16

→ Each unit of blood has a finite amount of Hb and can only transport a limited amount of oxygen

Question 17

What is the V/Q ratio?

Answer 17

→ The relationship between ventilation and perfusion

Question 18

What should the V/Q ratio normally be?

Answer 18

→ 1

Question 19

What is the cause of V/Q being >1?

Answer 19

→ Hypoperfusion

Question 20

What is the cause of V/Q being <1?

Answer 20

→ Hypoventilation

Question 21

What reduces ventilation-perfusion mismatching?

Answer 21

→ Homeostatic mechanisms

Question 22

What is hypoxic vasoconstriction for?

Answer 22

→ constriction of capillaries which diverts blood flow from poor to well ventilated alveoli

Question 23

How does hypoxic vasoconstriction occur?

Answer 23

→ If ventilation of a specific alveolus decreases
→ Pa Co2 will rise
→ Pa O2 will fall
→ Less oxygenation of blood flowing through innervating capillaries
→ decreased Pa O2 induces vasoconstriction = decreased blood flow
→ Blood is diverted to alveoli with more ventilation

Question 24

What is physiological dead space?

Answer 24

→ you have ventilation but not perfusion

Question 25

What happens to the V/Q to alveoli being underperfused due to an embolism and how can you compensate?

Answer 25

→V/Q ratio increases in alveoli that is not being perfused
→ Increased perfusion to other alveoli and their V/Q ratio decreases
→you can compensate by ventilating more

Question 26

What are diseases that cause physiologic dead space?

Answer 26

→ Heart failure
→ Blocked vessels
→ Loss/damage to capillaries

Question 27

What is the shunt effect?

Answer 27

→ Perfusion without ventilation

Question 28

What are diseases which cause the shunt effect?

Answer 28

→ Pneumonia
→ Acute lung injury
→ respiratory distress syndrome
→ atelectasis

Question 29

What is the respiratory exchange ratio (RER)?

Answer 29

→ The relationship between Co2 and O2 in inspired + expired air

Question 30

What is the equation for RER?

Answer 30

→ VCO2 produced/VO2 consumed

Question 31

How can the alveolar O2 pressure be calculated?

Answer 31

→ PaO2 = F1O2 x (Pb-PH2O) - (Pa Co2/RER)

Question 32

What is the difference in alveolar partial pressure and arterial partial pressure in a healthy individual?

Answer 32

→ <2kPa

Question 33

What is the A-a gradient?

Answer 33

→ Difference between arterial and alveolar partial pressure

Question 34

How do you know if the cause of hypoxaemia is hypoventilation?

Answer 34

→ PaCo2 > 6kPa

Question 35

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

Answer 35

→ A-a gradient should be less than 2 kPa