Gas exchange in the lungs

Question 1

Briefly describe how oxygen moves from the alveoli to the blood in the capillaries

Answer 1

1. O2 enters the alveolar airspace from the atmosphere
2. O2 then dissolves in the alveolar lining fluid
3. O2 diffuses through alveolar epithelium, basement membrane & then the capillary endothelial cells.
4. O2 dissolves in blood plasma
5. O2 binds to Haemoglobin molecule in the red blood cell

Question 2

Why must the rate of diffusion of oxygen from the alveoli to the capillaries be very quick?

Answer 2

Due to capillary blood flow being quite fast the oxygen only has a short time to oxygenate the RBC’s before blood enters the arterioles.

Question 3

Explain how diffusion defects can affect the partial pressure of oxygen within the blood?

Answer 3

Diffusion defect causes diffusion of oxygen from the alveoli to the capillaries to occur at a slower rate.
This means that less oxygen is able to diffuse into the capillaries resulting in a decreased oxygen concentration within the blood.
Diffusion defect also affects CO2 diffusion out of capillary so more CO2 stays within the blood.
This results in O2 contributing less than CO2 to the pressure of gases within the blood so partial pressure of oxygen decreased.

Question 4

How do you calculate the rate of diffusion?

Answer 4

Rate of diffusion = Surface area/Diffusion distance^2 x (Pa - Pc)

Question 5

If we apply this equation to the rate of diffusion of air from the alveoli to the capillaries what do these variables become?

Answer 5

Surface area = alveolar surface area

Distance = Epithelial & endothelial cell thickness + basement membrane thickness + fluid layer depth

(Pa - Pc) = Partial pressure gradient between alveolar air (Pa) & capillary blood (Pc)

Question 6

Name a disease that affects each of the variables within the equation for diffusion rate?

Answer 6

Hypoventilation (type II respiratory failure) - decreases the partial pressure gradient

Emphysema - decreases the alveolar surface area

Pulmonary fibrosis - Increases basement membrane thickness

Pulmonary oedema - Increases thickness of fluid layer

Both Pulmonary fibrosis and pulmonary oedema increase diffusion distance

Question 7

How does hypoventilation decrease the partial pressure gradient?

Answer 7

Person doesn’t breathe at a sufficient rate to perform gas exchange - person doesn’t breathe in enough oxygen/breathe out enough CO2 so partial pressures of CO2/O2 in alveoli aren’t high enough to produce sufficient gradients with capillaries.

Question 8

How does emphysema cause the alveolar surface area to decrease?

Answer 8

Causes damage to the overall structure of the alveoli which causes some alveoli to fuse together. This causes the overall surface area to volume ratio

Question 9

How does an increase in alveolar ventilation affect the partial pressures of O2/CO2 in the alveoli? Why is this the case?

Answer 9

Causes partial pressures of CO2/O2 within alveoli to more closely resemble the partial pressures of atmospheric CO2/O2.
PAO2 increases while PACO2 decreases.
This is because essentially you’re replacing alveolar air with atmospheric air at a faster rate.

Question 10

Why must blood flow through pulmonary capillaries (perfusion) be matched by alveolar ventilation to ensure efficient gas exchange?

Answer 10

Because there is a maximum amount of O2 each unit of blood can carry. This is because each unit of blood has a finite amount of haemoglobin molecules that the O2 can bind to.

Question 11

What is the V/Q ratio?

Answer 11

Ratio of the amount of oxygen that can be carried in a certain unit of pulmonary blood (perfusion) compared to the amount of oxygen that can be carried in a certain unit of dry air (alveolar ventilation).

Question 12

What does it mean to have a V/Q ratio of 1?

Answer 12

Means amount of oxygen carried by a specific unit of pulmonary blood is the same as the amount of oxygen carried in that same unit of dry air.

Question 13

What is the most likely cause of a V/Q ratio of > 1?

Answer 13

Hypoperfusion (‘dead space effect’)

Question 14

What is the most likely cause of a V/Q ratio of < 1?

Answer 14

Hypoventilation (‘shunt’)

Question 15

How can a decrease in ventilation (V/Q mismatch) lead to local hypoxia in capillary blood?

Answer 15

If ventilation of specific alveoli decreases, PACO2 will rise and PAO2 will fall.
This causes local hypoxia of the capillary blood that innervates that specific alveoli.

Question 16

How does hypoxic vasoconstriction help prevent V/Q mismatch?

Answer 16

Intrapulmonary arteries constrict in response to alveolar hypoxia, diverting capillary blood to alveoli that are better ventilated thus optimising ventilation/perfusion matching and systemic oxygen delivery.

Question 17

Does V/Q mismatch refer to changes in overall ventilation/perfusion of the lungs?

Answer 17

Not necessarily, V/Q mismatch can refer to situations where V/Q ratios vary substantially between alveolar units.

Question 18

Explain how a pulmonary embolism causes V/Q mismatch?

Answer 18

Embolism causes occlusion of a pulmonary artery. This leads to an increased V/Q ratio in the under-perfused alveoli (physiologic dead-space) that are innervated by the blocked vessel. This is because Ventilation to these alveoli hasn’t changed but perfusion has decreased.

Occlusion of the pulmonary artery causes more blood to be diverted to other pulmonary arteries. This causes V/Q ratio of the alveoli innervated by these vessels to decrease. This is because perfusion of these alveoli has increased but ventilation remains the same.

This increase in perfusion means that there isn’t enough oxygen within these alveoli for the amount of blood that is flowing through them. This leads to hypoxaemia and hypercapnia

Question 19

How does “physiological dead space” cause V/Q mismatch?

Answer 19

Reduced perfusion of certain alveoli in the lungs without changing their ventilation causes these under-perfused alveoli to become physiological dead space - no/reduced gas exchange occurs.

Physiological dead space causes an increase in the V/Q ratio of these under-perfused alveoli.

Question 20

How can V/Q mismatch due to the creation of physiological dead-space within the lungs be treated?
Why is this the case?

Answer 20

Can be treated using oxygen therapy. This is because it increases ventilation to the alveoli that have more blood flowing through them which causes the V/Q ratio of these alveoli to decrease.

Question 21

How does a “shunt” cause V/Q mismatch?

Answer 21

Reduced ventilation of alveoli or limits to diffusion cause a decrease in the V/Q ratio of these poorly-ventilated alveoli as perfusion of these alveoli aren’t affected.

Question 22

V/Q mismatch due to physiological dead space leads to hypoxaemia and hypercapnia. What condition does V/Q mismatch due to shunt lead to? Why this is the case?

Answer 22

It results in shunting. This is because poorly-ventilated Alveoli hardly contain any oxygen meaning that hardly any oxygen diffuses from the alveoli to the pulmonary capillaries.
This means pulmonary capillary blood is still deoxygenated by the time it mixes with blood that has been oxygenated at well-ventilated alveoli and travels back to the heart (shunting).

Question 23

Why isn’t oxygen therapy an effective treatment for V/Q mismatch caused by a shunt?

Answer 23

Oxygen therapy can only increase the amount of O2/ decrease the amount of CO2 in the functional airways/alveoli.
It can’t do this in the poorly-ventilated alveoli. Also, there’s only a finite amount that the oxygen within these functional alveoli can be increased by which doesn’t compensate for the fact that there’s no oxygen in the poorly-ventilated alveoli.
This only causes a small increase in the saturation of pulmonary capillary blood of the functional alveoli as under normal conditions blood is very highly saturated anyway.
However, there’s still no saturation of the pulmonary capillary blood of the poorly-ventilate alveoli so shunting still occurs.

Question 24

Give an example of a clinical situation in which you’s need to know the partial pressure of oxygen within the alveoli (PA02)?

Answer 24

Determining the cause of respiratory failure – either due to hypoventilation or poor oxygenation

Question 25

What is the alveolar gas equation?

Answer 25

𝑃𝐴𝑂2 = 𝐹𝐼𝑂2 ×𝑃𝐵 −𝑃𝐻2𝑂 − 𝑃𝑎𝐶𝑂2/𝑅𝐸𝑅

Question 26

What do each of the variables in the alveolar gas equation represent?

Answer 26

𝑃𝐴𝑂2 = Alveolar O2 pressure 
𝐹𝐼𝑂2 = Fraction of oxygen present in inspired gas 
𝑃𝐵 = Barometric pressure
𝑃𝐻2𝑂 = H2O vapour pressure 
𝑃𝑎𝐶𝑂2 = Arterial CO2 pressure 
𝑅𝐸𝑅 = Respiratory exchange ratio

Question 27

If you simplify the alveolar gas equation what are the 2 main properties used to calculate the partial pressure of oxygen in the alveoli?

Answer 27

Alveolar O2 pressure = Partial pressure of oxygen within inspired air - Amount of oxygen consumed by the body

Question 28

Why is the alveolar gas equation used to calculate the partial pressure of oxygen in the alveoli rather than a practical method?

Answer 28

Clinically it is very hard to actually sample gas directly from the alveoli

Question 29

What is the respiratory exchange ratio (RER)?

Answer 29

Ratio between the amount of CO2 produced and O2 consumed by the body during metabolism

Question 30

What is the equation for the respiratory exchange ratio?

Answer 30

𝑅𝐸𝑅 = 𝑉𝐶𝑂2 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑/ 𝑉𝑂2 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑

Question 31

What is the main variable that affects the RER?

Answer 31

The particular metabolic substrate being used as there’s a particular ratio for each type of metabolite.

Question 32

Once the partial pressure of O2 within the alveoli is calculated what is it used for?

Answer 32

Used to compare the partial pressure of O2 within alveoli and in arterial blood (A-a O2 gradient )

Question 33

What is a normal value for the A-a O2 gradient?

Answer 33

< 2KPa

Question 34

If the A-a O2 gradient is above 2 what does this suggest?

Answer 34

That the person has a gas exchange or oxygenation problem