Gas Exchange in the Lung

Question 1

After you exhale, what is the volume of air in the alveoli, and what is the volume in the dead space?

Answer 1

Alveoli - 2350ml.

Dead space - 150ml.

Question 2

How much atmospheric air is added when you inspire?

Answer 2

350ml.

Question 3

What is hypoxaemia?

Answer 3

Low PO2.

When partial pressure of O2 falls below 10 kPa.

Question 4

What level of O2 saturation does O2 have to be below for one to have hypoxaemia?

Answer 4

<94%.

Question 5

What is the normal range of PaCo2?

Answer 5

4.9 - 6.1 kPa.

Question 6

Describe type 1 and type 2 respiratory failures.

Answer 6

Type 1 - CO2 levels ok, but O2 levels are bad - hypoxaemic.

Type 2 - Patient is hypoxaemic and hypercapnic.

Question 7

What are the causes of hypoxaemia?

Answer 7

Hypoventilation.
Diffusion defects.
VQ mismatch.
Shunt (bad VQ mismatch).

Question 8

What is hypoventilation?

Answer 8

Not breathing enough - CO2 levels rise in blood.
Only cause of hypercapnia.
Could be seen in obesity - difficult to expand chest.
Seen in sleep apnoea - intermittent airway obstruction.
Drugs // stroke - both can affect central control of breathing.
Neuromuscular disease - makes breathing difficult.

Question 9

What is Fick’s Law of Diffusion?

Answer 9

There’s increased diffusion if there’s a greater concentration gradient, higher solubility, decreased molecular size and increased surface area.

Question 10

How do diffusion defects cause hypoxaemia?

Answer 10

Increased thickness - e.g oedema, fibrosis.
SA decreased in emphysema.
Concentration gradient affected - ventilation problem.

Question 11

What do V and Q stand for in VQ mismatch?

Answer 11

V - ventilation.

Q - Blood flow - perfusion.

Question 12

When can you expect dyspnoea (breathlessness)?

Answer 12

When rate of alveolar ventilation to blood flow is not 1..

Question 13

How can you confirm where lung ventilation and blood flow in the lungs are?

Answer 13

Ventilation - breathe in radioactive air.

Blood flow - radioactive substance in the blood.

Question 14

What is VQ mismatch?

Answer 14

Not a complete blockage, but obstruction to air flow in the lungs.

Question 15

What is a shunt?

Answer 15

When something is completely blocking airway into the lungs.

Question 16

What will supplying more oxygen do to someone with a shunt?

Answer 16

Nothing - one side won’t be able to receive it at all, and the other will already be fully saturated.

Question 17

What causes a shunt?

Answer 17

Oedema.
Pneumonia.
Developmental problem - e.g with the foramen ovale or ductus arteriousus.

Question 18

What is expiratory exchange ratio, and what is the normal value?

Answer 18

Measure of how much CO2 you’re producing compared to how much O2 you’re consuming.
Normal value - 0.8.

Question 19

What does the alveolar gas equation tell you?

Answer 19

Tells you what PAO2 based on PaCO2.

Question 20

How do the lungs cope with VQ mismatch?

Answer 20

Carry out hypoxic vasoconstriction.
This is where blood vessels in the lungs where ventilation is poor sense that there’s little oxygen. They constrict and divert blood away to a different area.
In COPD/fibrosis, the entire lung is always hypoxic, so there’s constant hypoxic vasoconstriction.
This causes major back-pressure, and the right heart will eventually fail.