S3) Gas Exchange in the Lungs

Question 1

Identify the 6 factors affecting the diffusion rate of a gas in a fluid

Answer 1

• Pressure difference (ΔP)
• Solubility of a gas in solution (S)
• Cross-sectional area of the fluid (A)
• Distance the gas molecules must diffuse (d)
• Molecular weight of the gas (MW)
• Temperature of the fluid (assume 37^o)

Question 2

Identify the equation for Fick’s first law of Diffusion

Answer 2

Question 3

Which is more soluble between CO₂ and O₂?

Answer 3

Carbon dioxide (20x)

Question 4

What is the result of CO₂’s greater solubility?

Answer 4

Greater diffusion coefficient – rate at which a substance diffuses

Question 5

What role does ΔP have in the diffusion of O₂

Answer 5

Diffusion coefficient is compensated by differences in partial pressures i.e. larger ΔP compensates for slower diffusion of O₂

Question 6

In a diseased lung, the diffusion of which substance is predominantly impaired?

Answer 6

Oxygen gas exchange is more impaired than CO₂ because of its slower diffusion rate

Question 7

Identify the 3 components of the diffusion barrier

Answer 7

• 5 cell membranes
• 3 layers of cytoplasm
• 2 layers of tissue fluid

Question 8

How does the distance in the blood gas barrier change during inhalation?

Answer 8

• Distance between blood and air (in alveoli) = 0.6 mm
• Distance decreases during inhalation as lung distends
• Allows fast and efficient diffusion

Question 9

Diffusion resistance depends on 2 factors.

Identify them

Answer 9

• Nature of barrier
• Nature of gas

Question 10

Describe how the nature of the barrier affects the diffusion resistance

Answer 10

• Permeability = (D x S)/thickness of membrane
• Larger molecules have small diffusion coefficients

Question 11

Which substance diffuses faster, CO₂ or O₂?

Why?

Answer 11

• O₂ is smaller so greater diffusion coefficient
• CO₂ is more soluble

CO₂ diffuses faster because gas exchange of O₂ is a limiting step

Question 12

Describe the gradients of partial pressure of O₂ in the returning blood and alveoli

Answer 12

PO₂ in alveolar gas > PO₂ in returning blood

So, oxygen diffuses into blood

Question 13

Describe the gradients of partial pressure of CO₂ in the returning blood and alveoli

Answer 13

PCO₂ in alveolar gas < PCO₂ in returning blood

So, carbon dioxide diffuses out of blood

Question 14

Identify 3 conditions which impair diffusion

Answer 14

• Fibrotic lung disease
• Pulmonary oedema
• Emphysema

Question 15

Explain how fibrotic lung disease impairs diffusion

Answer 15

Fibrotic lung disease: thickened alveolar membrane slows gas exchange

Question 16

Explain how pulmonary oedema impairs diffusion

Answer 16

Pulmonary oedema: fluid in the interstitial space increases diffusion distance

Question 17

Explain how emphysema impairs diffusion

Answer 17

Emphysema: destruction of alveoli reduces surface area for gas exchange

Question 18

What is anatomical dead space?

Answer 18

- Anatomical deadspace is the volume of air which is inhaled that does not take part in the gas exchange because it remains in the conducting airways

• Normally 0.15 L in adults

Question 19

What is alveolar dead space?

Answer 19

Distributive/alveolar deadspace involves air reaching the lungs that is not perfused or poorly perfused due to dead/damaged alveoli (0.12 L)

Question 20

What is physiological deadspace?

Answer 20

Dead space is the volume of a breath that does not participate in gas exchange (ventilation without perfusion)

Physiological dead space = anatomical + alveolar

Question 21

How can one calculate the dead space ventilation rate?

Answer 21

DSVR = Dead space volume x respiration rate

E.g. 0.15 x 15 = 2.25 L

Question 22

How can one calculate alveolar ventilation rate?

Answer 22

AVR = (tidal volume - dead space volume) x respiration rate

E.g. (0.5 - 0.15) x 15 = 5.25L

Question 23

How can one calculate lung perfusion?

Answer 23

• Lung perfusion (Q) = RV output
• It is the same as cardiac output (approx. 5 litres/min)

Question 24

What is the ideal value for V/Q ratio in the lungs?

Answer 24

V = Alveolar ventilation rate (approx. 5 l/min)

Q = Lung perfusion (approx. 5 l/min)

V/Q = 1 (ideally)

Question 25

An imbalance between alveolar ventilation and alveolar blood flow is described by ventilation-perfusion ratio.

Outline this

Answer 25

• If VA is 0, but there is still perfusion, VA/Q = 0
• If VA is normal, but perfusion is 0, VA/Q = infinity

Question 26

Define hypoxia and hypoxaemia

Answer 26

Hypoxia – low oxygen levels in body or tissues

Hypoxaemia – low pO₂ in arterial blood

Question 27

What is cyanosis?

Answer 27

Cyanosis is bluish discolouration due to unsaturated haemoglobin

Question 28

Which parts of the body can be affected by cyanosis?

Answer 28

• Can be peripheral (hands or feet) due to poor local circulation
• Can be central (mouth, tongue, lips, oral mucosa) due to poorly saturated blood in systemic circulation

Question 29

Why can it be difficult to detect cyanosis?

Answer 29

• Poor lighting
• Skin colouration

Question 30

What does pulse oximetry do?

Answer 30

Detects level of Hb saturation by detecting difference in absorption of light between oxygenated and deoxygenated Hb

Question 31

What are the limitations with pulse oximetry?

Answer 31

• Only detects pulsatile arterial blood
• Ignores levels in tissues and non-pulsatile venous blood
• Doesn’t say how much Hb present