S3) Gas Exchange in the Lungs Flashcards

1
Q

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

A
  • 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 37o)
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2
Q

Identify the equation for Fick’s first law of Diffusion

A
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3
Q

Which is more soluble between CO2 and O2?

A

Carbon dioxide (20x)

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4
Q

What is the result of CO2’s greater solubility?

A

Greater diffusion coefficient – rate at which a substance diffuses

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5
Q

What role does ΔP have in the diffusion of O2

A

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

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6
Q

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

A

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

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7
Q

Identify the 3 components of the diffusion barrier

A
  • 5 cell membranes
  • 3 layers of cytoplasm
  • 2 layers of tissue fluid
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8
Q

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

A
  • Distance between blood and air (in alveoli) = 0.6 mm
  • Distance decreases during inhalation as lung distends
  • Allows fast and efficient diffusion
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9
Q

Diffusion resistance depends on 2 factors.

Identify them

A
  • Nature of barrier
  • Nature of gas
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10
Q

Describe how the nature of the barrier affects the diffusion resistance

A
  • Permeability = (D x S)/thickness of membrane
  • Larger molecules have small diffusion coefficients
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11
Q

Which substance diffuses faster, CO2 or O2?

Why?

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

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

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12
Q

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

A

PO2 in alveolar gas > PO2 in returning blood

So, oxygen diffuses into blood

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13
Q

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

A

PCO2 in alveolar gas < PCO2 in returning blood

So, carbon dioxide diffuses out of blood

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14
Q

Identify 3 conditions which impair diffusion

A
  • Fibrotic lung disease
  • Pulmonary oedema
  • Emphysema
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15
Q

Explain how fibrotic lung disease impairs diffusion

A

Fibrotic lung disease: thickened alveolar membrane slows gas exchange

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16
Q

Explain how pulmonary oedema impairs diffusion

A

Pulmonary oedema: fluid in the interstitial space increases diffusion distance

17
Q

Explain how emphysema impairs diffusion

A

Emphysema: destruction of alveoli reduces surface area for gas exchange

18
Q

What is anatomical dead space?

A

- 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
19
Q

What is alveolar dead space?

A

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

20
Q

What is physiological deadspace?

A

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

Physiological dead space = anatomical + alveolar

21
Q

How can one calculate the dead space ventilation rate?

A

DSVR = Dead space volume x respiration rate

E.g. 0.15 x 15 = 2.25 L

22
Q

How can one calculate alveolar ventilation rate?

A

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

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

23
Q

How can one calculate lung perfusion?

A
  • Lung perfusion (Q) = RV output
  • It is the same as cardiac output (approx. 5 litres/min)
24
Q

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

A

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

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

V/Q = 1 (ideally)

25
An imbalance between alveolar ventilation and alveolar blood flow is described by ventilation-perfusion ratio. Outline this
- **If VA is 0**, but there is still perfusion, VA/Q = 0 - **If VA is normal,** but perfusion is 0, VA/Q = infinity
26
Define hypoxia and hypoxaemia
**Hypoxia** – low oxygen levels in body or tissues **Hypoxaemia** – low pO2 in arterial blood
27
What is cyanosis?
**Cyanosis** is bluish discolouration due to unsaturated haemoglobin
28
Which parts of the body can be affected by cyanosis?
- 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
29
Why can it be difficult to detect cyanosis?
- Poor lighting - Skin colouration
30
What does pulse oximetry do?
Detects level of Hb saturation by detecting difference in absorption of light between oxygenated and deoxygenated Hb
31
What are the limitations with pulse oximetry?
- Only detects pulsatile arterial blood - Ignores levels in tissues and non-pulsatile venous blood - Doesn’t say how much Hb present