Gas Exchange

Question 1

Gas Diffusion in Lungs
- Location
- A-C Membrane structure

Answer 1

Location: alveoli
A-C Membrane: alveoli-capillary membrane contains layer of surfactant, thin and large SA

Question 1

Diffusion Process
Pulmonary arteries vs veins

Answer 1

• Passive
• Driven by difference in partial pressure of gases either side of A-C membrane
• Pulmonary Arteries (away from heart, towards capillaries) = venous blood
• Pulmonary Veins (away from A-C membrane, towards heart) = arterial blood

Question 2

Influence of Partial Pressures on Diffusion

Answer 2

O2: moves from alveoli (higher PP) to blood (lower PP)
= PO2 < PAO2

CO2: moves from blood (higher PP) to alveoli (lower PP)
= PCO2 > PACO2

Question 3

Partial Pressure at Altitude

Answer 3

PAO2 decreases
PO2 remains constant

Altitude increases = Patm decreases = % of O2 in air volume remains constant (PO2), but amount of air molecules overall reduces
= less O2 inhaled overall
= reduced PA02

Reduced difference in PP of PAO2 and PO2:
= reduced driving force for diffusion
= slower diffusion rate
= reduced O2 in blood (PO2)

Question 4

Factors Influencing Diffusion Rate
- Proportional factors
- Inversely proportional factors

Answer 4

PROPORTIONAL:
- Diffusion coefficient
(how readily substances move across membrane; higher = faster diffusion)
- PP Gradient
(larger difference = greater driving force for diffusion)
- SA of gas exchange surface

INVERSELY PROPORTIONAL:
- Molecule size (larger = more resistance)
- Gas density (denser = more collisions = more resistance)
- Membrane thickness

Question 5

Oxygen Diffusion Speed and Limiting Factor

Answer 5

Speed = fast

Limiting factor = perfusion (amount of capillary blood flowing through A-C membrane, NOT diffusing capacity of A-C membrane itself)

Question 6

V/Q Mismatch
Compensatory Mechanism for Low V/Q

Answer 6

Low V/Q Units = less ventilation than perfusion

= O2 binding sites on haemoglobin not all filled
= Low PAO2 (not all blood returning to LA is fully oxygenated)
= Hypoxemia

Compensatory mechanism: Vasoconstriction

= reduces hypoxaemia effects of low V/Q units
= normal physiological response

Question 7

Diffusion of CO2

Answer 7

x20 faster than O2 diffusion

Question 8

Causes of abnormal PAO2 vs PACO2:

• Low PAO2
• High PACO2

Answer 8

LOW PAO2
- Gas exchange abnormalities (V/Q mismatch: most likely cause at rest)
(Diffusion impairment: most likely cause during exercise)
- Low ventilation
- Low inspired O2 (Altitude)

HIGH PACO2
- Low ventilation
- Not impacted by abnormal gas exchange

Question 9

Why is PO2 more affected by gas exchange abnormality than PCO2?

Answer 9

CO2 is more soluble than O2

= CO2 easier to dissolve in blood
= CO2 easier to transport

V/Q mismatch mechanisms (vasoconstriction) are more effective at maintaining normal PO2 than PCO2

Question 10

Hypoxaemia vs Hypoxia

• Define
• Causes

Answer 10

Hypoxaemia = low blood O2 (gas exchange abnormality, hypoventilation, altitude)

Hypoxia = low tissue O2 (hypoxaemia can cause hypoxia)

Question 11

Determinants of:
- Blood O2 Supply
- Tissue O2 Supply

Answer 11

BLOOD O2 SUPPLY:
- Efficiency of gas exchange
- [Hb] or abnormal Hb

TISSUE O2 SUPPLY:
- Blood O2 content (fixed)
- CO and local blood supply
- O2 unloading from Hb

(CO and O2 unloading can alter to meet increased tissue O2 demand)

Question 12

Four Causes of Hypoxia

Answer 12

1. Hypoxic: gas exchange abnormality, hypoventilation or altitude
2. Anaemic: low Hb = reduced O2 carrying capacity
3. Stagnant: insufficient CO or localised vascular blockage
4. Histotoxic: presence of substance blocking use of O2 in tissues (e.g. cyanide poisoning) - person has O2 but unable to use it