Diffusion of gases

Question 1

Significance of pressure difference

Answer 1

P driving gas across alveolar membrane = difference in partial pressure of gas in alveolus and blood flowing by in capillary

Question 2

Ventilation-perfusion limitations

Answer 2

• Gas crosses alveolar membrane
• Dissolves in blood
• Partial P of that gas quickly equilibrates with alveolar partial P
• Further transfer will be possible if more blood with lower partial P passes through alveolar capillary
• Gas crosses alveolar membrane bound to Hb => EXERTS NO PARTIAL P
• No eq. of partial P
• Gas’ ability to diffuse across membrane is limiting factor => DIFFUSION LIMITED

Question 3

What sort of affinity does CO have for Hb

Answer 3

Greater affinity than O2

CO binds tightly to Hb

PCO in blood doesn’t rise much, as it does not dissolve in blood => no exertion of partial pressure

DECREASED BACK DIFFUSION

Therefore

CO TRANSFER IS DIFFUSION LIMITED

Question 4

Nitrous oxide - diffusion or perfusion limited

Answer 4

PERFUSION LIMITED (dependent on capillary blood flow)

Diffuses into capillaries

Does not bind to Hb, as Hb has a low avidity for N2O

PN2O increases rapidly, and equilibrates with alveolar PN2O, so no more N2O transferred

Question 5

O2 - diffusion or perfusion limited

Answer 5

• O2 binds to Hb, although it has a lower avidity than CO
• PO2 starts high
• In a normal lung, PO2 in blood is equal with alveolar PO2
• Rapid O2 transfer - PERFUSION LIMITED

Question 6

Effect of abnormal lungs on O2 perfusion/diffusion limitations

Answer 6

Thickened barrier => diffusion impaired

Equilibrium may not be reached

Blood PO2 may not be equal to alveolar PO2

So ability to diffuse across the membrane is limiting factor

In NON-HEALTHY lung, O2 transfer is DIFFUSION LIMITED

Question 7

What does the RBC capillary reduce to during severe exercise

Answer 7

decreases from 0.75 -> 0.25 s

Question 8

How fast does capillary PO2 equate with P_AO2 in normal lungs

Answer 8

0.25 s

Question 9

What happens with a thickened blood gas barrier

Answer 9

Impaired diffusion

Blood PO2 doesn’t reach alveolar PO2

Question 10

What is CO used to measure

Answer 10

diffusion properties of the lungs

Question 11

Single breath method of measuring diffusing capacity

Answer 11

Single inspiration of CO mixture - hold for 10s - expire

Rate of diffusion = original CO conc - CO in expired air

normal value = 25 ml/min/mmHg

(x 2 or 3 during exercise)

Question 12

Why is the blood gas barrier in lungs ideal for diffusion

Answer 12

Area of barrier is large (50-100m2)

Thin membrane (0.3um) in many places

Question 13

Why does CO2 diffuse 20x more readily than O2, despite similar MW

Answer 13

CO2 has increased solubility in the tissues

Question 14

Pulmonary perfusion and pulmonary circulation

Answer 14

Pressure within pulmonary BVs

Pressure around pulmonary BVs

Pulmonary vascular resistance

Distribution of blood flow

Question 15

What are the 2 circulations in the lung

Answer 15

PULMONARY (100% blood flow)

BRONCHIAL

• part of systemic
• arises from aorta - 2%

Question 16

How much blood do lungs contain when upright/sitting

Answer 16

450 ml

Question 17

how much blood is in pulmonary capillaries

Answer 17

45 ml

most is in pulmonary venules

Question 18

Effect of pulmonary vasoconstriction

Answer 18

Adds 200-300 ml blood to systemic circulation in an emergency

Question 19

Where is the greatest pressure drop

Answer 19

Across pulmonary bed

Question 20

What is pulmonary vascular resistance

Answer 20

degree to which BVs impede flow of blood

PVR = P_{pulmonary artery} - P_{left atrium}/CO

= 15 - 5/6 = 1.7 mmHg/L/min

Systemic vascular R = 17 mmHg