Pulmonary diffusion and blood flow

Question 1

Dalton’s law

Answer 1

Part One states that:
‘the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture’.

Part two states that: ‘the pressure exerted by each gas (its partial pressure) is directly proportional to the percentage of gas in the mixture’

Question 2

Difference between atmospheric air and alveolar air.

Answer 2

The composition is not the same because:

• the atmospheric air we inspire is not humidified. Water evaporates from the surface of the respiratory passages and humidifies the air.
• air in the alveoli is also undergoing continuous exchange with the blood.

Question 3

Vapour pressure

Answer 3

At 37 degrees is 47 mmHg.

Question 4

Atmospheric air composition

Answer 4

Nitrogen: 597 mmHg
Oxygen: 159 mmHg
CO2: 0.3 mmHg
H20: 3.7 mmHg

Question 5

Humidifies air composition

Answer 5

Nitrogen: 563 mmHg
Oxygen: 149 mmHg
CO2: 0.29 mmHg
H20: 47 mmHg

Question 6

Alveolar air composition

Answer 6

Nitrogen: 569 mmHg
Oxygen: 104mmHg
CO2: 40mmHg
H2O: 47mmHg

Question 7

Fick principle

Answer 7

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Question 8

Henry’s law

Answer 8

States that ‘the amount of gas absorbed by a liquid is directly proportional to the positive pressure of the gas to which liquid is exposed and the solubility of the gas in the liquid’ (at constant temperature).

Question 9

Solubility co-efficient

Answer 9

Oxygen: 0.024
CO2 = 0.57

CO2 is 23 times more soluble than O2.

Question 10

Diffusion constant

Answer 10

Solubility/ √MW

The greater the molecular weight of a gas the lower the rate of diffusion (CO2 has a slightly higher MW than O2).

Question 11

PP gradient at respiratory membrane for O2 and CO2.

Answer 11

Oxygen = 104mmHg –> 40mmHg (gradient 64mmHg)

CO2 = 45mmHg –> 40mmHg (gradient 5mmHg)

Question 12

Effect of emphysema on surface area and diffusion distance.

Answer 12

Destruction of alveoli means less surface area for gas exchange.

PaO2 is normal or low. PAO2 is low.

Question 13

Effect of fibrotic lung disease on surface area and diffusion distance.

Answer 13

Thickened alveolar membrane slows gas exchange. Loss of lung compliance may decrease alveolar ventilation.

PaO2 normal or low. PAO2 is low.

Question 14

Effect of pulmonary oedema on surface area and diffusion distance.

Answer 14

Fluid in interstitial space increases diffusion distance. Arterial PCO2 may be normal due to higher CO2 solubility in water.

PAO2 is low.

Question 15

Limits to gas exchange.

Answer 15

Gas exchange is limited by the capacity of a gas to diffuse across the membrane (property of the gas, properties of the membranes).

Gas exchange may also be limited by the rate of perfusion. Circulation of blood through the capillaries is required to maintain concentration gradients at respiratory membrane.

Question 16

Perfusion limited gases

Answer 16

A gas with low solubility will increase the PP in the blood more quickly (faster equilibration with alveoli gas). Gas exchange will be limited by the rate at which ‘new’ blood flows through the capillaries maintaining the pressure gradient.

E.g. O2 and CO2.

Question 17

Diffusion limited gases

Answer 17

A gas with high solubility will increase the partial pressure more slowly (slow equilibration) and therefore gas exchange is limited by how much gas can diffuse across the membrane (diffusion limited).

E.g. CO.

Question 18

V/Q ratio when there is no ventilation.

Answer 18

VA/Q = 0

Units contribute to the venous admixture.

Question 19

V/Q ratio when there is no perfusion.

Answer 19

VA/Q = infinity.

Units become part of dead space.

Question 20

How does the pulmonary circulation accommodate change?

Answer 20

To accommodate change in pressure and flow pulmonary capillaries are capable of undergoing two distinct processes - recruitment and distension.

Question 21

Factors that influence distribution of blood flow

Answer 21

1. Gravity (hydrostatic pressure differences)
2. Alveolar pressure
3. Hypoxia (hypoxic pulmonary vasoconstriction)
4. Neural control (minor effect).

Question 22

Gravity and blood flow.

Answer 22

Low pressure circulation is influenced by gravity more than systemic circulation. In an upright person, blood flow decreases linearly from base to apex due to the effects of gravity. These regions will change depending on posture/position.

The more perfused regions have a higher pressure and cause distention and recruitment of capillaries to accommodate flow. This results in a lower resistance and better perfusion.

Question 23

Upper zone (zone 1)

Answer 23

Alveolar pressure exceeds arterial pressure, so resistance to flow is high and perfusion is absent or at a very low rate (does not usually occur in a healthy lung).

Question 24

Middle zone (zone 2)

Answer 24

Alveolar pressure lies intermediate to arterial and venous pressures, so resistance and perfusion are both moderate. only systolic flow occurs.

Question 25

Lowest zone (zone 3)

Answer 25

Both arterial and venous exceed alveolar pressure, so resistance to flow is low and perfusion rate is high and continuous.

Question 26

When is blood flow impaired in zone 1?

Answer 26

When there is decreased arterial pressure (e.g. haemorrhage) or increased alveolar pressure (e.g. breathing against positive pressure).

Question 27

Affect of hypoxia on blood flow.

Answer 27

A decrease in PO2 of alveolar gas will lead to hypoxic pulmonary vasoconstriction.

Local/automatic response, causing contraction of smooth muscle of small arterioles in affected area (in systemic circulation they would dilate).

This helps to distribute blood flow where it is more effective (i.e. to well ventilated alveoli).

Increased alveolar PCO2 has a similar response.

Question 28

Regional differences in V/Q ratio

Answer 28

The base of the lung receives the greater share of both the pulmonary ventilation and the pulmonary blood flow. Greater change in blood flow than ventilation.

The lower regions of the lung are relatively over-perfused. (V/Q = 0.7).

Upper regions of the lung are relatively over-ventilated. (V/Q = 3).

Question 29

Regional differences alveolar PO2/PCO2

Answer 29

The apex of the lung, being relatively over-ventilated (under perfused) has higher PO2 and lower PCO2 than at the base, which is relatively underventilated (over-perfused).

Question 30

Effect of exercise on V/Q ratio

Answer 30

Exercise, by recruiting more alvoli and more pulmonary capillaries throughout the whole of the lung, but more especially in the apical regions, produces a smaller range of V/Q ratios.