Gas Properties + Exchange In Lungs

Question 1

What happens when a gas mixture is in contact with a liquid?

When is the system in eqm?

Answer 1

• Gas molecules enter liquid to dissolve, some of which return to gas phase
• In eqm, when rate of gas entering solution= rate of gas leaving solution

Question 2

What is the partial pressure of gas in a liquid?

Answer 2

Pressure generated by collision of dissolved gas with the wall(s) of a container

Question 3

At eqm, how is the partial pressure of gas in a liquid related to partial pressure of the gas in gas phase

Answer 3

Equal partial pressures

Question 4

What affects the amount of a gas that can be dissolved in a specific volume of liquid

Answer 4

• Partial pressure of gas in gas phase
• Solubility coefficient of the gas

(Amount dissolved= SC*PP)

Question 5

What is the Solubility Coefficient of a gas?

What are they dependent on?

Answer 5

• Amount of a gas that will dissolve in a litre of plasma at 37 degrees, when exposed to a given partial pressure
• Temperature

Question 6

Different gases have different solubility coefficients.

Name 1 gas more soluble in plasma than O2

Answer 6

CO2 (By 20-21 times, despite larger molecular weight which does have a slowing effect)

Question 7

How do you determine Total Gas Content of a liquid if the gas combines chemically with it?

Answer 7

Amount of gas chemically bound + amount of gas dissolved/ in free solution

Question 8

What happens when gas is in contact with WATER

What happens when system is in eqm

Answer 8

• Some water evaporates to enter gas phase, some condenses to return to liquid phase
• When rate of evaporation= rate of condensation

Question 9

What is Saturated/ Water Vapour Pressure? (6.28kPa)

What is it dependent on?

Answer 9

• Pressure exerted by water in gaseous state, when in eqm

- Temperature only

Question 10

How do you calculate partial pressure of the humidified air in the airways?

Answer 10

Subtract water vapour pressure from atmospheric pressure, and multiply by volume percentage of O2

For reference: (101kPa-6.28kPa)*20.9%

Question 11

What 2 things determine Alveolar pO2 (PAO2)?

This is 13.8kPa, which is lower than in URT at 19.8kPa

Answer 11

• Rate at which O2 is taken up by blood
• Rate at which alveolar O2 is replenished by ventilation

(PAO2 can be changed by hypo/ hyperventilation)

Question 12

What 2 things determine PACO2? (5.3kPa)

Answer 12

• Rate at which CO2 enters alveoli from blood
• Rate at which CO2 is removed from alveolar gas by ventilation

(Hypo/ hyperventilation will change alveolar and therefore arterial PCO2)

Question 13

If 450ml is Tidal Volume, how much air reaches alveoli?

Therefore state the;

• Pulmonary Minute Ventilation/ Ventilation rate
• Alveolar Minute Ventilation/ Ventilation rare

Answer 13

300ml (30% stays in anatomical dead space)

• P: 12 breaths a minute-> 12*450-> 5400 ml/min
• A: 12””-> 12*300-> 3600 ml/min

Question 14

State the partial pressures of O2 and CO2 of blood leaving alveoli

Answer 14

pO2: 13.3kPa (Same as alveolar)
pCO2: 5.3kPa (Same as alveolar)

Question 15

What are the layers that must be crossed by gas diffusing from alveolar air to RBCs?

Answer 15

• Fluid film living alveolus
• Alveolar epithelial cell membranes
• Interstitial fluid
• Capillary endothelial cell membrane
• Plasma
• RBC membrane

(Overall thickness is 0.6 microns)

Question 16

How would lung disease causing a diffusion defect affect CO2 and O2 diffusion?

Answer 16

• O2: Reduced-> Hypoxaemia

- CO2: Unaffected directly (as CO2 always diffuses faster)

Question 17

What are 2 examples of types of diffusion defects

Answer 17

• Thickened barrier

- Reduced SA for gas exchange

Question 18

Name 3 diseases causing diffusion defects

Answer 18

• Interstitial Lung Disease: Thickened alveolar membrane + interstitial collagen deposition increases length of diffusion pathway
• Emphysema: Less alveoli= reduced SA for gas exchange
• Pulmonary Oedema: Fluid in interstitium and alveoli increases length of diffusion pathway

Question 19

What can we use to estimate diffusion resistance?

Answer 19

Can use CO due to its very high affinity for Hb

DLCO- Diffusing capacity of long for carbon monoxide

Question 20

With regards to Ventilation- Perfusion compare the terms ‘shunt’ and ‘deadspace’

Answer 20

Shunt: Perfusion of an unventilated alveolus- blood is wasted (V/Q ratio of 0)

Deadpsace: Ventilation of an unperfused alveolus- air is wasted

Question 21

Describe 2 examples of V/Q Mismatch

This can lead to hypoxaemia

Answer 21

• V/Q< 1: Perfusion greater than Ventilation, will be limited gas exchange
• V/Q> 1: Ventilation greater than Perfusion, will be limited gas exchange

Question 22

Under normal conditions, there is a small degree of V/Q Mismatch.

Give 2 reasons why

Answer 22

• Lung perfusion is somewhat dependent on gravity

- Ventilation is dependent on lung stretch, which is also gravity dependent IN THE UPRIGHT POSITION

Question 23

What is the V/Q ratio of a healthy resting lung

What does it increase to during exercise?
Give 2 reasons how?

Answer 23

0.8-0.9

Increases to 1.0

• Increased blood flow to lung
• Increased recruitment of alveoli in lung bases

Question 24

How does increased ventilation affect O2 and CO2 concentrations

What about in people with severe lung disease?

Answer 24

O2: Increased pO2, but only a little extra total O2 content

CO2: Significantly more eliminated from blood, due to steeper slop of CO2 curve

With lung disease, also causes poor CO2 elimination-> Hypoxaemia as well as Hypercapnia

Question 25

Describe the compensatory mechanism that acts to avoid V/Q mismatch

Answer 25

• Localised lung hypoxaemia causes arterial vasoconstriction to that region
• Blood diverted to better ventilated alveoli

Question 26

What are 3 limitations to the V/Q Compensation mechanism?

Answer 26

• Diverted blood has a limit to how much extra O2 can be carried before Hb is saturated AND not all blood is diverted
• Less well ventilated alveoli to divert blood to, in people with severe lung disease
• Certain lung diseases disable lungs ability to effectively vasoconstrict the vessels supplying poorly/ non-ventilated alveoli

Question 27

Does over-ventilation of one alveolus fully compensate for the disturbances created by under-ventilating the other alveolus?

Answer 27

No