Gas Properties + Exchange In Lungs Flashcards
What happens when a gas mixture is in contact with a liquid?
When is the system in eqm?
- 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
What is the partial pressure of gas in a liquid?
Pressure generated by collision of dissolved gas with the wall(s) of a container
At eqm, how is the partial pressure of gas in a liquid related to partial pressure of the gas in gas phase
Equal partial pressures
What affects the amount of a gas that can be dissolved in a specific volume of liquid
- Partial pressure of gas in gas phase
- Solubility coefficient of the gas
(Amount dissolved= SC*PP)
What is the Solubility Coefficient of a gas?
What are they dependent on?
- Amount of a gas that will dissolve in a litre of plasma at 37 degrees, when exposed to a given partial pressure
- Temperature
Different gases have different solubility coefficients.
Name 1 gas more soluble in plasma than O2
CO2 (By 20-21 times, despite larger molecular weight which does have a slowing effect)
How do you determine Total Gas Content of a liquid if the gas combines chemically with it?
Amount of gas chemically bound + amount of gas dissolved/ in free solution
What happens when gas is in contact with WATER
What happens when system is in eqm
- Some water evaporates to enter gas phase, some condenses to return to liquid phase
- When rate of evaporation= rate of condensation
What is Saturated/ Water Vapour Pressure? (6.28kPa)
What is it dependent on?
- Pressure exerted by water in gaseous state, when in eqm
- Temperature only
How do you calculate partial pressure of the humidified air in the airways?
Subtract water vapour pressure from atmospheric pressure, and multiply by volume percentage of O2
For reference: (101kPa-6.28kPa)*20.9%
What 2 things determine Alveolar pO2 (PAO2)?
This is 13.8kPa, which is lower than in URT at 19.8kPa
- 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)
What 2 things determine PACO2? (5.3kPa)
- 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)
If 450ml is Tidal Volume, how much air reaches alveoli?
Therefore state the;
- Pulmonary Minute Ventilation/ Ventilation rate
- Alveolar Minute Ventilation/ Ventilation rare
300ml (30% stays in anatomical dead space)
- P: 12 breaths a minute-> 12*450-> 5400 ml/min
- A: 12””-> 12*300-> 3600 ml/min
State the partial pressures of O2 and CO2 of blood leaving alveoli
pO2: 13.3kPa (Same as alveolar)
pCO2: 5.3kPa (Same as alveolar)
What are the layers that must be crossed by gas diffusing from alveolar air to RBCs?
- Fluid film living alveolus
- Alveolar epithelial cell membranes
- Interstitial fluid
- Capillary endothelial cell membrane
- Plasma
- RBC membrane
(Overall thickness is 0.6 microns)
How would lung disease causing a diffusion defect affect CO2 and O2 diffusion?
- O2: Reduced-> Hypoxaemia
- CO2: Unaffected directly (as CO2 always diffuses faster)
What are 2 examples of types of diffusion defects
- Thickened barrier
- Reduced SA for gas exchange
Name 3 diseases causing diffusion defects
- 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
What can we use to estimate diffusion resistance?
Can use CO due to its very high affinity for Hb
DLCO- Diffusing capacity of long for carbon monoxide
With regards to Ventilation- Perfusion compare the terms ‘shunt’ and ‘deadspace’
Shunt: Perfusion of an unventilated alveolus- blood is wasted (V/Q ratio of 0)
Deadpsace: Ventilation of an unperfused alveolus- air is wasted
Describe 2 examples of V/Q Mismatch
This can lead to hypoxaemia
- V/Q< 1: Perfusion greater than Ventilation, will be limited gas exchange
- V/Q> 1: Ventilation greater than Perfusion, will be limited gas exchange
Under normal conditions, there is a small degree of V/Q Mismatch.
Give 2 reasons why
- Lung perfusion is somewhat dependent on gravity
- Ventilation is dependent on lung stretch, which is also gravity dependent IN THE UPRIGHT POSITION
What is the V/Q ratio of a healthy resting lung
What does it increase to during exercise?
Give 2 reasons how?
0.8-0.9
Increases to 1.0
- Increased blood flow to lung
- Increased recruitment of alveoli in lung bases
How does increased ventilation affect O2 and CO2 concentrations
What about in people with severe lung disease?
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
