Properties of gases and diffusion Flashcards
1
Q
What is the kinetic theory of gases?
A
- Gases are a collection of molecules moving randomly around a space
- Pressure is generated by collisions of molecules with a surface
- More frequent and harder collisions = higher pressure
2
Q
What is Boyle’s Law?
A
- Pressure of a gas is inversely proportional to its volume
3
Q
What is Dalton’s Law?
A
- In a mixture of gases, total pressure = sum of partial pressures of individual gases
4
Q
What is atmospheric pressure?
A
- 101kPa at sea level
- At high altitude atmospheric pressure is lower
5
Q
What is partial pressure of O2 at sea level?
A
- 21.1 kPa
- pO2 = fraction of O2 in air (0.209) x atmospheric pressure (101kPa)
5
Q
What is partial pressure of O2 at sea level?
A
- 21.1 kPa
- pO2 = fraction of O2 in air (0.209) x atmospheric pressure (101kPa)
6
Q
What happens when air enters the upper respiratory tract?
A
- It is humidified
- Water molecules exert air vapour pressure - this is affected by temperature
- At body temperature, saturated vapour pressure = 6.28 kPa
- Water vapour displaces 6.28 kPa of atmospheric gas pressure
- 101 kPa - 6.28 = 94.72 kPa (total gas pressure in upper respiratory tract)
7
Q
How do gases diffuse in the body?
A
- From an area of high partial pressure to an area of low partial pressure
- Not affected by concentrations of gases in gas mixtures or liquids
8
Q
What is the difference between PAO2 and PaO2?
A
- PAO2 and PACO2 refer to pressures of these gases in alveoli
- PaO2 and PaCO2 refer to pressures of gases in arterial blood
9
Q
What is the pO2 and pCO2 in alveolar air?
A
- pO2 = 13.3. kPa (lower than inhaled air)
- pCO2 = 5.3 kPa (higher than inhaled air)
Because:
- Inhaled air mixes with residual air
- Gas exchange is always happening
- Blood equilibrates to these levels
10
Q
Why is alveolar pO2 different from upper respiratory tract pO2?
A
- When we breathe, we do not replace all air in the lungs
- We inhale 350ml of fresh air every time we breathe in
- This represents ~ 10% of all the air in the lungs
- Fresh air is diluted by old air in lungs
- Older air has O2 being continually extracted, and CO2 constantly being added
- Therefore, PAO2 is lower than pO2 in URT
11
Q
Outline how tidal volume affects how much air reaches the respiratory portion of the lungs?
A
- Typically we inhale/exhale ~500ml air at rest (tidal volume)
- In healthy lungs ~30% (150ml) of normal tidal volume fills anatomical dead space
- 350ml of air reaches respiratory portion of lung - this is alveolar ventilation
12
Q
Outline the behaviour of a gas as it dissolves in a liquid?
A
- Equilibrium is reached when rate of gas entering water = rate of gas leaving water
- At equilibrium partial pressure of the gas in the liquid = partial pressure of the gas in the air above it
13
Q
What is KH?
A
- Henry’s constant
- The solubility of a gas in a liquid at a defined temperature
- In the human body KH is the solubility of a gas at body temp in plasma
14
Q
What is the equation that gives us the amount of a dissolved gas?
A
- Amount dissolved = partial pressure x solubility coefficient of that gas
- E.g. for O2 = 0.01 x 13.3 = 0.13 mmol O2 dissolved
15
Q
How does O2 binding to haemoglobin affect its equilibrium?
A
- The reaction of Hb and O2 must be completed before equilibrium can be reached and partial pressure established
- Initially O2 dissolves in plasma and binds to Hb in RBCs
- Once Hb is fully saturated, O2 continues to dissolve in plasma until equilibrium is recahed
16
Q
What kinds of O2 does blood plasma contain?
A
- Both dissolved and Hb bound O2
- PO2 is a measure of dissolved O2 in blood
17
Q
How is O2 moved into tissues?
A
- Dissolved O2 is available to diffuse into tissues down its partial pressure gradient
- As dissolved O2 leaves the blood, it is replaced by O2 bound to Hb
18
Q
What allows gas exchange to occur at the lungs?
A
- Low oxygen partial pressure and high CO2 partial pressure in venous blood allows oxygen to diffuse from lungs into blood and CO2 to diffuse from blood into lungs
19
Q
Why is alveolar gas composition steady?
A
- Amount of O2 brought in by ventilation = amount of O2 diffusing into blood
- Amount of CO2 removed by ventilation = amount of CO2 diffusing from blood into alveolus
20
Q
What are the values for PO2 and PCO2 in venous blood?
A
- pO2 = ~6.0 kPa
- pCO2 = ~ 6.1 kPa
- pO2 is lower in mixed venous blood than in alveoli
- pCO2 is higher in mixed venous blood than in alveoli
21
Q
What are the components of the diffusion barrier?
A
- Fluid film lining alveolus
- Epithelial cell of alveolus
- Interstitial space
- Endothelial cell of capillary
- Plasma
- Red blood cell membrane
22
Q
Which factors affect the rate of diffusion?
A
- Partial pressure difference across membrane
- Surface area available for diffusion
- Thickness
- Diffusion coefficient of the individual gas
23
Q
Diffusion also depends on the properties of the individual gas. Which properties are relevant?
A
- The solubility of the gas in the liquid - greater solubility = faster rate of diffusion
- Molecular weight of gas - greater molecular weight = slower rate diffusion
24
Compare the diffusion of O2 with the diffusion of CO2
- CO2 has a greater molecular weight than O2 but is much more soluble
- CO2 diffuses 20x faster than O2
- Larger difference in partial pressures compensates for slower diffusion of O2
- In a diseased lung, O2 gas exchange is more impaired than CO2 gas exchange
25
Outline the factors affecting gas exchange in the lungs
- Surface area of alveolar capillary membrane is about 100m^2
- Barrier <0.4 micrometers thick
- Oxygen exchange complete in 1/3 of the time blood spends in the capillary
- Have plenty of reserve for exercise
26
How is the thickness of the membrane in the lungs affected by disease?
- Increases as a result of oedema fluid in the interstitial space and in alveoli
- Lung fibrosis increases thickness of alveolar capillary membrane
27
How is the surface area of the membrane in the lungs affected by disease?
- Decreased by removal of an entire lung
- Emphysema decreases surface area
28
When is gas exchange optimal?
- V/Q ratio of individual alveolar units is approximately 1
29
What do we mean when we say that ventilation and perfusion should be matched throughout the lung?
- There are many alveoli that all may have widely differing amounts of ventilation and perfusion in different alveolar units
- Alveoli with increased ventilation should have increased perfusion and vice versa
30
What happens when PAO2 is low?
- Hypoxic vasoconstriction of pulmonary arterioles occurs
- This diverts blood to better ventilated alveoli
31
What can cause V/Q mismatch with V/Q greater than 1?
- Asthma
- COPD early stages
- Pneumonia
- RDS in new-born (some alveoli not expanded)
- Pulmonary oedema (fluid in alveoli)
32
What is the pulmonary shunt?
- Triggered by a smooth muscle reflex as a consequence of low O2 concentration
- Minimised by normal reflex pulmonary hypoxic vasoconstriction
- diverts blood away from poorly ventilated areas
33
Why is there no use in prescribing 100% inspired oxygen to a patient with pulmonary shunting?
- 100% inspired oxygen is unable to overcome the hypoxia caused by shunting
- The alveoli being oxygenated with 100% O2 have no perfusion due to the shunt, so O2 will not pass into the capillaries
34
What are the consequences of inadequate ventilation of an alveolar unit?
- PACO2 increases
- PAO2 falls
- A new steady state is established
- Diffusion continues at this new level
- Blood equilibrates at the new alveolar PAO2 and PACO2
- In mixed blood pO2 is low and pCO2 is high
35
Outline VQ mismatch where perfusion is affected but ventilation is ok
- A decrease in perfusion relative to ventilation is an example of increased dead space
- E.g. pulmonary embolus blocking a capillary
- Can be corrected by supplying 100% inspired oxygen - blood will redistribute between other capillaries that are exchanging gases without issue.
