Properties of gases and diffusion Flashcards
What is the kinetic theory of gases?
- 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
What is Boyle’s Law?
- Pressure of a gas is inversely proportional to its volume
What is Dalton’s Law?
- In a mixture of gases, total pressure = sum of partial pressures of individual gases
What is atmospheric pressure?
- 101kPa at sea level
- At high altitude atmospheric pressure is lower
What is partial pressure of O2 at sea level?
- 21.1 kPa
- pO2 = fraction of O2 in air (0.209) x atmospheric pressure (101kPa)
What is partial pressure of O2 at sea level?
- 21.1 kPa
- pO2 = fraction of O2 in air (0.209) x atmospheric pressure (101kPa)
What happens when air enters the upper respiratory tract?
- 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)
How do gases diffuse in the body?
- 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
What is the difference between PAO2 and PaO2?
- PAO2 and PACO2 refer to pressures of these gases in alveoli
- PaO2 and PaCO2 refer to pressures of gases in arterial blood
What is the pO2 and pCO2 in alveolar air?
- 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
Why is alveolar pO2 different from upper respiratory tract pO2?
- 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
Outline how tidal volume affects how much air reaches the respiratory portion of the lungs?
- 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
Outline the behaviour of a gas as it dissolves in a liquid?
- 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
What is KH?
- 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
What is the equation that gives us the amount of a dissolved gas?
- Amount dissolved = partial pressure x solubility coefficient of that gas
- E.g. for O2 = 0.01 x 13.3 = 0.13 mmol O2 dissolved
How does O2 binding to haemoglobin affect its equilibrium?
- 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
What kinds of O2 does blood plasma contain?
- Both dissolved and Hb bound O2
- PO2 is a measure of dissolved O2 in blood
How is O2 moved into tissues?
- 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
What allows gas exchange to occur at the lungs?
- 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
Why is alveolar gas composition steady?
- 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
What are the values for PO2 and PCO2 in venous blood?
- 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
What are the components of the diffusion barrier?
- Fluid film lining alveolus
- Epithelial cell of alveolus
- Interstitial space
- Endothelial cell of capillary
- Plasma
- Red blood cell membrane
Which factors affect the rate of diffusion?
- Partial pressure difference across membrane
- Surface area available for diffusion
- Thickness
- Diffusion coefficient of the individual gas
Diffusion also depends on the properties of the individual gas. Which properties are relevant?
- The solubility of the gas in the liquid - greater solubility = faster rate of diffusion
- Molecular weight of gas - greater molecular weight = slower rate diffusion
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
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
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
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
When is gas exchange optimal?
- V/Q ratio of individual alveolar units is approximately 1
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
What happens when PAO2 is low?
- Hypoxic vasoconstriction of pulmonary arterioles occurs
- This diverts blood to better ventilated alveoli
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)
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
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
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
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.
