Session 3: Gas Exchange - Properties of Gases and Diffusion in the Lung Flashcards
Boyle’s Law
Pressure of a fixed quantity of gas at a constant temperature is inversely proportional to its volume.
Dalton’s Law
This explains the partial pressure of a gas in a gas mixture. Each component gas exerts a partial pressure in proportion to its volume percentage in the mixture.
Explain the partial pressure of a gas in liquid.
When a gas mixture is in contact with a liquid, gas molecules enter the liquid to dissolve. At sometime molecules which entered the liquid will leave to return to the gas phase. Collision of the gas molecules with the walls of the container generates a pressure within the liquid, this is partial pressure of a gas in the liquid.
When is a system of partial pressure of a gas in a liquid at equilibrium?
When the rate of a gas coming out of solution is equal to the rate at which it enters the solution. At equilibrium, the partial pressure of the gas in the liquid is equal to the partial pressure of that gas in the gas phase in contact with the liquid.
Explain Henry’s Law.
The amount of a gas that dissolves in a specific volume of liquid is proportional to the partial pressure of that gas in gas phase and its solubility coefficient. Py x cy = ny
What is the solubility coefficient?
Amount in mmol of a gas that will dissolve in a litre of plasma at 37 degrees celsius when exposed to a given partial pressure.
What is a normal partial pressure of oxygen (pO2) in the alveolar air?
13.3 kPa
Given the normal partial pressure of oxygen in alveolar air, and the solubility coefficient of oxygen is 0.01. What is the O2 content of plasma?
13.3 x 0.01 = 0.13 mmol/litre
Is the solubility coefficient the same for all gases?
No
Which is higher; solubility coefficient of O2 or CO2? What does this mean?
CO2 is higher. That CO2 is much more soluble in plasma than O2. This means that a given volume of plasma would accommodate more CO2 molecules within it before equilibrium has been reached.
Is the O2 content of the blood actually 0.13 mmol/litre?
No, just the plasma. For the whole blood we need to take into account the gas which has been dissolved and then combined chemically with the liquid. Amount bound to haemoglobin is approx 8.8 mmol/l. This gives an O2 content of the blood to be 8.93 mmol/l. When the O2 is bound to haemoglobin it doesn’t add to the partial pressure.
What is atmospherical partial pressure of O2?
101 x 0.21 = 21.3 kPa
What is the pO2 in the respiratory tract?
20 kPa
How come the pO2 of the respiratory tract is lower than atmospheric?
Because water is added to the air. This is saturated vapour pressure.
Explain saturated vapour pressure.
When a gas comes in contact with water, some water will evaporate (gas) and some will condense (water). There will be an equilibrium where rate of condensation is the same as the rate of evaporation. At this equilibrium the gas mixture is saturated with water vapour and the pressure that exerts is the saturated vapour pressure. This will take away a fraction of the total pressure of 101 kPa that air is at sea level.
What is SVP at 37 degrees?
6.28 kPa
Calculate the pO2 in the respiratory tract.
101 kPa - 6.28 kPa = 94.72 kPa. 94.72 kPa x 0.21 = 19.9 kPa.
Why is the pO2 of the alveoli 13.3 kPa and not 19.9 kPa?
Because alveolar pO2 is determined by the rate at which O2 is taken up by the blood and the rate at which it is replenished by alveolar ventilation. This means that there is a balance between perfusion and ventilation.
Does hyper or hypoventilation change alveolar pO2?
Yes
Normal partial pressure of CO2 in the alveoli
5.3 kPa
Does hyper or hypoventilation affect alveolar pCO2?
Yes
Three factors that affects gas exchange/rate of diffusion
Area available for exchange (surface area) Resistance to diffusion (distance) Gradient of partial pressure
How does surface area affect gas exchange?
Huge number of alveoli cause a large surface area to around 70 m2. This enables more area for gas exchange.
Diffusion pathway for O2.
Alveolar film of fluid
Alveolar epithelium
Alveolar interstitium
Endothelium of capillary
Plasma of blood
Red cell membrane
In total 5 cell membranes, 3 layers of cytoplasm and 2 layers of tissue fluid and plasma.
Overall thickness of the diffusion pathway
0.6 microns
What factors affects the rate of diffusion in disease?
Thickness of membrane/space
Surface area
Diffusion coefficient of the gas
What else is the rate of diffusion affected by?
Solubility of the gas in the water.
How does the rate of diffusion differ between O2 and CO2?
Since CO2 is much more soluble it will have a higher rate of diffusion. The higher gradient of partial pressure of O2 evens this out a little bit. However the rate of diffusion for CO2 is still 21 times faster.
What does the rate of diffusion difference between O2 and CO2 mean in a patient with lung disease causing a diffusion defect?
That O2 will be much more affected than CO2. This means that the patient will be hypoxic but not necessarily have any issues with the exchange of CO2.
Give examples of diseases causing diffusion defects.
Interstitial lung disease
Pulmonary oedema
Emphysema
Explain how interstitial lung disease can cause decrease in rate of diffusion.
Excessive deposition of collagen in the interstitial space along with thickening of the alveolar walls causes lengthening of the diffusion pathway.
Explain how pulmonary oedema can cause decrease in rate of diffusion.
Fluid in the interstitium and alveolus increases the length of the diffusion pathway.
Explain how emphysema can cause a decrease in rate of diffusion.
Destruction of alveolar walls results in large airspaces. This will reduce the total surface area that is available for gas exchange.
What is diffusion resistance measured by?
Carbon Monoxide Transfer Factor
Explain the Carbon Monoxide Transfer Factor method.
Measuring CO uptake following a single maximal breath of a gas mixture containing air, 14% Helium and 0.1% Carbon monoxide.
Inhaled CO is used because it has a very high affinity for Hb. This means that almost all CO entering the blood will bind to Hb and hardly any CO will be found in the plasma.
This means that the concentration gradient for paCO across the alveolar capillary membrane is maintained for the entire time blood remains in contact with alveolar gas.
The amount of CO transferred from alveoli to the blood is an estimate of the diffusion resistance of the barrier.
