Principles of gas exhange Flashcards
What is ECMO?
A treatment that uses a pump to circulate blood through an artificial lung back into the bloodstream of a very ill baby.
Equation for rate
Rate of diffusion directly proportional to A x ΔP
What is diffusion dependent on
Diffusion is also dependent on the physicochemical properties of the gas and nature of the membrane (thickness, physicochemical properties- what the membrane is made up of).
Pressure gradient
The pressure gradient is the difference in partial pressures of the respective gases in the alveolus and the blood.
Partial pressure of a gas in a mixture of gases
The partial pressure of a gas in a mixture of gases is the pressure that it would exert if it was the only gas in the container
What is Dalton’s law?
The partial pressure of a gas in a gas mixture = Total pressure x fractional concentration of the gas
Why does partial pressure decrease the further you go up?
Higher you go up the earth less partial pressure- 20 tonnes of air above us, further up less air so less partial pressure.
Is partial pressure of oxygen in alveoli lower then in room air? Why?
Yes- Lower than in room air because our gases we breathe in mix with our gases we breathe out
Why does inspired air need to be humidified?
Inspired air is humidified in the upper airway- need to do this as dry air causes drying out of tissues- which causes it to become the saturated vapour pressure of water
What happens to the partial pressure in the alveoli when CO2 is added?
In the alveoli oxygen is taken up while carbon dioxide is added- CO2 dilutes the amount of O2 coming in so partial pressure drops
Why does the body consume more oxygen than it produces CO2?
This is due to the R quotient dependent on your diet e.g. carbohydrates for every molecule of oxygen you produce 1 molecule of CO2 – diets have R quotient of about 0.8 means for every 1 molecule of CO2 you produced you consume 1.2 molecules of oxygen as fat has less oxygen than carbohydrate so need more water to produce oxygen and CO2.
What effect does humidification have on partial pressure?
• At 37oC the saturated vapour pressure of water ≈ 6.3 kPa
• i.e., the partial pressure of water vapour is 6.3 kPa as it gets down alveolus
• Therefore, in fully humidified air at 37oC the partial pressure of oxygen =
(101.3 (partial pressure at sea level) – 6.3) x 0.21 (fractional conc. of oxygen) = 19.95 kPa
What effect does CO2 have on partial pressure?
- A typical value of the alveolar partial pressure of CO2 is 5 kPa
- If one CO2 molecule was produced for every O2 molecule consumed the alveolar partial pressure of oxygen would be 19.95 – 5 = 14.95 kPa
- But, typically 1.25 x as many O2 molecules are consumed as CO2 molecules produced – respiratory quotient as produce less carbon dioxide than oxygen consumed
- Thus, alveolar partial pressure of oxygen = 19.95 – 6.25 = 13.7 kPa
What is a shunt?
A shunt is a hole or a small passage which moves, or allows movement of, fluid from one part of the body to another.
What is a cardiac shunt?
Where the alveoli oxygen conc. is 13.5kPa and by the time it gets back to the heart, have a cardiac shunt where some of the blood from the right V drops straight into the L atrium which is deoxygenated so when blood comes out of artery is 13.3 kPa.
