Lecture 4 - Principles of gas exchange Flashcards
How are O2 andCO2 eliminated from the blood?
through simple diffusion
rate of diffusion is…
proportional to the surface area and pressure gradient
what is diffusion also dependent on
on the physiochemical properties of the gas and nature of the membrane (thickness etc)
What is lung tissue made of
simple squamous epithelial cells
What is pressure gradient
difference in the partial pressures of the respective gases in the alveolus and the blood.
partial pressure of a gas in a mixture of gases is the pressure it would exert it is was the only gas in the contained
Universal gas equation
pV=nRT
What is daltons law
The partial pressure of a gas in a gas mixture = Total pressure x fractional concentration of a gas
Partial pressures of gases in the atmosphere
Nitrogen - 79% PP at high altitude - 44.2
Carbon dioxide - always 0
Oxygen - 215 and at high pp 11.8%
Why is partial pressure of oxygen in alveoli less than in room air?
inspired air is humidified in the upper airway
In the alveoli, oxygen is taken up while co2 is added/
body consumed more o2 molecules than it produces CO2
At 37 degrees celcius, the saturated vapour pressure of water is?
6.3 kPa that is also the partial pressure
What is the partial pressure of oxygen in fully humidified air at 37 degrees celcius
[101.3 (total partial pressure at sea level) - 6.3 (PP of water)] x 0.21 (fraction concentration of oxygen = 19.95 kPa . this is less than atmospheric partial pressure
What is the effect of CO2
co2 decreases alveolar partial pressure of oxygen further as 1.25x as many O2 molecules are consumed as CO2 molecules produced.
Alveolar partial pressure of oxygen = 19.95 - (1.25x5) = 13.7 kPa
Alveolar partial pressure of CO2
5 kPa
What is the relationship between partial pressure and solubility
partial pressure of a gas in solution is inversely proportional to its solubility. greater the solubility, more molecules can be accommodated for a given partial pressure
Compare CO2 solubility to that of oxygen
it is 24 times more soluble than oxygen
Rate of diffusion of O2 and CO2
In theory, CO2 transfer is much more efficient than O2 transfer
What happens when the blood is non-oxygenated in a healthy person at rest in terms of partial pressures
PO2 is less hence there is more solubility - 5.3kPa
PCO2 is 6.1 kPa which is more than oxygen so it leaves the blood to the alveoli to get oxygenated
What happens to partial pressures when the co2 leaves the blood to enter the aveoli and oxygen re-enters the blood
the partial pressure of oxygen increases as oxygenated diffuses back into the blood and co2 leaves it
so the reoxygenated blood has same partial pressures as the alveoli -
p02= 13.3 kPa and PCO2 = 5.3 kPa
what happens to partial pressures in a healthy person during exercise
po2 of non-oxygenated blood is less than at rest - 4.3 kPa
Oxygen enters blood from alveoli to reinstate partial pressure to 13.3 kPa
what happens to partial pressures in a person with pulmonary fibrosis during exercise
the partial pressure of oxygen in non-oxygenated blood is still less - 4.3 kPa
Due to alveolar thickening during pulmonary fibrosis, there is less diffusion of oxygen back into the blood and so pO2 = 9.5 kPa
Clinical measurement of diffusing capacity
oxygen diffusing capacity = oxygen uptake/ (alveolar pO2 - mean pulmonary capillary pO2)
What is carbon monoxide diffusing capacity
carbon monoxide uptake /alveolar Pco
mean pulmonary capillary PC0 is effectively zero because of its high affinity for haemoglobin and thats why it is not there in the equation
Alveolar capillary destruction is
emphysema
Alveolar collapse
atelactasis
Thickening of the alveolar wall
alveolar fibrosis
Alveolar consolidation (filling with liquid can be due to swelling caused by infection)
pneumonia
Frothy secretions
pulmonary edema
