What is Dalton's law?
In mixture of gases- each component exerts partial pressure
What unit do we use to measure pressure in medicine?
What % oxygen does air contain?
If the partial pressure of oxygen in alveolar air is 13.3% and the air is allowed to equilibrate with the blood in the pulmonary capillaries, what will be the partial pressure of oxygen in the blood leaving the alveolus?
(Atmospheric pressure= 101.1 kPa)
Partial pressure of oxygen in the blood leaving the alveolus: 13kPa
Partial pressure of a gas and content of dissolved gas in a liquid are not the same. Henry's law shows the relationship between them. What is Henry's law?
Dissolved gas in liquid (mmol/L) = partial pressure of gas (kPa) x solubility coefficient of gas
What is the solubility coefficient?
Amount of gas that will dissolve in 1L of plasma at 37 degrees C, when exposed to given partial pressure
How does the solubility coefficent of CO2 differ from that of O2?
Solubility coefficent of CO2 > Solubility coefficent of O2
What is the standard vapour pressure of water vapour at body temperature (37 degrees C) ?
Where does inhaled air become saturated with water vapour?
Upper respiratory tract.
The normal value of partial pressue of oxygen in the alveolar gas is about 13.3 kPa. Since atmospheric partial pressure of oxygen is about 21 kPa, explain the difference.
- Inhaled air saturated with water vapour- partial pressure of O2 = 19.8 kPa
- In alveoli- O2 taken up by blood- so partial pressure lower
What is the normal value for the partial pressure of CO2 in the alveolar gas?
What is meant by 'mixed venous blood'?
Blood from superior vena cava, inferior vena cava and coronary sinus
(ie deoxgenated venous blood from body)
What are the partial pressures of oxygen and carbon dioxide in the mixed venous blood reaching the capillaries?
Name the 3 factors which determine the the rate of gas exchange in the alveoli.
- Area available for exchange
- Resistance to diffusion
- Gradient of partial pressure
There are 5 key structures which the gas must diffuse through from entering the alveoli to entering a red blood cell. What are these structures?
What is the thickness of this barrier overall?
- Alveolar epithelia cell
- Interstitial fluid
- Capillary endothelilal cell
- Red cell membrane
Thickness: 0.6 micron
How much faster does carbon dioxide diffuse compared to oxygen?
21 x faster
Diseases causing diffusion defects will affect oxygen concentration first and will only start to affect carbon dioxide concentration towards the end stages of the disease. Why is this?
CO2 diffuses 21 times faster than O2 for given gradient.
Once the blood enters an alveolar capillary, how long does it take for the blood to become fully saturated with oxygen?
(1/3rd of way along)
Identify some lung diseases which cause diffusion defects:
- Interstitial lung disease
- Pulmonary oedema (fluid in interstitium and alveolus)
- Emphysema- reduces total SA
How is diffusion resistance calculated?
Measuring CO uptake
following single, maximal breath of gas mixture
containing 14% helium and 0.1% CO
(almost all CO entering blood binds to Hb)
(Difference between PaCO and PACO does not change for whole time blood in contact with alveoli)
amount transfered from alveoli to blood= estimate of diffusion distance
Convert 101 kPa to mmHg and atms.
101 kPa = 760 mmHg = 1 atm
What is oxygen tension?
Partial pressure of oxygen in liquid
(Tension= partial pressure of gas in liquid)
The presence of haemoglobin in the blood increases the oxygen carrying capacity of the blood by how much?
What is the solubility co efficient for CO2 and Oxygen? What are their partial pressures in the alveoli?
O2= 0.13 mmol/L
Partial pressure in alveoli is 13.3 kPa
CO2= 0.23 mmol/L
Partial pressure in alveoli is 5.3 kPa