4. Properties of gases and Gas exchange in the lung Flashcards
what is boyle’s law?
The pressure of a fixed quantity of gas at a constant temperature (e.g. body temperature) is inversely proportional to its volume.
What is Gas pressure caused by?
the collisions of gas particles with the walls of the container
What is partial pressure?
The pressure exerted by a particular gas in a mixture of gases (proportional to its volume percentage in the mixture)
What is the atmospheric pressure At sea level?
101.1 kPa = 1 atmosphere = 760 mmHg
What percentage of air is made up of oxygen and therefore what is its partial pressure?
20.9%. pO2 = 101.1 x 0.209 = 21.1 kPa
Describe how to calculate Partial Pressure of a Gas in a Gas Mixture
Multiply the fractional percentage (percent converted to a decimal) of the individual gas by the total pressure exerted by the mix of gases
(% gas A)(total pressure) = partial pressure gas A
What should be assumed when calculating partial pressure of a gas in a gas micture?
Assume there are no chemical reactions between the gases
What is dalton’s law?
In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.
What is atmospheric pressure?
• pressure exerted by the weight of the air above the earth in the atmosphere
How does the atmospheric pressure change at higher altitudes?
goes lower
Which gas makes up 78% of air?
nitrogen
What happens to inhaled air in the nostril?
When we breathe in air it is moistened – water molecules in vapour form are added to air
What is the result of moistening air in the airways?
Water vapour pressure – called saturated vapour pressure- displaces a proportion of the total pressure of the mixture of gases – in this case, total atmospheric pressure
What is the saturated water pressure dependant upon?
temperature - does not change with altitude
What is the saturated water pressure at body temp?
6.28 kPa
How does the saturated water pressure affect the calculation of Partial Pressure of a Gas in a Gas Mixture?
- need to subtract the water vapour pressure from the total pressure of the dry gas mixture to arrive at the total pressure of gas mixture in a moist environment
- therefore 101(kPa) – 6.28 (kPa) = 94.7 kPa =total pressure of gas mixture in a moist environment
- The other gases are still in the same ratios as in dry air
• pO2 = 94.7 kPa x .209 = 19.8 kPa= partial pressure oxygen in URT
What is alveolar partial pressure of oxygen?
13.3 kPa
Why is the alveolar partial pressure of oxygen different in URT and alveoli?
- Oxygen from alveolar air constantly diffusing into pulmonary circulation
- Alveolar air only partially replaced with each breath - certain amount of air remain in alveoli and not fully emptied so the Fresh air diluted by residual already in the lungs which is constantly having its oxygen removed
What determines alveolar pO2?
Rate at which O2 is taken up by the blood and the rate at which it is replenished by alveolar ventilation
What determines alveolar pCO2?
Alveolar pCO2 is determined by the rate at which CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation
is the alveolar partial pressure of carbon dioxide different in URT and alveoli?
yes
What is the typical value for tidal volume, how much of this reaches the alveoli and what happens to the rest?
450ml, 300ml reaches alveoli, 150ml (30%) remains in anatomical dead space
What is the anatomical dead space?
Volume of non-gas exchange surface in the respiratory system (trachea, bronchi, bronchioles)
Compare Pulmonary ventilation and alveolar ventilation
- Pulmonary ventilation - amount of air we bring into our lungs
- alveolar ventilation - volume of air that reaches respiratory portion of lung after minusing the 30% that remain in the anatomical dead space
Compare Pulmonary Minute Ventilation and alveolar Minute Ventilation
• Pulmonary Minute Ventilation – We breathe ≈ 12 times a minute - the total volume of air moved is 450 ml × 12 = 5400 ml/min
• Alveolar Minute Ventilation (ml/min) is the rate of air flow reaching the alveoli
– 12 breathes/minute × 300 ml = 3600 ml/min
What happens when inspired gases come in contact with body fluids?
gas molecules will enter fluid and dissolve in the liquid
What determines amt of gas that dissolves in liquid?
directly proportional to the partial pressure of that gas - higher the partial pressure,
When is equilibrium reached in a gas-liquid interface?
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.
(rate gas entering liquid = rate gas leaving liquid)
Down what gradient will A gas in air dissolve into a fluid/
along its partial pressure gradient – from high partial pressure to low
What two factors determine the rate out of liquid to gas?
- conc of dissolved gas
2. solubility of gas for liquid medium
When equilibrium is reached in a gas-liquid interface, will the concentration of the gas be the same in the ags phase and liquid phase?
No
- if a molecule has a high solubility to the liquid, a higher concentration gradient is needed for the molecule to move out of the liquid and for equilibrium to be reached - liquid has higher conc
- if a molecule has a low solubility to the liquid, only a low conc gradient is needed for it to move out of the liquid to gas, and to reach equilibrium - gas has higher conc
explains why ALVEOLAR gas partial pressures determine ARTERIAL gas partial pressures
- Alveolar partial pressure oxygen > partial pressure of oxygen in the venous blood arriving at the alveoli
- Oxygen gas diffuses and dissolves down its partial pressure gradient into the blood
- Equilibrates with blood
- Partial pressure oxygen in blood *leaving alveoli (which will become arterial blood) is now equal to partial pressure oxygen in alveoli
- The partial pressure of oxygen in arterial blood is the equilibrium derived partial pressure established at the air-fluid interface – i.e. at the alveolar-blood interface
What factors determine Dissolved Gas Content?
- Depends on the partial pressure of the gas
- And
- The solubility coefficient of that gas
What is the solubility coefficient?
Amount (in mmol) of a gas that will dissolve in a
litre of plasma at 370C (body temperature), when exposed to a given partial pressure
