Gases and Gas Exchange Flashcards
What is Boyle’s law?
Pressure of a gas is inversely proportional to its volume
In a mixture of non reacting gases, what is the total pressure exerted?
The sum of the partial pressures of the individual gases
Why is atmospheric pressure lower at high altidudes?
Amount of air pressing down is less
What is the partial pressure of oxygen in atmospheric air at sea level?
21.1 kPa
What happens to gases when they come into contact with body fluids?
Gas molecules will enter water to dissolve in liquid
What do water molecules entering the air exert?
Vapour pressure
What is saturated vapour pressure (SVP) at body temperature?
6.28 kPa
Where does inhaled air become saturated with water?
In the upper respiratory tract
How does water vapour affect the partial pressure of other gases?
Pressure of the rest of the gases = 101-6.28 = 94.7kPa
Still in same ratios as in dry air so eg:
pO2 = (101-6.28) x 20.9% = 19.8 kPa
How do you calculate amount of gas dissolved?
Partial pressure x solubility coefficient of gas
Solubility coefficient of oxygen in plasma is 0.01/mmol/L/kPa at body temperature
How much dissolved oxygen does plasma have per litre?
0.13 mmol/L
In alveolar air, what is the partial pressure of oxygen?
13.3 kPa
In alveolar air, what is the partial pressure of CO2?
5.3 kPa
What factors affect the rate of diffusion in a fluid?
Pressure difference, solubility of the gas in solution, cross-sectional area of fluid, distance molecles must diffuse, molecular weight of gas, temperature
True or False:
Oxygen is far more soluble than carbon dioxide so it diffuses faster in a liquid
False
Carbon dioxide is much more soluble than oxygen so diffuses in a liquid 20 times faster than oxygen - smaller gradient
What is the diffusion barrier?
5 cell membranes 3 layers of cytoplasm 2 layers of tissue fluid - epithelial cell of alveolus - tissue fluid - endothelial cell of capillary - plasma - red cell membrane
Describe the PO2 in the alveoli
Lower in alveoli compared to external environment
- continous diffusion of oxygen across alveolar wall
- diluting effect of CO2 entering the alveoli as it travels in the opposite direction
Higher in alveoli than in capillaries, so oxygen diffuses into blood
Why does emphysema affect the rate of diffusion?
Results in alveoli combining
Decreases surface area
Decreases rate of diffusion
How does fibrosis affect rate of diffusion?
Decreases diffusion rate
Causes thickening of exchange surface
Larger distance
What things can increase diffusion distance?
Pulmonary Oedema
In lung diseases that interfere with the rate of diffusion, why is PO2 low but PCO2 often normal?
CO2 crosses alveolar membrane more easily than oxygen and is more soluble
