3.2 Gas Exchange Flashcards
What are some factors affecting diffusion rate of a gas in a fluid?
Pressure difference Solubility of the gas in solution Cross-sectional area of the fluid Distance that the molecules need to diffuse Molecular weight of the gas Temperature of fluid (body assume 37)
What can the diffusion coefficient be used to determine?
The relative rates at which different gases will diffuse across the same membrane at the same pressures
Eg CO2 diffuses x20 more quickly than O2
How can lower diffusion coefficients be compensated for?
Larger partial pressures
Which gas exchange is more impaired in a diseased lung and why?
Oxygen exchange more impaired
Because it has a slower diffusion rate
Describe the diffusion barrier for diffusion through gas to alveolar wall
5 cell membranes, 3 layers of cytoplasm, 2 layers of tissue fluid
Epithelial cells of alveolus Tissue fluid Endothelial cell of capillary Plasma Red cell membrane
What is the distance for diffusion between the blood and air in the alveoli and why is this relevant?
0.6 micrometers
Distance decreases during inhalation as lung distends
Allows fast and efficient diffusion
What does diffusion resistance depend on?
Nature of the barrier
Nature of the gas
How do you work out permeability using diffusion coefficient?
Permeability = (Diffusion coefficient of molecules in the membrane X Solubility in the lipid bilayer) / Thickness of membrane
Describe the composition of dry air
Mixture of O2 (21%) and N2 (79%)
Variable amount of H20 depending on relative humidity and temperature
101kPa
Describe the composition of tracheal/inhaled air
Inspired air always saturated with water
Water content depends on temperature
- At body temp, water is 6.28kPa
Means that oxygen and nitrogen are slightly diluted
Describe the movement of oxygen from the air to the blood in terms of partial pressures
PO2 lower in alveoli than external environment
- Continuous diffusion of O2 across alveolar wall
- Due to diluting effect of CO2 in alveoli
PO2 is higher in alveoli than capillaries so O2 diffuses into blood
O2 combines with Hb and travels in bloodstream and oxy-haemoglobin
Describe the movement of carbon dioxide from blood to air
Enters RBC as a waste product and reacts with water -> Form carbonic acid
CA dissociates to bicarbonate IONS and hydrogen ions
Bicarbonate ions diffuse to plasma
H+ buffered by Hb in RBC
PCO2 in capillaries higher thats alveoli -> Exhaled
What are the normal partial pressures of oxygen and carbon dioxide in the lung?
PO2: 13.3kPa
PCO2: 5.3kPa
What are the partial pressures of oxygen and carbon dioxide in the alveolar capillaries in comparison to alveolar air?
The same
Equilibrium
Why does oxygen diffuse into the blood and carbon dioxide diffuse out?
PO2 in alveolar gas > PO2 in returning blood
PCO2 in alveolar gas < PCO2 in returning blood
What are the factors affecting rate of gas diffusion through the respiratory membrane?
Thickness of the membrane
Surface area
Diffusion coefficient of the gas in the substance of the membrane
Pressure difference of the gas over the two sides of the membrane
What is the diffusion impairment in fibrotic lung disease?
Thickened alveolar membrane
Slows gas exchange
PO2 low
PCO2 normal - crosses alveolar membrane more easily
What is the diffusion impairment in pulmonary oedema?
Fluid in the interstitial space increases diffusion distance
PO2 low -> increased diffusion distance
PCO2 normal due to higher CO2 solubility in water
Why is some air wasted in every breath?
Because air enters and leaves by the same airways
Last air in is first air out so it doesn’t reach the alveoli
What is the serial/anatomical dead space?
Due to the portion of the airways where there is no gaseous exchange -> Conducting portion
150ml
What is the distributive dead space?
170ml
Some parts of the lung are not airways or do not support gas exchange (dead/damaged alveoli or alveoli with poor perfusion)
Add this to the serial dead space
How do you calculate alveolar ventilation rate?
(Tidal volume - Anatomical dead space) X Respiration rate
What is the ideal ventilation-perfusion ratio?
1
V = 350 X 14 = 4.9L/min Q = RV (CO) output = 5L
5/5=1
What is the V/Q ratio if V is 0 but there is still perfusion?
0
