Week 3 - gas exchange and lung function tests Flashcards
What is Fick’s first law of diffusion?
Flux of molecules across a barrier is proportional to the permeability of the molecules times the transfer surface area over which diffusion can occur times the concentration gradient
What factors affect the rate of diffusion?
- Pressure difference
- Solubility of the gas in solution
- The cross-sectional area of the fluid
- The distance the molecules must diffuse
- The molecular weight of the gas
- Temperature of the fluid
What does the rate of diffusion depend on when gases pass through other gases?
- The rate of diffusion is inversely proportional to the square root of its molar mass
- So lighter gases diffuse more rapidly
- O2 is a smaller molecule than CO2 so would tend to diffuse more quickly
What does the rate of diffusion depend on when gases pass liquids?
- Rate of diffusion is dependent on the solubility of the gas
- CO2 is much more soluble than O2, so diffuses in a liquid 20 times faster than oxygen
- – So the gradient is smaller
How do you compensate for the difference in diffusion coefficient between CO2 and O2?
By the differences in partial pressures:
- Larger pO2 compensates for the slower diffusion of O2
- In a diseased lung, oxygen gas exchange is thus more impaired than CO2 because of oxygen’s slower diffusion rate
What factors affect the rate of gas diffusion through the respiratory membrane?
- Thickness of membrane
- – Increase as a result of oedema fluid in the interstitial space and in alveoli (slows rate)
- Surface area of membrane
- – Decreased by removal of an entire lung
- – Emphysema results in alveoli combining, decreasing surface area
- Diffusion coefficient of gas in the substance of the membrane
- Pressure difference of the gas between the 2 sides of the membrane
What are the layers making up the diffusion barrier at the air-blood interphase?
- Epithelial cell of alveolus
- Tissue fluid
- Endothelial cell of capillary
- Plasma
- Red cell membrane
The barrier is 0.6μm thick
What is serial (anatomical) dead space?
- The volume of the airways
- Can be measured by nitrogen washout
- Typically about 0.15L
What is distributive dead space?
Some parts of the lung (that are not airways) do not support gas exchange
- Dead/damaged alveoli
- Alveoli with poor perfusion
What is physiological dead space?
Serial dead space + distributive dead space (i.e. the total!)
- Typically 0.17L
How quickly is alveolar air replaced and why is this important?
Multiple breaths are required to totally exchange alveolar air
- Important, as it guards against sudden changes in blood gas levels
- If respiration is temporarily interrupted, blood gas levels and pH are unaffected
What is the alveolar ventilation rate?
- The amount of air that actually reaches the alveoli
- To calculate AVR, you need to allow for ‘wasted’ ventilation of dead spaces
- AVR = PVR – DSVR = (TV x RR) – (DSV x RR)
What is simple spirometry?
- How it is done:
- – Patient fills their lungs from the atmosphere
- – They breath out as far and fast as possible through a spirometer
- Allows the measurement of many lung volumes and capacities
- – Vital capacity is particularly significant
- – Tables can be used to predict the vital capacity of an individual of known, age, sex and height
What factors affect vital capacity?
- Inspiration = compliance of the lungs and force of inspiratory muscles
- Expiration = airway resistance
What is residual volume?
Volume remaining after maximal expiration
- Cannot be measured by spirometry
- Contributes to the total lung capacity, so important
What are capacities?
2 or more lung volumes added together
- Volumes over the cycle can change depending on the pattern of breathing, but capacities are fixed
What is vital capacity?
- Measured from max inspiration to max expiration
- About 5L in a typical adult
- VC = IRV + TV +ERV
- Often changes in disease
What is inspiratory capacity?
- Biggest breath that can be taken from resting expiratory level
- Typically about 3L
- IC = IRV + TV
What is functional residual capacity?
- Volume of air in the lungs at resting expiratory level
- Typically 2L
- FRC = ERV + RV
What is the total lung capacity?
- Volume of gas in the lungs at the end of maximal inspiration
- Typically 5.8L
- TLC = IRV + TV + ERV + RV
How do you measured forced vital capacity?
- FVC = the maximum volume that can be expired from full lungs
- Plot a graph of volume expired over time, following spirometry
- FVC is the highest point on the graph
How do you measure forced expiratory volume in 1 second?
- The volume expired in the 1st second of expiration from full lungs
- Affected by how quickly air flow slows down
- – .: It is low if the airways are narrowed
- Again, plot a graph of volume expired over time
- – Find the volume expired after 1 second
- If FEV1 is > 70% of FVC then it is normal
What is a restrictive deficit?
- Maximal filling of the lungs is determined by the balance between the maximum inspiratory effort and the force of recoil of the lungs
- If the lungs are unusually stiff, or inspiratory effort is compromised by muscle weakness, injury or deformity, then a restrictive deficit is produced
- FVC is reduced
- FEV1 > 70% FVC so normal
What is an obstructive deficit?
- During expiration, particularly when forced, the small airways are compressed
- – This increases flow resistance, eventually to the point where no more air can be driven out of the alveoli
- If the airways are narrowed, then expiratory flow is compromised much earlier in expiration, producing an obstructive deficit
- FEV1 is reduced
- FVC is relatively normal
