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What is a gas?
a substance that is in its gaseous phase above its critical temperature
Critical temperature is the temperature above which a gas cannot be liquefied no matter how high the pressure.
What is a vapour?
In contrast to a gas, a vapour is in its gaseous phase but below its critical temp and can be returned to liquid form with increase in pressure
What is Pressure?
Pressure is defined as "the force per unit area acting at right angles to the surface under consideration
Pressure = Force/Area
- The unit of pressure is the Pascal.
- Pressure is the consequence of molecular bombardment of the surface by the gas.
- Kinetic energy is transferred to the surface and a force is produced that creates the pressure.
- If the volume falls, the pressure goes up because the area for collisions fall and so more kinetic energy transfer per unit area, and thus an increase in pressure.
What is temperature?
“This is a measure of the average kinetic energy in a system and translates to the degree of hotness or coldness of that system
- Temperature is measured in our clinical practice in the Celsius or Fahrenheit scales.
- In Physics it is the Kelvin.
- The divisions of the Kelvin and Celsius scale are the same but the start points differ.
- 0oC is 273K, so body temperature is 310K on this scale.
What is Critical Temperature?
- The temperature above which a vapour cannot be liquefied by any amount of pressure.
- Above this temperature, the substance is a gas; below it, a vapour.
- O2 -118°C
- N2O - 36.5°C
- CO2 - 31.0°C
Define Tidal Volume (TV):
amount of air entering and leaving with normal breath.~500ml resting
Inspiratory Reserve Volume (IRV):
volume that can be inspired above TV(TLC – (TV + FRC)) (~2000 ml)
Expiratory Reserve Volume (ERV):
Additional volume that can be expiredfrom a normal end expiratory breath to RV (1500ml)
Residual Volume (RV):
gas in lungs after maximal expiration (1500-1900ml)
Volume able to be inspired from FRC
Vital Capacity (VC):
Maximal forced expiration from TLC (4500-5000ml)
Total Lung Capacity ( TLC):
total lung volume (around 6000ml)
Draw a lung volume diagram
Define FRC and factors influencing it:
- The amount of air remain in the lung after a quiet expiration.
- The equilibrium point of the lung and chest wall
- Factors influencing FRC
- • Height
- • Obesity reduces FRC
- • Gender: female have FRC 10% less than male
- • Age: increase by around 16 mL per year
- • Diaphragm tone: FRC decrease in paralysed patient ~400 mL
- • Posture: The abdominal content can push against the diaphragm
- • Lung disease:
- • Increase recoil decrease FRC fibrotic lung disease
- • Decrease recoil increase FRC asthma, emphysema
This is the amount of air in the respiratory system that does not participate in gas exchange.
- •Anatomical dead space: This is the volume of air in the conducting airway.
- •Alveolar dead space: This is the volume of air in un-perfused alveoli.
- •Physiological dead space: This is sum of anatomical and alveolar dead space.
- Airways and alveoli in the dependent part of the lungs are always smaller than those at the top.
- As the lung volume is reduced closer to residue volume the dependent airway begins to collapse.
- •The lung volume at which dependent airways begins to collapse is known as the closing capacity (CC).
- •Closing volume = CC-RV
- •CC increases with age generally less than FRC in young adult
- •FRC = CC supine age 44
- •FRC = CC erect age 66
- •Independent of body position
- Static compliance: The change in volume per unit pressure changewhen the flow of gas has ceased
- • Measured by inspiration to a certain volume and then relaxing against a closed airway for as long as possible, intrapleural pressure is measure by oesophagus
- Dynamic compliance: Measure of volume V pressure changeduring normal breathing without breath holding.
- • Calculation is made when there is no gas flow at mouth at the end of insp & exp parts.
- • It is frequency dependent and decrease with resp rate due to lung units have different time constant.
Compliance per unit volume of the lung i.e.static compliance/FRC
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