Define the following terms (40%)
1. Saturated Vapour Pressure of Water
2. Absolute Humidity
3. Relative Humidity
4. Latent heat of vaporisation
Briefly outline how the humidity of air is altered during inspiration and expiration by the respiratory tract (60%)
It is essential that candidates read and respond to the question asked of them. The first part of
the question required simple, accurate definitions, which the majority of candidates were
unable to provide. Marks were awarded for the definition only. Descriptions of
measurement, potential effects on oxygenation, etc. were not asked and gained no marks.
The core of the second part required an outline of the exchange of heat and moisture through
the upper airways and bronchial tree, culminating in fully saturated gas at core body
temperature by level of the 2nd generation bronchi. Whilst the question asked ‘humidity’ and
not temperature, correct definitions in the first part would have dictated a joint outline of
both. The effects of surface area, the nasal turbinates, mucosal secretion and blood flow were
all relevant. The contribution to insensible moisture and heat loss should have been
mentioned. No candidate considered the effects of respiratory rate, mouth versus nose
breathing, or dry medical gases versus room air.
Syllabus – B1k2d
Reference: Nunn pages15 and 26
- Saturated vapour pressure is the pressure exerted by a vapour in equilibrium with liquid of the same substance. It is influenced by temperature.
- Increasing temperature increases the saturated vapour pressure
- Increasing pressure increases the temperature at
- For water, at room temperature, the saturated vapour pressure is approximately 17.5 mmHg. It increases to 47 mmHg at body temperature.
- Absolute humidity is the mass of water vapour present in a given volume of air.
- Relative humidity is the percentage ratio of the mass of water vapour in a given volume of air to the mass required to saturate that given volume of air at the same temperature.
- Latent heat of vapourisation is the heat required to convert a substance from liquid to vapour at a given temperature. Latent heat of vapourisation decreases as ambient temperature increases and is reduced to zero at the critical temperature of that substance.
Inspiratory heating and humidification of the gas mixture
- Inspired gas has a water content of around 10g/kg (50% humidity, 22° C)
- The change in temperature and humidity occurs during respiration due to the effects of the upper airway structures on the inspired air mixture.
- Inspired gas passes through the convoluted air passages of the nasopharynx and pharynx which generates turbulence.
- This turbulence increases evaporative heat exchange between the air and the mucosa; such that at the posterior nasal cavity the relative humidity is already 85%
- In the lower pharynx, the temperature is about 33° C and relative humidity approaches 100%
- The inspired air achieves body temperature at the isothermic saturation boundary, around 5cm beyond the carina
- Alveolar gas has a water content of around 47g/kg (100% humidity, 37° C)
Expiratory reclamation of heat and moisture
- Expired gas passes over the cooler upper airway mucosa, and returns some of its heat to it
- Expired air at the nares is usually 32° C and close to 100% humidified
- Some of the water is also reclaimed by the process of condensation
- This process is highly dependent on the temperature of the ambient air; the cooler the ambient air the more moisture is reclaimed.
- In hot environments, humidity cannot be reclaimed and the net water loss increases
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