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
Inspiratory heating and humidification of the gas mixture
Expiratory reclamation of heat and moisture
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