2009 Second Sitting Paper 2 Question 19

2009-2-19 Number 19 Question: 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%) (11% pass)

SVP The water vapour pressure when the air is fully saturated. Depends on both pressure and Temperature = 47mmHg at STP

Absolute Humidity The amount of water vapour present in a given volume of gas (units g H2O/m3 or mgs H2O /L) < 100% saturation ◦ Absolute humidity is temperature independent 100% saturation ◦ Absolute humidity is temperature dependent due to ΔSVP fully saturated air ▪ at 0 °C contains 4.8 mg/L; ▪ at 20 °C contains 17 mg/L; ▪ at 37 °C contains 44 mg/L

Relative Humidity the amount of water vapour present in the gas expressed as a percentage of the amount of water vapour that would be present if the gas were saturated with water vapour.

Latent heat of vaporisation. the heat required to convert 1g of a substance from the liquid phase to the gaseous phase at a given temperature (expressed in Jg-1)

Respiratory tract humidification: INSPIRED AIR (During nose breathing) Air is warmed by the radiant heat from nasal blood supply. Moisture evaporates from the epithelia to increase the relative humidity of the inspired air to ~90% Mouth breathing reduces the relative humidity of inspired air to 60-70% At the lungs, it reaches the isothermic saturation boundary where it achieves BTPS (body temperature and pressure, saturated with water vapour) conditions. This usually occurs at the second generation of bronchi.

EXPIRED AIR Expired gas transfers heat back to the cooler trachea and nasal mucosa. As the saturated gas cools, it can hold less water vapour (its saturated water vapour pressure falls) Condensation occurs on the mucosal surfaces, where the liquid water is reabsorbed. Reabsorption reduces potential airway water losses from 300ml/day to ~150ml/day

Tracheal temperature and humidity fall with an increase in respiratory rate (ie, the isothermic saturation boundary moves more away from the upper airway)

Gladwin 2016 2009 Second Sitting Paper 2 Question 19

Examiner Comments

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.

Gladwin 2016