How do you lose fluid through respirations?

Mechanism of Respiratory Fluid Loss

Fluid is lost through respiration primarily by evaporation of water from the airway surface liquid (ASL) into inspired air, which is then exhaled as water vapor, with the amount lost determined by the temperature and humidity gradient between inspired and expired air. 1

Primary Mechanism: Evaporative Water Loss

• During inspiration, dry air passes over the moist surfaces of the nasal passages, pharynx, trachea, and bronchi, where water evaporates from the airway surface liquid into the inspired air to humidify and warm it 1
• The airway mucosa acts as a heat and moisture exchanger, with water molecules transferring from the epithelial surface into the air stream down a concentration gradient 1
• High ventilation causes respiratory water loss with cooling of the airways and a temporary increase in osmolarity of the airway surface liquid due to loss of ASL volume 1

Quantitative Aspects of Respiratory Water Loss

• At rest under normal conditions, respiratory water loss is approximately 15-20 ml/hour, but this increases dramatically with increased minute ventilation 2, 3
• During physical exercise at a heart rate of 140 bpm, exhaled water increases approximately four-fold compared to rest, reaching about 60-70 ml/hour 3
• In patients with fever above 39°C, minute ventilation increases by approximately 25% (1.0 L/m²), resulting in an additional respiratory water loss of approximately 4.6 g/hour or 110 g/24 hours 2

Environmental Factors Affecting Respiratory Water Loss

• When inspired air temperature is 35°C with 75% humidity, water loss is only about 7 ml/hour, whereas when temperature drops to -10°C with 25% humidity, respiratory water loss increases to approximately 20 ml/hour 3
• The temperature and humidity of inspired air directly determine the amount of water that must be added to saturate the air to body temperature conditions 4
• Breathing warm dry air (30°C with 0 kPa water vapor pressure) causes a transient fall in the mass of water vapor lost per liter of ventilation and in expired air relative humidity during the first 15 minutes, suggesting the mucosa becomes dehydrated and hyperosmotic 4

Physiological Adaptations and Heat Exchange

• On inhalation, the nasal passages and upper airway surfaces are cooled by the passing air; on exhalation, heat from the exhaled air is transferred back to these cooler surfaces, resulting in exhaled air at temperatures below body core temperature 5, 6
• This countercurrent heat exchange mechanism can reduce exhaled air temperature to near ambient temperature during cooler conditions, substantially reducing water loss 5, 6
• The hygroscopic properties of nasal surfaces allow them to give off water vapor during inhalation and recapture water from respiratory air during exhalation, particularly under conditions of dehydration 5

Clinical Implications

• Dehydration of the ASL causes a transient increase in ion content and osmolarity when water evaporates faster than it can be returned by condensation or from the epithelium or submucosa 1
• This reduction in ASL volume decreases mucociliary clearance, which has been demonstrated in vivo by marked reduction during dry air breathing in both asthmatic patients and healthy subjects 1
• The combination of cooling and desaturation mechanisms can provide water savings of up to 60% relative to exhalation of fully saturated air at body temperature 5

Common Pitfalls to Avoid

• Do not underestimate respiratory water loss in febrile patients, though dermal losses from sweating (500-1000 g/24 hours) typically exceed the additional respiratory losses of approximately 110 g/24 hours 2
• Recognize that humidification in mechanical ventilation to greater than 80% relative humidity essentially eliminates water loss from evaporation through the respiratory tract, which can lead to positive free water retention 1
• During exercise or hyperventilation, the mechanism for humidifying expired gas can be overwhelmed by thermal stress, leading to transient dehydration of the airway mucosa that lasts at least 15 minutes 4