Water leaves the body via the lungs during exhalation.

The idea

Water is lost from the body by several routes, and one of these is the lungs. The air breathed in becomes warmed and moistened as it passes through the respiratory tract. Water evaporates from the moist surfaces of the alveoli into the air in the lungs. During exhalation, this water vapour is carried out of the body in the exhaled air. The loss is continuous but varies with breathing rate, temperature and humidity: faster or deeper breathing, dry air and cold air tend to increase it. This route removes water but not urea, so it contributes to water balance rather than to nitrogen balance.

What earns the marks

• Water evaporates from the moist surfaces of the lungs into the air in the alveoli.
• The water vapour is removed from the body when air is exhaled.
• The lungs are a route of water loss, not a route of urea loss.
• The rate of water loss through the lungs increases with faster or deeper breathing.
• Exhaled air contains more water vapour than inhaled air.

Common mistakes and how to put them right

• Stating that water is lost from the lungs during inhalation.

  Put it right: Water vapour leaves the body during exhalation.

• Confusing water loss with urea loss.

  Put it right: The lungs lose water but urea is removed mainly by the kidneys.

• Saying that the lungs produce water.

  Put it right: Water evaporates from moist lung surfaces and is carried out in exhaled air.

• Ignoring the effect of breathing rate.

  Put it right: More frequent or deeper breathing increases water loss.

Exam tips

• Name the process as evaporation from the moist surfaces of the lungs.
• State clearly that the water leaves during exhalation, not inhalation.
• Link the loss to breathing rate or environmental conditions if the question asks for factors.
• Keep this route separate from urea loss, which occurs mainly in urine.