There is no control over water, ion or urea loss by the lungs or skin.

The idea

Water leaves the body via the lungs during exhalation, while water, ions, and urea are lost through the skin in sweat. Crucially, there is no control over these losses. For example, when you exercise on a hot day, you sweat heavily to cool down, losing water, ions, and urea as a passive consequence of temperature regulation, not to maintain internal balance. Similarly, breathing rate dictates water loss from the lungs, independent of the body's hydration status. Because the lungs and skin cannot regulate these losses, the kidneys must actively adjust the volume and concentration of urine to maintain the body's water, ion, and nitrogen balance.

What earns the marks

• Water is lost from the lungs as water vapour in exhaled air.
• Water, ions, and urea are lost from the skin in sweat.
• The loss of water, ions, and urea from the lungs and skin is uncontrolled and not adjusted to maintain internal balance.
• The kidneys are responsible for the controlled loss of water, ions, and urea by adjusting urine production.

Common mistakes and how to put them right

• Thinking the skin controls water or ion balance by sweating.

  Put it right: Sweating is for temperature regulation, and the resulting water and ion loss is uncontrolled.

• Believing urea is breathed out from the lungs.

  Put it right: Urea is lost in sweat from the skin and in urine from the kidneys, while the lungs only lose water and carbon dioxide.

• Assuming all water loss is regulated.

  Put it right: Only the kidneys actively control water loss, whereas lung and skin losses are unregulated.

Exam tips

• Clearly distinguish between the routes: state that lungs lose water, while skin loses water, ions, and urea.
• Use the term 'uncontrolled' for losses via the lungs and skin, contrasting this with the 'controlled' regulation by the kidneys.