Excretion by Lungs, Liver and Skin: Other Excretory Organs

Excretion is removal of metabolic waste from the body. The kidneys are the main organs, but the lungs, liver and skin also remove important wastes every day. Together they keep blood composition, water and temperature steady.

Understanding these organs matters because failure of any one affects the whole body. Carbon dioxide builds up if lungs fail, toxic substances accumulate if the liver fails, and sweat glands help prevent overheating while also losing salts that must be replaced.

Key Concepts

• Lungs: Remove carbon dioxide produced by cellular respiration and lose some water vapour in exhaled air.
• Liver: Converts toxic ammonia into less toxic urea through the ornithine cycle, breaks down old red blood cells, and inactivates drugs and alcohol.
• Skin: Its sweat glands release sweat containing water, sodium chloride and tiny amounts of urea and lactic acid.
• Sebaceous glands: Oil glands in skin that secrete sebum; they remove some lipids but are not true excretory organs for nitrogen waste.
• Homeostasis: Maintenance of stable internal conditions such as pH, temperature and salt balance.

How It Works

1. In tissues, glucose oxidation produces carbon dioxide. It diffuses into blood, reaches the lungs and is exhaled.
2. Protein breakdown produces ammonia, which is highly toxic. Liver cells convert ammonia and carbon dioxide into urea, which blood carries to the kidneys for urine formation.
3. The liver also splits haemoglobin from worn-out red cells into bile pigments such as bilirubin, sent to the gut in bile and lost in faeces.
4. When body temperature rises, sweat glands secrete sweat onto skin. Evaporation cools the body, while small quantities of salts and urea leave with it.

Worked Example

A resting adult exhales about 200 millilitres of carbon dioxide per minute. In one day: 200 × 60 × 24 = 288,000 millilitres = 288 litres of carbon dioxide removed by the lungs. For water loss, if a person sweats 500 millilitres on a hot day and sweat has about 0.9 percent salt equivalent losses plus water, most loss is water for cooling, showing skin is mainly a temperature regulator and only a minor excretory route compared with about 1.5 litres of urine daily.

Common Misconceptions

• Sweating removes most toxins: Wrong. Sweat is mostly water and salt; the kidneys and liver remove most wastes.
• Lungs excrete only oxygen waste: Wrong wording. Lungs take in oxygen and remove carbon dioxide and water vapour.
• Liver makes urine: Wrong. The liver makes urea; the kidneys filter it out into urine.

Real-World Applications

Ventilators support breathing when lungs cannot remove carbon dioxide. Liver function tests guide treatment of jaundice and hepatitis. Excess sweating in heat requires water and electrolyte replacement to avoid dehydration and cramps. Dialysis replaces kidney function but does not replace liver detoxification.

Thus excretion is a team effort: lungs clear carbon dioxide, the liver detoxifies and makes urea, and skin assists with water, salts and cooling. The kidneys remain the chief organ for nitrogenous waste.