Excretion in Humans
Cambridge (CIE) IGCSE Biology
The Big Idea: Your body produces toxic waste products (carbon dioxide and urea) every second. Your lungs and kidneys work together to detect, filter, and safely excrete these poisons — and they do this with incredible precision.
Quick Summary
- Excretion is the removal of toxic waste products from metabolism, excess substances, and materials your body doesn't need.
- Two main waste products: Carbon dioxide (from breathing cells) and urea (from broken-down amino acids).
- The lungs excrete CO₂ by breathing it out during exhalation.
- The kidneys filter the blood to remove water, salts, and urea — forming urine.
- Three kidney processes: ultrafiltration (forcing small molecules out), selective reabsorption (bringing useful ones back), and urine formation.
- The liver breaks down amino acids and converts the toxic nitrogen part into less toxic urea.
- This is NOT the same as digestion (egestion) — that's removing undigested food, this is removing metabolic waste.
1. What Is Excretion?
The Definition
Excretion
The removal from the body of waste substances produced by metabolic reactions (chemical reactions inside cells), toxic materials, and substances in excess of the body's requirements.
Think of it this way: excretion is the body's garbage collection service. Every living cell in your body is busy carrying out chemical reactions — respiring, building proteins, breaking down glucose. These reactions generate waste. Some of it is toxic. Some of it is just excess. Your body has to get rid of it, or it will poison you.
Why Excretion Matters: Two Key Waste Products
Carbon Dioxide (CO₂)
Every time a cell uses oxygen to release energy from glucose (aerobic respiration), it produces CO₂ as a byproduct.
Why must we excrete CO₂?
CO₂ dissolves in water to form a weak acid. In high concentrations, it lowers the pH of your cells. This damages enzymes — the proteins that control all your metabolic reactions. A damaged enzyme can't do its job. So if CO₂ builds up, your metabolism slows down and you could die. That's why you must breathe it out constantly.
Urea
Your liver breaks down excess amino acids (the building blocks of protein). When it does, it creates a toxic chemical called ammonia. The liver immediately converts this into a less toxic form: urea. Urea is still toxic in high concentrations and must be excreted via the kidneys.
What happens if urea isn't excreted? Cell death, reduced response to insulin (leading to diabetes), and deposits inside blood vessels that can block them.
Key Distinction: Excretion vs. Egestion
Excretion: Removal of metabolic waste (products of chemical reactions inside your cells). Examples: CO₂, urea, excess water.
Egestion: Removal of undigested food waste that was never absorbed. This happens at the anus.
These are completely different processes. Examiners love catching students who mix them up.
2. The Excretory System: The Organs Involved
| Organ |
Main Excretory Job |
How It Works |
| Lungs |
Excrete carbon dioxide |
Blood carrying CO₂ reaches the lungs. CO₂ diffuses out of the blood and into the alveoli. You breathe it out during exhalation. |
| Kidneys |
| Liver |
Key Terms to Memorise