Excretion
The Excretory System: Removing Waste from the Body
Excretion is the process of removing waste substances that are produced inside cells. It includes carbon dioxide from respiration, urea from the breakdown of proteins, and excess water. These substances have been inside the body’s cells or bloodstream and must be removed to keep the body healthy.
It’s important to note that faeces is not considered excretion. Food that travels through the digestive system but never enters the body’s internal cells is not part of excretion.
When we eat food, proteins are broken down into smaller molecules, and excess protein is converted into urea in the liver. Urea is a toxic substance. It travels in the blood to the kidneys.
The kidneys filter the blood, removing urea and excess water. These substances form a liquid called urine. Urine passes through the ureters to the bladder, where it is stored before exiting the body through the urethra.
In addition to excreting waste, the kidneys play a vital role in maintaining the balance of water and salts in the body. This helps regulate blood pressure and supports proper cellular function.
Each kidney contains millions of tiny filtration units called nephrons. These filter the blood and form urine. The kidneys constantly adjust how much water is lost in urine to keep the internal environment stable.
Important Concept
Urea is produced when the liver breaks down excess amino acids. The kidneys remove urea from the blood and help maintain water and salt balance — a process essential for homeostasis.
Common Mistake
Thinking that faeces is an excretory product is incorrect. Faeces is not formed from substances that were part of the body’s cells or bloodstream. It is undigested food that never entered the internal environment of the body, so it is not considered part of excretion.
Correct understanding: Excretion removes waste made inside the body’s cells, like urea, carbon dioxide, and excess water — not leftover food from digestion.
Questions
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Another name for the excretory system is the urinary system.
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The body eliminates water through urine, sweat, and exhalation (water vapor in breath).
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The water needed by the body is absorbed and used for essential functions such as digestion, circulation, and regulating body temperature.
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No, water consumption can vary depending on activity level, temperature, and health condition.
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Carbon dioxide is excreted from the body through the lungs during exhalation.
Note
Some diseases like kidney stones can cause severe pain and pose serious health risks. If untreated, they may lead to kidney failure or even death. Using mineral water from safe sources can help prevent such conditions.