The Excretory System
Kidneys filter blood to make urine.
About this topic
Excretion removes metabolic waste. The kidneys filter the blood: in each nephron, blood is filtered at the glomerulus, useful substances like glucose are reabsorbed, and water balance is adjusted under ADH hormone control, leaving urea, excess water and salts as urine. Urine flows through the ureters to the bladder and out through the urethra.
Urea is made in the liver from excess amino acids. Lungs excrete CO₂ and skin excretes sweat. Kidney failure is treated with dialysis or transplant.
Each kidney holds about a million nephrons. Blood pressure forces water, glucose, salts and urea out of the glomerulus into the Bowman's capsule; glucose and much water are then reabsorbed, leaving urea and excess salts in the urine. If you are dehydrated, the pituitary releases more ADH, making the kidneys reabsorb more water, so urine is darker and less.
Urea is made in the liver from the breakdown of excess amino acids. When kidneys fail, dialysis filters the blood artificially, or a transplant can be performed. The lungs excrete carbon dioxide and the skin excretes water and salts in sweat. A common confusion is excretion with egestion: excretion removes metabolic waste made by cells, while egestion removes undigested food.
Key ideas
- Nephrons filter
- Water and salt balance
- Urea is waste from protein
Quick check
Use cases: where The Excretory System helps
- Homework: review the key idea behind The Excretory System before you start a problem set.
- Exam revision: use the quick check to test whether you can recall The Excretory System without notes.
- Visual learning: play with the interactive lab to see how The Excretory System changes when you adjust its inputs.
- Teaching a friend: explain The Excretory System in your own words, then compare your version with the key ideas.
Biology underpins medicine, farming, conservation and biotechnology.
Frequently asked questions
What is The Excretory System?
Kidneys filter blood to make urine.
What will I learn in The Excretory System?
Nephrons filter. Water and salt balance. Urea is waste from protein.
Is The Excretory System part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the The Excretory System lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study The Excretory System?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.