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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Excretory Organs
Your body is constantly working to get rid of waste products that build up from all the chemical reactions happening inside you. This process is called excretion and it's super important for keeping you healthy. Without it, harmful substances would build up and make you ill!
Key Definitions:
Imagine your body as a busy factory. As it works, it produces waste that needs to be removed. If the rubbish isn't taken out, the factory can't work properly! Your body needs to get rid of:
Don't mix these up in your exam! Excretion is removing waste products made inside your body from chemical reactions. Egestion is getting rid of undigested food that never actually became part of your body - it just passed through your digestive system and came out as faeces.
Your body has several organs that work together to remove different types of waste products. Each one specialises in dealing with particular substances.
The kidneys are bean-shaped organs about the size of your fist, located just below your ribcage on either side of your spine. They're the stars of the excretory show!
Each kidney contains about a million tiny filtering units called nephrons. Blood enters the kidneys through the renal artery and leaves via the renal vein. Urine leaves through the ureter to the bladder.
The nephron has two main parts:
The kidneys perform three main processes:
The end product is urine, which contains excess water, salts and urea.
Your kidneys filter about 180 litres of fluid every day - that's enough to fill a bathtub! But you only pee out about 1-2 litres because most of the water gets reabsorbed. Your kidneys filter all your blood about 60 times every day!
The liver is the largest internal organ in your body and has over 500 different jobs! When it comes to excretion, it has several crucial roles.
When we eat more protein than we need, the liver breaks down the excess amino acids. This creates ammonia, which is very toxic. The liver quickly converts this ammonia into urea through a process called the urea cycle. The urea then travels in the bloodstream to the kidneys, which filter it out into urine.
While we often think of the lungs as organs for breathing in oxygen, they're also crucial for excreting carbon dioxide - a waste product of cellular respiration.
When cells respire, they produce carbon dioxide as a waste product. This CO₂ diffuses from the cells into the bloodstream, where most of it combines with water to form carbonic acid. This is carried to the lungs, where it breaks down again and the CO₂ diffuses out of the blood into the air sacs (alveoli). When you breathe out, you're actually excreting this waste gas!
The lungs also help remove small amounts of water in the form of water vapour - you can see this on cold days when your breath forms a cloud.
Your skin is actually your body's largest organ and it plays a role in excretion through sweat production.
Sweat is produced by sweat glands in your skin. While its main purpose is to cool you down, sweat also contains waste products:
When you exercise hard, your muscles produce more lactic acid and some of this is removed through sweat. This is why sweat can sometimes smell - it's not just water!
All these organs don't work in isolation - they form an interconnected system that keeps your internal environment balanced.
Remove urea, excess salts and water
Regulate blood pH and water levels
Produces urea from amino acids
Detoxifies blood
Removes old red blood cells
Remove carbon dioxide
Excrete water and some salts
Help regulate body temperature
When kidneys fail, waste products build up in the blood, causing symptoms like fatigue, swelling and eventually coma if untreated. Patients need dialysis - a process where blood is filtered through a machine that acts like an artificial kidney. The machine removes waste products and excess water, then returns the cleaned blood to the body. Patients typically need dialysis three times a week for 3-4 hours each session. While dialysis saves lives, it only does about 10% of the work of healthy kidneys, showing just how amazing these organs really are!
When excretory organs don't work properly, serious health problems can develop.
Your excretory organs work tirelessly to remove waste products and maintain homeostasis. The kidneys filter blood and produce urine, the liver detoxifies and creates urea, the lungs remove carbon dioxide and the skin excretes salts and water through sweat. Together, they keep your internal environment clean and balanced, allowing all your body systems to function properly.
Remember to be specific about what each organ excretes:
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