Human Excretory System: Organs, Functions, Urine Formation and Kidney Health
Your body is like a busy city. Every cell works, uses food, makes energy, and produces waste. If waste is not removed, it can harm the body. The human excretory system works like a cleaning and filtering system. It removes harmful wastes and helps keep the body’s water and salt levels balanced.
This topic is important in Biology because it connects many ideas: blood, kidneys, urine formation, lungs, skin, liver, and health habits. These Biology notes explain the excretory system in simple language for Classes 6–10.
What excretion means
Excretion is the process by which the body removes metabolic wastes.
Metabolic wastes are wastes formed inside cells during life processes. For example:
Cells break down food to release energy.
This produces carbon dioxide.
Proteins are broken down in the body.
This produces nitrogenous wastes such as urea.
Extra water and salts may collect in the body.
These must be removed or balanced.
Excretion is necessary because harmful substances can build up in the blood and damage organs.
Why excretion is necessary
Excretion helps the body in many ways.
It removes poisonous wastes.
It controls the amount of water in the body.
It maintains the balance of salts and minerals.
It helps keep blood composition steady.
It supports normal cell function.
It helps maintain body temperature through sweating.
A simple example is a water filter at home. If the filter is not cleaned, impurities collect and the water becomes unsafe. In a similar way, the kidneys filter the blood and remove wastes.
Excretion is different from egestion
Many students confuse excretion and egestion. They are not the same.
Excretion | Egestion |
Removal of metabolic wastes formed inside cells | Removal of undigested food from the digestive system |
Wastes are produced by body activities | Wastes are food materials that were not digested |
Main organs include kidneys, lungs, skin and liver | Main organ involved is the large intestine, and waste leaves through the anus |
Examples include urea, carbon dioxide, extra salts and water | Example is faeces |
Part of the excretory system | Part of the digestive system |
Easy memory tip
Excretion removes wastes made by cells.
Egestion removes undigested food.
Main parts of the human excretory system
The human excretory system includes:
Two kidneys
Two ureters
One urinary bladder
One urethra
These organs work together to form, carry, store and remove urine.
Organ | Main function |
Kidneys | Filter blood and form urine |
Ureters | Carry urine from kidneys to urinary bladder |
Urinary bladder | Stores urine temporarily |
Urethra | Carries urine out of the body |
The main kidney function is to clean the blood by removing urea, extra water and extra salts.
Main excretory organs and their functions
The kidneys are the main excretory organs, but they are not the only ones. The lungs, skin and liver also help in excretion.
Excretory organ | Waste removed | How waste leaves the body |
Kidneys | Urea, extra water, extra salts | As urine |
Lungs | Carbon dioxide and water vapour | During exhalation |
Skin | Water, salts and small amounts of urea | As sweat |
Liver | Bile pigments and harmful substances after processing | Through bile and later faeces or urine |
This means the excretory system is not only about urine. The body uses different organs to remove different wastes.
Kidneys and their structure
The kidneys are two bean-shaped organs. They are found in the back part of the abdomen, one on each side of the backbone.
Each kidney is reddish-brown and about the size of a fist in adults. The right kidney is usually slightly lower than the left because the liver is on the right side.
Main parts of a kidney
The structure of kidney includes these important parts.
Part | Meaning and function |
Renal capsule | Outer covering that protects the kidney |
Cortex | Outer region where many filtering units begin |
Medulla | Inner region containing tube-like structures |
Renal pelvis | Funnel-shaped area where urine collects |
Renal artery | Brings waste-containing blood to the kidney |
Renal vein | Carries cleaned blood away from the kidney |
Ureter | Takes urine from kidney to urinary bladder |
Functions of kidneys
The kidneys do much more than make urine.
They help to:
Remove urea from blood
Remove extra water
Remove extra salts
Maintain water balance
Maintain salt balance
Help control blood pressure
Keep the blood clean
Maintain a stable internal environment
This stable internal condition is called homeostasis.
Ureters
The ureters are two narrow tubes. One ureter comes from each kidney.
Their job is to carry urine from the kidneys to the urinary bladder. Urine does not simply fall down due to gravity. The walls of the ureters make wave-like movements that push urine forward. These movements are called peristalsis.
Urinary bladder
The urinary bladder is a muscular bag. It stores urine for some time.
When the bladder fills, stretch receptors send signals to the brain. This creates the feeling that urine needs to be passed.
The bladder can expand as it fills and shrink after urine leaves.
Urethra
The urethra is the tube that carries urine from the urinary bladder to the outside of the body.
A ring-like muscle called a sphincter helps control the release of urine. This is why urine does not usually leak out all the time.
Nephron and its structure
A nephron is the basic filtering unit of the kidney. Each kidney has a very large number of nephrons. These tiny units filter blood and form urine.
A nephron is microscopic, so it cannot be seen with the naked eye.
Main parts of a nephron
Part of nephron | Function |
Bowman’s capsule | Cup-shaped part that receives filtered fluid |
Glomerulus | Network of tiny blood capillaries where filtration happens |
Proximal convoluted tubule | Reabsorbs useful substances such as glucose, water and salts |
Loop of Henle | Helps in water and salt balance |
Distal convoluted tubule | Adds some extra wastes and adjusts salt levels |
Collecting duct | Collects urine from many nephrons and carries it towards the renal pelvis |
Simple way to understand a nephron
Imagine a school water purifier.
The filter stops dirt.
Useful water passes through.
Some minerals may be adjusted.
Clean water comes out.
A nephron also filters, selects and adjusts. It removes wastes but keeps useful materials in the blood.
How blood is filtered in the kidneys
Blood enters each kidney through the renal artery. This blood contains useful substances as well as wastes.
Inside the kidney, blood reaches the glomerulus of each nephron. The glomerulus has very thin walls. Blood pressure forces small substances out of the blood and into Bowman’s capsule.
This filtered fluid contains:
Water
Urea
Salts
Glucose
Amino acids
Other small dissolved substances
Blood cells and large proteins usually remain in the blood because they are too large to pass through.
Steps of urine formation
Urine formation happens in three main stages:
Filtration
Reabsorption
Secretion
Stage | Where it mainly happens | What happens | Result |
Filtration | Glomerulus and Bowman’s capsule | Water and small dissolved substances move out of blood | Filtered fluid is formed |
Reabsorption | Tubules of nephron | Useful substances move back into blood | Glucose, much water and needed salts are saved |
Secretion | Later parts of tubules | Extra wastes and ions move from blood into tubule | Urine becomes more concentrated |
Filtration
Filtration is the first step. Blood pressure pushes water and small molecules from the glomerulus into Bowman’s capsule.
This fluid is not urine yet. It still contains useful substances like glucose and amino acids.
Reabsorption
Reabsorption means taking useful substances back into the blood.
The body reabsorbs:
All or almost all glucose in a healthy person
Amino acids
Much of the water
Needed salts
This step is very important. Without reabsorption, the body would lose too much water and nutrients.
Secretion
Secretion adds extra unwanted substances from the blood into the nephron tubule.
This helps remove:
Extra hydrogen ions
Extra potassium ions
Some medicines
Other waste substances
After these steps, the fluid becomes urine.
Composition of urine
Urine is mostly water. It also contains dissolved wastes.
Typical urine contains:
Water
Urea
Salts
Uric acid in small amounts
Creatinine
Pigments that give urine its yellow colour
The colour of urine can change. Pale yellow urine often shows good hydration. Dark yellow urine may mean the body needs more water, though foods, medicines and health conditions can also affect urine colour.
How kidneys maintain water and salt balance
The kidneys act like smart controllers. They decide how much water and salt should stay in the body.
When the body has less water
This may happen after sweating, exercise or not drinking enough water.
The kidneys reabsorb more water back into the blood. As a result:
Less urine is produced.
Urine becomes more concentrated.
Urine may look darker.
When the body has too much water
This may happen after drinking a lot of water.
The kidneys allow more water to leave in urine. As a result:
More urine is produced.
Urine becomes more dilute.
Urine looks lighter.
This balancing process helps protect cells from swelling or shrinking.
Other organs involved in excretion
Lungs
The lungs remove carbon dioxide and water vapour.
Carbon dioxide is produced during respiration in cells. Blood carries it to the lungs. When we breathe out, carbon dioxide leaves the body.
Skin
Skin removes sweat through sweat glands.
Sweat contains:
Water
Salts
Small amounts of urea
Sweating also helps cool the body on hot days or during exercise.
Liver
The liver performs many important functions. In excretion, it helps by breaking down harmful substances and converting ammonia into urea.
Ammonia is very toxic. The liver changes it into urea, which is less harmful. The kidneys then remove urea in urine.
The liver also removes old red blood cell pigments through bile.
Common disorders of the excretory system
This section is for learning only. If someone has pain, fever, blood in urine, swelling or difficulty passing urine, they should speak to a doctor or trusted adult.
Kidney stones
Kidney stones are hard deposits that can form when certain minerals crystallise inside the urinary system.
They may cause:
Severe pain in the side or back
Pain while passing urine
Blood in urine
Nausea
Frequent urge to urinate
Drinking enough water may reduce the chance of some types of stones, but treatment depends on the type and size of the stone.
Kidney failure
Kidney failure means the kidneys cannot filter blood properly.
Wastes and extra water may build up in the body. Serious kidney failure needs medical treatment. Some people may need dialysis or a kidney transplant.
Possible signs can include swelling, tiredness, reduced urine, nausea and high blood pressure. These signs can have many causes, so a doctor must diagnose the condition.
Urinary tract infections
A urinary tract infection, or UTI, happens when harmful microbes grow in the urinary tract.
Common symptoms may include:
Burning while passing urine
Frequent urination
Lower abdominal pain
Fever in some cases
Cloudy or strong-smelling urine
Good hygiene, drinking enough water and not holding urine for very long may help reduce risk.
Dialysis and how it works
Dialysis is a treatment used when kidneys cannot filter blood well enough.
A dialysis machine works like an artificial kidney. It removes wastes and extra fluid from the blood.
Basic steps in dialysis
Blood is taken from the body through a tube.
It passes through a dialyser, also called an artificial kidney.
Waste substances move out of the blood into a special fluid.
Cleaned blood returns to the body.
Dialysis does not cure kidney failure, but it helps remove wastes when kidneys are not able to do their job properly.
Healthy habits for kidney health
Good daily habits can support kidney health.
Drink enough water, especially in hot weather and after playing sports.
Do not hold urine for too long.
Eat a balanced diet with fruits, vegetables, pulses, grains and enough protein.
Avoid too much salty packaged food.
Stay active through walking, games or exercise.
Maintain good toilet hygiene.
Take medicines only as advised by a doctor or trusted adult.
Get medical help for burning urination, blood in urine or long-lasting pain.
Water is important, but drinking extreme amounts of water is not healthy either. The body needs balance.
Interesting facts about the excretory system
The kidneys filter blood many times a day.
Urine is made continuously, even when we are asleep.
The bladder stores urine, but it does not make urine.
The yellow colour of urine comes from pigments formed after breakdown of haemoglobin.
Healthy urine usually does not contain blood cells or large proteins.
The liver changes toxic ammonia into urea.
Sweating is both an excretory and cooling process.
The nephron is tiny, but it performs a very important job.
Common misconceptions about the excretory system
Misconception | Correct idea |
Only kidneys remove wastes | Lungs, skin and liver also help in excretion |
Urine is stored in kidneys | Urine is formed in kidneys but stored in the urinary bladder |
Excretion and egestion are the same | Excretion removes metabolic wastes, egestion removes undigested food |
Sweating removes only water | Sweat also contains salts and small amounts of urea |
More water is always better | The body needs the right amount of water, not too little or too much |
Urine formation is only filtration | It also includes reabsorption and secretion |
Urea is made in kidneys | Urea is made in the liver and removed by kidneys |
Exam tips for Classes 6–10
Draw the human excretory system neatly and label kidneys, ureters, urinary bladder and urethra.
In kidney diagrams, label cortex, medulla, renal pelvis, renal artery, renal vein and ureter.
For nephron diagrams, label Bowman’s capsule, glomerulus, tubules, loop of Henle and collecting duct.
Do not write that the bladder makes urine. It stores urine.
Remember the order of urine flow.
Kidney → Ureter → Urinary bladder → Urethra → Outside body
For urine formation, remember:
Filtration → Reabsorption → Secretion
Quick revision notes
Excretion removes metabolic wastes from the body.
The kidneys are the main excretory organs.
The basic unit of kidney is the nephron.
Blood enters the kidney through the renal artery.
Cleaned blood leaves through the renal vein.
Urine passes from kidneys to bladder through ureters.
Urine is stored in the urinary bladder.
Urine leaves the body through the urethra.
Urine formation has three steps: filtration, reabsorption and secretion.
Lungs remove carbon dioxide.
Skin removes sweat.
Liver changes ammonia into urea.
Dialysis helps clean blood when kidneys fail.
20 solved examples and questions
1. What is excretion?
Excretion is the removal of metabolic wastes from the body.
2. Name the main organs of the human excretory system.
The main organs are kidneys, ureters, urinary bladder and urethra.
3. What is the function of kidneys?
Kidneys filter blood and remove urea, extra water and extra salts to form urine.
4. What is a nephron?
A nephron is the basic filtering unit of the kidney.
5. Where is urine stored?
Urine is stored in the urinary bladder.
6. Which tube carries urine from kidney to bladder?
The ureter carries urine from kidney to urinary bladder.
7. Which tube carries urine out of the body?
The urethra carries urine out of the body.
8. What is the first step of urine formation?
Filtration is the first step of urine formation.
9. What happens during reabsorption?
Useful substances like glucose, water and needed salts return to the blood.
10. What happens during secretion?
Extra wastes and ions move from blood into the nephron tubule.
11. Name one waste removed by lungs.
The lungs remove carbon dioxide.
12. Name one waste removed by skin.
Skin removes water, salts and small amounts of urea in sweat.
13. Where is urea formed?
Urea is formed in the liver.
14. Is egestion the same as excretion?
No. Egestion removes undigested food. Excretion removes metabolic wastes.
15. Why is urine yellow?
Urine is yellow because of pigments formed from the breakdown of haemoglobin.
16. What is dialysis?
Dialysis is a treatment that cleans blood when kidneys cannot filter properly.
17. What are kidney stones?
Kidney stones are hard deposits formed from minerals in the urinary system.
18. What can happen if kidneys fail?
Wastes and extra water can build up in the body. Medical treatment is needed.
19. Why should we not hold urine for a very long time?
Holding urine for too long may increase discomfort and can raise the risk of urinary problems.
20. Give one healthy habit for kidneys.
Drink enough water and maintain good toilet hygiene.
25 practice questions
Define excretion.
Define egestion.
Write two differences between excretion and egestion.
Name the main parts of the human excretory system.
What is the function of the ureter?
What is the function of the urinary bladder?
What is the function of the urethra?
What is the basic unit of the kidney?
Name two parts of a nephron.
10. What is glomerulus?
11. What happens in Bowman’s capsule?
12. Name the three stages of urine formation.
13. What is reabsorption?
14. Why is secretion important?
15. Name the artery that brings blood to the kidney.
16. Name the vein that carries blood away from the kidney.
17. What is the main nitrogenous waste in human urine?
18. Name two substances found in urine.
19. How do lungs help in excretion?
20. How does skin help in excretion?
21. What is the role of liver in excretion?
22. What is dialysis?
23. Write two symptoms of urinary tract infection.
24. Write two habits that support kidney health.
25. Draw and label a neat diagram of the human excretory system.
Answer key
Removal of metabolic wastes from the body.
Removal of undigested food from the digestive system.
Excretion removes wastes made by cells, egestion removes undigested food. Excretion uses organs like kidneys and lungs, egestion uses the digestive tract.
Kidneys, ureters, urinary bladder and urethra.
It carries urine from kidney to urinary bladder.
It stores urine temporarily.
It carries urine out of the body.
Nephron.
Any two: Bowman’s capsule, glomerulus, tubule, loop of Henle, collecting duct.
10. A network of tiny blood capillaries inside Bowman’s capsule.
11. Filtered fluid collects there after filtration.
12. Filtration, reabsorption and secretion.
13. Return of useful substances from nephron tubules to blood.
14. It adds extra wastes and ions into the tubule for removal.
15. Renal artery.
16. Renal vein.
17. Urea.
18. Any two: water, urea, salts, creatinine, uric acid.
19. They remove carbon dioxide and water vapour.
20. It removes sweat containing water, salts and a little urea.
21. It converts toxic ammonia into urea and processes harmful substances.
22. A treatment that cleans blood when kidneys cannot work properly.
23. Any two: burning urination, frequent urination, lower abdominal pain, cloudy urine, fever.
24. Any two: drink enough water, avoid holding urine, eat balanced food, reduce excess salt, maintain hygiene.
25. Diagram should show two kidneys, two ureters, urinary bladder and urethra with correct labels.
10 frequently asked questions
1. What is the main function of the excretory system?
The main function is to remove metabolic wastes and maintain water and salt balance.
2. Which organ makes urine?
The kidneys make urine.
3. Which organ stores urine?
The urinary bladder stores urine.
4. What is the role of the nephron?
The nephron filters blood and forms urine through filtration, reabsorption and secretion.
5. Is sweat a waste?
Yes. Sweat contains water, salts and small amounts of urea.
6. Why do we pass less urine on hot days?
We lose more water through sweat, so kidneys save water and produce less urine.
7. Why do we pass more urine after drinking more water?
The body has extra water, so kidneys remove more water in urine.
8. Can urine show health problems?
Yes. Blood, protein, excess sugar, unusual colour or pain during urination may show a health problem. A doctor should check it.
9. What is dialysis used for?
Dialysis is used when kidneys cannot clean blood properly.
10. What is the best way to keep kidneys healthy?
Drink enough water, eat balanced food, avoid excess salt, stay active and do not ignore urinary symptoms.
Conclusion
The human excretory system keeps the body clean from the inside. The kidneys filter blood, nephrons form urine, ureters carry urine, the bladder stores it, and the urethra removes it. Lungs, skin and liver also help remove wastes.
For exams, focus on three big ideas: excretion removes metabolic wastes, the nephron is the working unit of the kidney, and urine formation includes filtration, reabsorption and secretion. If these are clear, the whole chapter becomes much easier to learn and revise.
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