Your Kidneys Filter Everything, Then Take Most of It Back
Learn the excretory products and the organ that removes each, label the urinary system and the parts of a kidney, follow urine formation through a nephron, and understand how dialysis works.
Why do your kidneys filter far more than they throw away?
Because filtering is crude and must be corrected afterwards. The kidney pushes almost everything small out of the blood, useful or not, and then takes back the glucose, most of the water and the salts the body still needs.
Filtering first and reclaiming later is simpler than filtering selectively. This page covers everything in the ICSE Class 7 Biology chapter on excretion: the excretory products and their organs, the urinary system, the nephron and urine formation, and dialysis.
Filtering first and reclaiming later is simpler than filtering selectively. This page covers everything in the ICSE Class 7 Biology chapter on excretion: the excretory products and their organs, the urinary system, the nephron and urine formation, and dialysis.
What are the excretory products, and which organ removes each?
Excretion is the removal of the harmful nitrogenous and other waste products formed during the body's chemical activities.
Four organs share the work:
- Kidneys — remove urea, excess water and excess salts, as urine. These are the main excretory organs.
- Lungs — remove carbon dioxide and water vapour, in exhaled air.
- Skin — removes water, salts and a little urea, as sweat.
- Liver — converts the ammonia produced from the breakdown of proteins into the less harmful urea, and removes bile pigments and worn-out red blood cells.
Everyday evidence is easy to find: sweat tastes salty, and breathing onto a cold mirror leaves a film of water.
The liver is the organ students misplace. It does not pass waste out of the body — it prepares waste, turning highly poisonous ammonia into urea, which the kidneys can then safely remove. Naming the liver as an excretory organ without that explanation loses the mark.
Four organs share the work:
- Kidneys — remove urea, excess water and excess salts, as urine. These are the main excretory organs.
- Lungs — remove carbon dioxide and water vapour, in exhaled air.
- Skin — removes water, salts and a little urea, as sweat.
- Liver — converts the ammonia produced from the breakdown of proteins into the less harmful urea, and removes bile pigments and worn-out red blood cells.
Everyday evidence is easy to find: sweat tastes salty, and breathing onto a cold mirror leaves a film of water.
The liver is the organ students misplace. It does not pass waste out of the body — it prepares waste, turning highly poisonous ammonia into urea, which the kidneys can then safely remove. Naming the liver as an excretory organ without that explanation loses the mark.
What are the parts of the urinary system?
The urinary system has four parts, in the order urine travels:
- Kidneys — a pair of dark red, bean-shaped organs on either side of the backbone, in the abdomen. They filter the blood and make urine. The right kidney sits slightly lower than the left, because the liver takes up room above it.
- Ureters — two narrow tubes, one from each kidney, carrying urine down to the bladder.
- Urinary bladder — a muscular, stretchable bag that stores urine temporarily.
- Urethra — the tube through which urine leaves the body, controlled by a ring of muscle.
Inside, a kidney has three regions:
- Cortex — the outer, darker region, containing the filtering capsules.
- Medulla — the inner, lighter region, made of cone-shaped pyramids.
- Pelvis — the funnel-shaped hollow space where urine collects before entering the ureter.
Blood arrives through the renal artery and leaves through the renal vein.
The point worth noticing is that nothing is added or removed after the kidney. The ureters, bladder and urethra only carry and store urine — its composition is fixed by the time it leaves the kidney, which is why disease of the kidney and disease of the bladder cause quite different problems.
- Kidneys — a pair of dark red, bean-shaped organs on either side of the backbone, in the abdomen. They filter the blood and make urine. The right kidney sits slightly lower than the left, because the liver takes up room above it.
- Ureters — two narrow tubes, one from each kidney, carrying urine down to the bladder.
- Urinary bladder — a muscular, stretchable bag that stores urine temporarily.
- Urethra — the tube through which urine leaves the body, controlled by a ring of muscle.
Inside, a kidney has three regions:
- Cortex — the outer, darker region, containing the filtering capsules.
- Medulla — the inner, lighter region, made of cone-shaped pyramids.
- Pelvis — the funnel-shaped hollow space where urine collects before entering the ureter.
Blood arrives through the renal artery and leaves through the renal vein.
The point worth noticing is that nothing is added or removed after the kidney. The ureters, bladder and urethra only carry and store urine — its composition is fixed by the time it leaves the kidney, which is why disease of the kidney and disease of the bladder cause quite different problems.
How does a nephron make urine?
A nephron is the structural and functional unit of the kidney, and each kidney contains about a million of them. Urine is made in two stages.
Each nephron has a cup-shaped Bowman's capsule enclosing a knot of capillaries called the glomerulus, followed by a long coiled tubule surrounded by blood capillaries.
Stage 1 — filtration. Blood enters the glomerulus under pressure, and the small molecules are forced out into the Bowman's capsule. This filtrate contains water, glucose, salts, urea and amino acids. Large things — blood cells and proteins — are too big to pass and stay in the blood.
Stage 2 — selective reabsorption. As the filtrate travels along the tubule, the surrounding capillaries take back what the body needs: all the glucose, most of the water, and the amino acids and useful salts.
What is left — urea, excess salts and excess water — continues as urine into the collecting duct, the pelvis and the ureter.
The word selective carries the whole idea. Both useful and waste substances are filtered out together, and it is reabsorption that separates them.
This also explains a familiar medical test. Glucose in urine is abnormal, because every bit of it should have been reabsorbed — so finding it means there was more than the tubule could take back.
Each nephron has a cup-shaped Bowman's capsule enclosing a knot of capillaries called the glomerulus, followed by a long coiled tubule surrounded by blood capillaries.
Stage 1 — filtration. Blood enters the glomerulus under pressure, and the small molecules are forced out into the Bowman's capsule. This filtrate contains water, glucose, salts, urea and amino acids. Large things — blood cells and proteins — are too big to pass and stay in the blood.
Stage 2 — selective reabsorption. As the filtrate travels along the tubule, the surrounding capillaries take back what the body needs: all the glucose, most of the water, and the amino acids and useful salts.
What is left — urea, excess salts and excess water — continues as urine into the collecting duct, the pelvis and the ureter.
The word selective carries the whole idea. Both useful and waste substances are filtered out together, and it is reabsorption that separates them.
This also explains a familiar medical test. Glucose in urine is abnormal, because every bit of it should have been reabsorbed — so finding it means there was more than the tubule could take back.
What is in normal urine, and what happens when kidneys fail?
Normal urine is a pale yellow liquid, slightly acidic, with a characteristic smell. Its composition is roughly:
- Water — about 95 percent
- Urea — the chief nitrogenous waste
- Salts — mainly sodium chloride
- Uric acid, creatinine and traces of pigments
Normal urine contains no glucose, no protein and no blood cells. Finding any of these points to a disorder.
The skin assists excretion through its sweat glands. Sweat is water with dissolved salts and a little urea; as it evaporates it also cools the body, which is why you sweat most in the heat and after exercise. On a humid Chennai afternoon sweat cools you poorly, because it cannot evaporate quickly.
When the kidneys fail, urea builds up in the blood and becomes poisonous. Dialysis by an artificial kidney takes over the job:
- Blood is drawn from an artery in the patient's arm and passed through long tubes of a selectively permeable membrane.
- These tubes sit in a dialysing fluid containing the correct amounts of glucose and salts but no urea.
- Urea and excess salts diffuse out of the blood into the fluid, because of the difference in concentration, while glucose and needed salts stay put.
- The cleaned blood is returned to a vein in the same arm.
The elegance of the design is in the fluid. It has no urea, so urea always moves outward, and it has the right amount of glucose, so no glucose is lost — the machine copies selective reabsorption using nothing but diffusion.
- Water — about 95 percent
- Urea — the chief nitrogenous waste
- Salts — mainly sodium chloride
- Uric acid, creatinine and traces of pigments
Normal urine contains no glucose, no protein and no blood cells. Finding any of these points to a disorder.
The skin assists excretion through its sweat glands. Sweat is water with dissolved salts and a little urea; as it evaporates it also cools the body, which is why you sweat most in the heat and after exercise. On a humid Chennai afternoon sweat cools you poorly, because it cannot evaporate quickly.
When the kidneys fail, urea builds up in the blood and becomes poisonous. Dialysis by an artificial kidney takes over the job:
- Blood is drawn from an artery in the patient's arm and passed through long tubes of a selectively permeable membrane.
- These tubes sit in a dialysing fluid containing the correct amounts of glucose and salts but no urea.
- Urea and excess salts diffuse out of the blood into the fluid, because of the difference in concentration, while glucose and needed salts stay put.
- The cleaned blood is returned to a vein in the same arm.
The elegance of the design is in the fluid. It has no urea, so urea always moves outward, and it has the right amount of glucose, so no glucose is lost — the machine copies selective reabsorption using nothing but diffusion.
Exam tip
Exam tip: naming both stages of urine formation
Excretion questions are marked on named structures and named stages, so be specific.
Give both stages with their locations: filtration in the glomerulus and Bowman's capsule, then selective reabsorption in the tubule. One stage earns half the marks.
Use the word selective, and say what is reabsorbed — all the glucose, most of the water, useful salts and amino acids.
When naming excretory organs, pair each with its product: kidneys and urea, lungs and carbon dioxide, skin and sweat, liver and urea formation plus bile pigments.
For dialysis, mention that the dialysing fluid contains no urea but the correct glucose concentration. That is the detail the mark is for.
And remember that normal urine contains no glucose and no protein — a favourite one-mark question.
Give both stages with their locations: filtration in the glomerulus and Bowman's capsule, then selective reabsorption in the tubule. One stage earns half the marks.
Use the word selective, and say what is reabsorbed — all the glucose, most of the water, useful salts and amino acids.
When naming excretory organs, pair each with its product: kidneys and urea, lungs and carbon dioxide, skin and sweat, liver and urea formation plus bile pigments.
For dialysis, mention that the dialysing fluid contains no urea but the correct glucose concentration. That is the detail the mark is for.
And remember that normal urine contains no glucose and no protein — a favourite one-mark question.
Did you know
Why is a fever more tiring on a humid day than a dry one?
Because cooling depends on sweat evaporating, not on sweat being produced.
Sweat carries heat away only as it turns into vapour. In dry air that happens fast, so you cool quickly and the sweat seems to vanish. In humid air the surrounding air is already loaded with water vapour, so the sweat sits on your skin and cools you far less.
You lose the same water and salts either way — you simply get less cooling in return, which is exactly why a humid day feels more exhausting than a hotter dry one.
Sweat carries heat away only as it turns into vapour. In dry air that happens fast, so you cool quickly and the sweat seems to vanish. In humid air the surrounding air is already loaded with water vapour, so the sweat sits on your skin and cools you far less.
You lose the same water and salts either way — you simply get less cooling in return, which is exactly why a humid day feels more exhausting than a hotter dry one.
Key takeaways
Excretion in humans: quick revision
- Excretion removes nitrogenous wastes: kidneys remove urea, water and salts; lungs carbon dioxide and water vapour; skin sweat; and the liver converts ammonia into urea.
- The urinary system runs kidneys, ureters, urinary bladder, urethra; a kidney has a cortex, medulla and pelvis.
- The nephron is the kidney's functional unit, with a Bowman's capsule enclosing the glomerulus, followed by a tubule.
- Filtration forces small molecules out under pressure, leaving blood cells and proteins behind; selective reabsorption takes back all the glucose, most water and useful salts.
- Normal urine is about 95 percent water with urea and salts, and contains no glucose, protein or blood cells.
- Dialysis passes blood along a selectively permeable membrane in a fluid with no urea but the correct glucose level, so urea diffuses out while glucose stays.
You will remember all of this far better after answering five questions on it than after reading it twice.
- The urinary system runs kidneys, ureters, urinary bladder, urethra; a kidney has a cortex, medulla and pelvis.
- The nephron is the kidney's functional unit, with a Bowman's capsule enclosing the glomerulus, followed by a tubule.
- Filtration forces small molecules out under pressure, leaving blood cells and proteins behind; selective reabsorption takes back all the glucose, most water and useful salts.
- Normal urine is about 95 percent water with urea and salts, and contains no glucose, protein or blood cells.
- Dialysis passes blood along a selectively permeable membrane in a fluid with no urea but the correct glucose level, so urea diffuses out while glucose stays.
You will remember all of this far better after answering five questions on it than after reading it twice.