Excretory system questions are among the most direct in biology. Which organ, which unit, which part filters, what the normal output is. The answers never change and the whole topic fits on a few tables. This post covers the organs of excretion, the kidney and the nephron, urine formation, and the diseases and terms exams ask about, with 15 previous year questions.
Key Points (Quick Revision)
- Nephron is the structural and functional unit of the kidney — about one million per kidney
- Filtration happens in the glomerulus, inside Bowman's capsule
- Urea is made in the liver, not the kidney, and removed by the kidney
- ADH (vasopressin) controls water reabsorption; less ADH means more urine
- Order of flow: kidney → ureter → urinary bladder → urethra
1. Organs of excretion
The kidneys are the main excretory organs, but they are not the only ones, and this classification is asked directly.
| Organ | What it excretes |
|---|---|
| Kidneys | Urea, uric acid, creatinine, excess water and salts, as urine |
| Lungs | Carbon dioxide and water vapour |
| Skin | Water, salts and a little urea, as sweat |
| Liver | Bile pigments; also produces urea from ammonia |
The liver makes urea; the kidney removes it. That division is the single most common trap in this topic. Ammonia produced from the breakdown of proteins is converted to less toxic urea in the liver through the ornithine cycle, and the kidney then filters it out of the blood.
Humans are described as ureotelic, meaning the main nitrogenous waste excreted is urea. Birds and reptiles are uricotelic, excreting uric acid, and most fish are ammonotelic, excreting ammonia directly into water. That three-way classification is asked as its own question.
2. The kidney and the nephron
| Part | What it does |
|---|---|
| Nephron | The structural and functional unit; about one million in each kidney |
| Glomerulus | A tuft of capillaries where blood is filtered under pressure |
| Bowman's capsule | The cup that surrounds the glomerulus and collects the filtrate |
| Malpighian body | Glomerulus and Bowman's capsule together |
| Proximal and distal tubules | Reabsorption of useful substances and secretion of wastes |
| Loop of Henle | Concentrates the urine by reabsorbing water and salts |
| Collecting duct | Final water reabsorption; carries urine towards the ureter |
The kidney is bean-shaped, lies in the abdominal cavity on either side of the vertebral column, and the right kidney sits slightly lower than the left because of the liver above it — a detail papers like.
Glomerulus for filtration, loop of Henle for concentration is the pairing to fix. The filtrate leaving Bowman's capsule is close to plasma without the large proteins; almost all of the glucose, amino acids and most of the water are reabsorbed before urine leaves the nephron.
3. How urine is formed
Urine formation happens in three steps, and the order is asked directly.
| Step | Where | What happens |
|---|---|---|
| 1. Glomerular filtration | Glomerulus and Bowman's capsule | Blood is filtered under pressure; water, salts, glucose, amino acids and urea pass through, large proteins and blood cells do not |
| 2. Selective reabsorption | Tubules and loop of Henle | Glucose, amino acids, most water and needed salts are taken back into the blood |
| 3. Tubular secretion | Distal tubule | Extra wastes and ions are added to the filtrate from the blood |
Glucose is normally reabsorbed completely, which is why glucose in urine points to diabetes mellitus. That link between a normal process and a disease is one of the most asked connections in this topic.
Two hormones control the final concentration. ADH, also called vasopressin, increases water reabsorption — more ADH means less and more concentrated urine, less ADH means more dilute urine. Aldosterone, from the adrenal cortex, controls sodium reabsorption.
4. Diseases and terms
| Term | What it means |
|---|---|
| Dialysis | Artificial filtering of blood when the kidneys fail; the artificial kidney works on the principle of diffusion |
| Kidney stones | Hard deposits, commonly of calcium oxalate |
| Nephritis | Inflammation of the nephrons |
| Diabetes mellitus | Glucose in urine, due to insulin problems |
| Diabetes insipidus | Large volumes of dilute urine, due to lack of ADH — not a sugar disease |
| Uraemia | Excess urea in the blood |
| Micturition | The act of passing urine |
Diabetes mellitus against diabetes insipidus is the pair papers use as a trap. They share a name but not a cause — mellitus involves glucose and insulin, insipidus involves water and ADH.
Normal urine is about 95 per cent water, with urea as the main solid waste, and its yellow colour comes from the pigment urochrome. An adult typically passes between one and two litres a day, varying with intake and climate.
5. Previous year questions
- What is the structural and functional unit of the kidney? — Nephron
- About how many nephrons are there in each kidney? — About one million
- Where does filtration of blood take place in the nephron? — Glomerulus
- The glomerulus is enclosed in which structure? — Bowman's capsule
- Which organ produces urea? — The liver
- Which organ removes urea from the blood? — The kidney
- Which part of the nephron concentrates urine? — Loop of Henle
- What is the correct order of urine flow? — Kidney, ureter, urinary bladder, urethra
- Which hormone controls water reabsorption in the kidney? — ADH or vasopressin
- Lack of ADH causes which condition? — Diabetes insipidus
- Presence of glucose in urine indicates what? — Diabetes mellitus
- An artificial kidney works on which principle? — Diffusion
- Which pigment gives urine its yellow colour? — Urochrome
- Humans excrete which main nitrogenous waste? — Urea, so humans are ureotelic
- Which kidney is positioned slightly lower? — The right kidney
6. How to revise this topic
Learn it as three cards. Card one is the parts — nephron, glomerulus, Bowman's capsule, loop of Henle — with one function each. Card two is the three steps of urine formation in order. Card three is the disease and term list.
Then fix the three traps that cost the most marks. The liver makes urea and the kidney removes it. Diabetes mellitus is about glucose and insulin while diabetes insipidus is about water and ADH. And more ADH means less urine, not more — candidates reverse this constantly.
7. Frequently Asked Questions
What is the functional unit of the kidney?
The nephron. Each kidney has about a million of them, and each consists of a glomerulus inside Bowman's capsule followed by tubules and the loop of Henle.
Does the kidney produce urea?
No. Urea is produced in the liver from ammonia formed during protein breakdown. The kidney filters it out of the blood and excretes it in urine, which is why the two are often confused.
What is the difference between diabetes mellitus and diabetes insipidus?
Mellitus involves glucose and insulin, and shows up as glucose in the urine. Insipidus involves water balance and is caused by a lack of ADH, producing large volumes of dilute urine. They share a name but not a cause.
What does ADH do?
ADH, also called vasopressin, increases water reabsorption in the kidney. More ADH means less and more concentrated urine; less ADH means more dilute urine. Candidates frequently reverse this.
On what principle does an artificial kidney work?
Diffusion. In dialysis, wastes move from the blood across a membrane into the dialysing fluid because of the concentration difference, doing the job the failed kidneys cannot.
For more General Science topics in simple language, see our Human Body Systems and Endocrine System and Hormones posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.