Excretion in Humans
Summary: Excretion is the removal of metabolic waste products (carbon dioxide, urea, excess water, salts) from the body. The main excretory organs are the lungs (CO2), kidneys (urea, excess water, salts), and skin (water, salts via sweat). The kidneys contain millions of nephrons which filter blood by ultrafiltration in the glomerulus/Bowman’s capsule, then selectively reabsorb glucose, amino acids, some water, and some salts back into the blood. Urine (urea, excess water, excess salts) passes to the bladder. ADH controls water reabsorption in the collecting duct (osmoregulation). Tags: igcse biology excretion kidney nephron ultrafiltration osmoregulation ADH Created: 2026-07-16 Last Updated: 2026-07-16
1. Excretion vs Egestion
Excretion is the removal of metabolic waste products — substances that have been produced by chemical reactions in cells (metabolism), and substances that are present in excess of the body’s requirements.
Egestion is the removal of undigested food (faeces) from the digestive system. This material has never entered the body’s cells — it is NOT a metabolic product.
| Feature | Excretion | Egestion |
|---|---|---|
| What is removed? | Metabolic waste (CO2, urea, excess water, salts) | Undigested food (fibre/cellulose, bacteria, dead gut cells) |
| Where does it come from? | Metabolic reactions in cells | Food that was never absorbed |
| Is it a characteristic of life (MRS GREN)? | Yes (E = Excretion) | No |
2. Excretory Organs and Their Products
| Organ | Excretory Products | Source of Waste |
|---|---|---|
| Lungs | Carbon dioxide (CO2), water vapour | CO2 from aerobic respiration in all body cells; water vapour from moist lung surfaces |
| Kidneys | Urea, excess water, excess salts (ions) | Urea from deamination of excess amino acids in the liver; water and salts from diet and metabolic processes |
| Skin | Water, mineral salts (small amounts of urea) | Sweat glands excrete water and salts for thermoregulation; small amount of urea is also lost |
The liver’s role in excretion:
- Deamination: Excess amino acids are broken down in the liver. The amino group (NH2) is removed and converted to ammonia (toxic), then quickly converted to urea (less toxic). Urea is released into the blood and transported to the kidneys for excretion
- Detoxification: The liver breaks down toxic substances including alcohol, drugs, and hormones, converting them into forms that can be excreted by the kidneys
3. Kidney Structure
Each kidney contains approximately one million microscopic filtration units called nephrons.
| Structure | Description and Function |
|---|---|
| Cortex (outer region) | Contains the glomeruli (capillary knots), Bowman’s capsules, and the convoluted portions of nephrons. Site of ultrafiltration and selective reabsorption |
| Medulla (inner region) | Contains the loops of Henle and collecting ducts. Site of water and salt regulation |
| Renal artery | Branches from the aorta — delivers blood (containing urea, water, salts) to the kidney at high pressure |
| Renal vein | Carries filtered blood (lower urea, normal water/salt levels) away from the kidney |
| Ureter | Tube carrying urine from the kidney to the bladder |
| Bladder | Muscular sac that stores urine until it is expelled |
| Urethra | Tube carrying urine from the bladder out of the body |
4. The Nephron — Functional Unit of the Kidney
The nephron is the microscopic unit where blood is filtered and urine is formed. Each kidney contains ~1 million nephrons.
Structure of a Nephron
| Part | Description |
|---|---|
| Glomerulus | A knot of blood capillaries inside the Bowman’s capsule. Blood enters at high pressure through the afferent arteriole (wide) and leaves through the efferent arteriole (narrower) — the pressure difference forces fluid out |
| Bowman’s capsule | A cup-shaped structure surrounding the glomerulus. Collects the filtrate (fluid filtered out of the blood) |
| Proximal convoluted tubule (PCT) | First coiled section. Most selective reabsorption occurs here — all glucose, all amino acids, most water, and most salts are reabsorbed back into the blood |
| Loop of Henle | U-shaped section descending into the medulla and ascending back. Creates a concentration gradient in the medulla — important for water reabsorption later |
| Distal convoluted tubule (DCT) | Second coiled section. Further fine-tuning of salt and water balance |
| Collecting duct | Receives urine from several nephrons. Water reabsorption here is controlled by ADH (antidiuretic hormone). Urine passes to the ureter |
Ultrafiltration (Glomerulus + Bowman’s Capsule)
- Blood enters the glomerulus through the afferent arteriole (wider) at high pressure
- The efferent arteriole is narrower, creating a pressure build-up in the glomerulus
- This high pressure forces small molecules out of the blood through the capillary walls and into the Bowman’s capsule:
- Filtered out: water, glucose, amino acids, urea, salts (ions), vitamins, hormones
- Retained in blood: red blood cells, white blood cells, platelets, large plasma proteins (e.g. albumin, fibrinogen) — too large to pass through the capillary wall
- The fluid collected in the Bowman’s capsule is called the glomerular filtrate
Selective Reabsorption (Proximal Convoluted Tubule)
As the filtrate flows through the PCT:
- All glucose is reabsorbed by active transport back into the blood capillaries surrounding the nephron
- All amino acids are reabsorbed by active transport
- Most water is reabsorbed by osmosis (following the solutes)
- Most salts (ions) are reabsorbed by active transport and diffusion
- Urea is NOT reabsorbed — it remains in the filtrate (except some passive reabsorption)
The filtrate leaving the PCT contains mainly water, urea, and some salts.
Water Regulation — Osmoregulation (Collecting Duct)
The amount of water reabsorbed in the collecting duct is controlled by ADH (antidiuretic hormone).
| Condition | ADH Level | Collecting Duct Permeability | Water Reabsorbed | Urine Produced |
|---|---|---|---|---|
| Dehydrated / high blood solute concentration | High ADH released from pituitary gland | Walls become more permeable | More water reabsorbed into blood | Small volume, concentrated (dark yellow) |
| Hydrated / low blood solute concentration | Low ADH released | Walls become less permeable | Less water reabsorbed | Large volume, dilute (pale/colourless) |
Negative feedback mechanism: Blood water concentration decreases → detected by hypothalamus → pituitary releases more ADH → collecting duct reabsorbs more water → blood water concentration returns to normal → ADH release reduced.
5. Composition of Urine
| Component | Present in Urine? | Notes |
|---|---|---|
| Water | Yes (large amount) | Volume varies depending on hydration |
| Urea | Yes | Main nitrogenous waste from amino acid breakdown |
| Salts (Na+, K+, Cl-) | Yes (excess) | Homeostatic regulation of blood salt concentration |
| Glucose | No (should not be present) | All glucose is reabsorbed in the PCT. Glucose in urine indicates diabetes |
| Amino acids | No (should not be present) | All reabsorbed in the PCT |
| Proteins | No (should not be present) | Too large to pass through glomerular filtration barrier. Protein in urine indicates kidney damage |
Related Notes
- Characteristics and Classification of Living Organisms — Excretion as one of the seven characteristics of life (MRS GREN)
- Transport in Animals — Blood transports urea from the liver to the kidneys; plasma composition
- Human Nutrition — Deamination of excess amino acids in the liver produces urea
- Biological Molecules — Proteins are made of amino acids; nitrogen is excreted as urea
- Coordination and Response — ADH as a hormone; negative feedback in osmoregulation
- Human Influences on Ecosystems — Eutrophication from excess nitrates in water systems
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Excretion / Kidneys and osmoregulation, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 41: Osmotic Regulation and Excretion, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 13: Excretion in Humans, Cambridge Assessment International Education
- CK-12 Biology for High School — Excretory System chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Excretion is the same as egestion” | Excretion removes metabolic waste (urea, CO2); egestion removes undigested food (faeces) that never entered cells |
| ”The kidneys filter the blood by sieving out all small molecules” | Ultrafiltration is pressure-driven, but useful substances (glucose, amino acids) are later actively reabsorbed — they are not lost in urine |
| ”Urine contains glucose” | In a healthy person, all glucose is reabsorbed in the PCT. Glucose in urine is a sign of diabetes |
| ”Urea is made in the kidneys” | Urea is made in the liver by deamination of excess amino acids. The kidneys only excrete it |
| ”The more water you drink, the more ADH is released” | ADH is released when the blood is too concentrated (dehydrated). Drinking more water causes the pituitary to release less ADH |
| ”The skin is not an excretory organ” | The skin excretes water, salts, and a small amount of urea through sweat — it is an excretory organ, primarily for thermoregulation |