What is excretion?
Cells produce harmful by-products while doing their work. Breaking down proteins produces nitrogen wastes like ammonia, urea and uric acid. Excretion is the biological process of removing these harmful metabolic wastes from the body.
Single-celled organisms simply let wastes diffuse out through their surface into water. Complex animals need special organs.
Excretion vs egestion: egestion is removing undigested food through the anus, which is not a waste made by cells.
The human excretory system
- Two kidneys: bean-shaped, in the abdomen on either side of the backbone. They filter blood.
- Two ureters: tubes that carry urine from kidneys to the bladder.
- Urinary bladder: a muscular bag that stores urine. It is under nervous control, so we can hold urine for some time.
- Urethra: the tube through which urine leaves the body.
Blood enters each kidney through the renal artery and cleaned blood leaves through the renal vein.
Nephron: structure and how urine is formed
Each kidney has about a million tiny filtering units called nephrons. A nephron has:
- Glomerulus: a tight bunch of capillaries.
- Bowman's capsule: a cup around the glomerulus that collects the filtrate.
- Tubule (with a loop in the middle) leading to a collecting duct.
Three steps
- Filtration: blood pressure pushes water, glucose, amino acids, salts and urea from the glomerulus into Bowman's capsule. Cells and big proteins stay behind.
- Selective reabsorption: along the tubule, useful glucose, amino acids, salts and a major part of the water are taken back into the blood around the tubule. How much water is taken back depends on how much extra water the body has.
- Urine: the leftover urea, some salts and extra water flow into the collecting duct → ureter → bladder.
The kidneys filter roughly 180 litres of filtrate a day, yet only about 1–2 litres leave as urine, because most is reabsorbed.
Artificial kidney (haemodialysis)
Infection, injury or low blood flow can damage kidneys. When they fail, poisonous wastes build up and can cause death. An artificial kidney removes nitrogen wastes by dialysis: the patient's blood is passed through long tubes with semi-permeable walls, bathed in a dialysing fluid that has the same osmotic pressure as blood but no nitrogen wastes. Wastes pass out of blood into the fluid, and the cleaned blood is returned to the patient.
Unlike a real kidney, dialysis does not reabsorb anything. For lasting kidney failure, a kidney transplant from a matching donor (often a relative) is an option.
Excretion in plants
Plants have no excretory organs, and use different methods:
- Gases: oxygen (from photosynthesis) and CO₂ (from respiration) leave through stomata.
- Extra water: removed by transpiration.
- Falling leaves: many wastes are stored in leaves that later fall off.
- Cell vacuoles: some wastes are stored in vacuoles.
- Bark: wastes stored in old xylem and bark that peel off.
- Resins and gums: stored especially in old xylem.
- Into the soil: some wastes are released around the roots.
Exam tip: nephron diagram and urine formation (5 marks), dialysis, and methods of plant excretion are frequent questions.
Key formulas and definitions
- Excretion: removal of harmful nitrogen wastes (urea, uric acid, ammonia)
- Kidney → ureter → urinary bladder → urethra
- Nephron = glomerulus + Bowman's capsule + tubule + collecting duct
- Urine formation: filtration → selective reabsorption → urine
- Dialysis: blood cleaned through a semi-permeable membrane
- Plant excretion: stomata, transpiration, falling leaves, bark, resins, gums, vacuoles
Worked examples
1. Why does urine not normally contain glucose?
Glucose is filtered out in the glomerulus, but it is fully taken back into the blood by the tubule during selective reabsorption. If glucose appears in urine, the blood sugar is too high for the tubule to take all of it back, as in diabetes.
2. Why does a person produce less urine in summer?
In summer the body loses much water as sweat. To save water, the nephrons take back more water from the filtrate, so less and more concentrated urine is formed.
3. What is the difference between a real kidney and a dialysis machine?
Both remove nitrogen wastes by filtration. A real kidney also reabsorbs useful substances like glucose, salts and water; the dialysis machine does not reabsorb anything.
4. Why are blood cells not found in normal urine?
They are too big to pass through the walls of the glomerulus into Bowman's capsule, so they stay in the blood. Blood in urine points to damage in the kidney or urinary tract.
5. Name three ways plants get rid of wastes.
Through stomata (gases), by shedding leaves and bark that store wastes, and by storing resins and gums in old xylem. Extra water is lost by transpiration.
Common mistakes
- Calling removal of faeces 'excretion'. It is egestion; excretion is about metabolic wastes like urea.
- Writing that urine is made in the bladder. Urine is made in the nephrons of the kidney and only stored in the bladder.
- Saying all filtered water is lost as urine. Most of it is reabsorbed in the tubule.
- Mixing up ureter (kidney → bladder) and urethra (bladder → outside).