Maarg Manthan · Topic 3.4

Excretory System: Excretion in Plants and Animals

Science & Technology › Human Physiology – Organ Systems · Topic 3.4

Excretion is the removal of metabolic waste products from the body, such as urea, uric acid, ammonia, carbon dioxide and excess salts. In humans, the main excretory organs are the two kidneys, which filter blood and form urine, and the urine passes through the ureters to the urinary bladder and out through the urethra. Other organs also remove wastes: the lungs remove carbon dioxide, the skin removes water, salts and some urea in sweat, and the liver changes toxic ammonia into urea and breaks down old red cells into bile pigments. The functional unit of the kidney is the nephron, and each kidney has about one million nephrons. Animals have different excretory structures, and plants excrete in different ways.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

Main organs Kidneys, ureters, bladder, urethra
Functional unit Nephron (about 1 million per kidney)
Main nitrogenous waste in humans Urea
Daily filtrate About 180 litres
Daily urine About 1.5 litres
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the fourth topic of the macro on human physiology and organ systems. It describes how wastes leave the body and leads into the control of water and temperature in the next topic. Previous topic: Reproductive System: Asexual and Sexual Reproduction in Plants, Animals and Humans. Next topic: Osmoregulation, ADH and Formation of Urine; Thermoregulation.

What Is the Structure of the Kidney and Nephron?

Part	Function
Kidney	A bean-shaped organ about 10 to 12 centimetres long, with an outer cortex and inner medulla. The renal artery brings blood and the renal vein takes it away
Glomerulus and Bowman’s capsule	Together called the renal corpuscle or Malpighian body. Blood is filtered under pressure into the capsule
Proximal convoluted tubule	Reabsorbs most of the useful substances, including glucose, amino acids and about two thirds of water and salts
Loop of Henle	Creates a salt gradient in the medulla that helps to concentrate urine
Distal convoluted tubule	Regulates salts, acidity and water under hormones
Collecting duct	Takes urine to the renal pelvis, and can reabsorb more water under the hormone ADH

How Is Urine Formed?

Ultrafiltration: blood pressure in the glomerulus pushes water, glucose, salts and urea into the Bowman’s capsule, while blood cells and large proteins stay in the blood. About 180 litres of filtrate form each day.
Selective reabsorption: useful substances return to the blood from the tubule, so that more than 99 per cent of the filtrate is reabsorbed and only about 1.5 litres leaves as urine.
Tubular secretion: the tubule adds extra hydrogen ions, potassium ions and some drugs to the filtrate.
Urine: pale yellow because of the pigment urochrome, about 95 per cent water, with urea, uric acid, creatinine and salts, and slightly acidic.
Storage and release: urine collects in the bladder and is released by the urethra under voluntary control after about the age of two.

How Do Other Organisms Excrete, and What Else Should You Remember?

Ammonotelic, ureotelic, uricotelic: bony fishes and tadpoles excrete ammonia, which needs much water. Mammals and adult amphibians excrete urea, and birds, reptiles and insects excrete uric acid, which saves water.
Amoeba and Paramecium: a contractile vacuole removes excess water.
Flatworms: flame cells remove wastes, and earthworms have nephridia.
Insects: Malpighian tubules collect wastes from the body fluid and empty them into the gut.
Crustaceans: green glands such as those of prawns excrete wastes.
Plants: excrete gases through stomata and lenticels, store wastes in vacuoles, bark and fallen leaves, and release resins, gums and latex.
Dialysis: an artificial kidney removes wastes from the blood of patients with kidney failure, by haemodialysis through a machine or by peritoneal dialysis. The Pradhan Mantri National Dialysis Programme provides services in public hospitals in India.
Disorders: kidney stones of calcium oxalate, glomerulonephritis, urinary infections and kidney failure, which is often due to diabetes or high blood pressure.
Transplant: a healthy kidney from a donor replaces a failed one. In India, the Transplantation of Human Organs and Tissues Act, 1994 regulates donation.
Excretory system in brief: the excretory system of humans removes wastes and keeps the composition of blood steady, and the liver, skin and lungs share this task with the kidneys.
Kidney functions beyond excretion: kidneys make the hormone erythropoietin that stimulates red blood cell production, activate vitamin D and release renin for blood pressure control.

Why Does It Matter for the Exam?

Structure questions: parts of the nephron and the order of urine formation.
Comparison questions: excretory organs and wastes in different animals.
Health link: dialysis, kidney stones and organ donation, as in Osmoregulation, ADH and Formation of Urine; Thermoregulation.
Policy link: national programmes on dialysis and organ donation.
Common errors: confusing excretion with egestion, which is the removal of undigested food.

As an analytical point, the kidney is an organ of balance as well as of waste removal. It controls the water, salt and acid levels of blood, which is why kidney failure affects the whole body.

Why Is This Topic Important?

Facts Nephron and urine steps asked
Compare Animal excretory organs
Link Dialysis and donation

What Came Next?

The next topic covers osmoregulation, ADH, urine formation and thermoregulation. Next topic: Osmoregulation, ADH and Formation of Urine; Thermoregulation.

Key People and Terms

Nephron Functional unit of kidney
Glomerulus Cluster of capillaries in nephron
Ultrafiltration Pressure filtration of blood
Urea Main nitrogenous waste of mammals
Dialysis Artificial removal of blood wastes
Excretion Removal of metabolic wastes

Exam Corner

Points to Remember

About 1 million nephrons per kidney.
180 litres filtrate; 1.5 litres urine daily.
Mammals: urea; birds: uric acid.
Insects: Malpighian tubules.
Reabsorption mostly in the proximal tubule.

Do Not Confuse With

Excretion and egestion Metabolic waste versus undigested food
Ureotelic and uricotelic Urea versus uric acid
Haemodialysis and peritoneal dialysis Machine versus abdominal lining

Memory Hook

Filter, reabsorb, secrete: urine’s three steps; urea in mammals, uric acid in birds, fishes ammonia let go.

Mains Angle

Questions ask for the working of the kidney and kidney disease. These points add depth.

Process Filtration, reabsorption
Diversity Animal wastes
Disease Dialysis, stones

A Question You May Face

An original practice question, not a past paper question.

“Explain the structure of the nephron and the process of urine formation, and describe the role of dialysis in kidney failure.”

How to Answer

Organs Kidney, ducts.
Nephron Parts.
Steps Filter, reabsorb.
Disease Dialysis.

GS Relevance

Prelims: organs, nephron parts, nitrogenous wastes, excretory organs of animals, dialysis. GS Paper 3: science and technology, health.

Frequently Asked Questions

What is the functional unit of the kidney?

The nephron is the structural and functional unit of the kidney, and each kidney has about one million of them. A nephron has a renal corpuscle made of the glomerulus and Bowman's capsule, and a long tubule with a proximal convoluted tubule, a loop of Henle and a distal convoluted tubule.

How is urine formed?

Urine is formed in three steps. Ultrafiltration pushes water and small molecules out of the glomerulus into Bowman's capsule, selective reabsorption returns useful substances such as glucose and most of the water to the blood, and tubular secretion adds extra ions and drugs, leaving about 1.5 litres of urine a day.

What are the nitrogenous wastes of different animals?

Bony fishes and tadpoles excrete ammonia and are called ammonotelic, mammals and adult amphibians excrete urea and are called ureotelic, and birds, reptiles and insects excrete uric acid and are called uricotelic. Uric acid needs very little water, which suits animals in dry habitats.

What is dialysis?

Dialysis is a treatment that removes wastes and extra fluid from the blood of people whose kidneys have failed. In haemodialysis, blood passes through a machine with a semi-permeable membrane, and in peritoneal dialysis the lining of the abdomen acts as the filter, and India provides services in public hospitals under a national programme.

How do plants get rid of wastes?

Plants do not have excretory organs. They release oxygen and carbon dioxide through stomata and lenticels, store wastes in vacuoles and in tissues such as bark, shed them in fallen leaves, and release substances such as gums, resins and latex that can be removed from the plant.

PYQ Practice — Statement Analysis

1 Malpighian tubules are the excretory organs of insects.

True

They empty wastes into the gut.

2 Birds excrete mainly urea.

False

They excrete uric acid.

3 Selective reabsorption takes place mainly in the proximal convoluted tubule.