Cells produce wastes during metabolism. Excretion removes these wastes and helps maintain suitable levels of water and dissolved substances in the body. In humans, the kidneys are the main organs responsible for removing nitrogenous waste and regulating the composition of body fluids.
Table of Contents
Types of Nitrogenous Waste
Animals differ in the principal nitrogenous waste they release.
| Mode of excretion | Main waste | Examples | Water requirement |
|---|---|---|---|
| Ammonotelism | Ammonia | Many bony fish and aquatic amphibians | High |
| Ureotelism | Urea | Humans and other mammals | Moderate |
| Uricotelism | Uric acid | Birds and many reptiles | Low |
Ammonia is highly toxic and needs plenty of water for removal. Uric acid can be excreted with relatively little water. Humans convert much of the ammonia produced during metabolism into urea in the liver; the kidneys then remove the urea from blood.
Human Excretory System
The human urinary system consists of two kidneys, two ureters, a urinary bladder and a urethra.
Path of urine: Kidneys → Ureters → Urinary bladder → Urethra → Outside the body
Each kidney has an outer cortex and an inner medulla. Its functional units are called nephrons.
Structure of a Nephron
A nephron begins with a renal corpuscle, made of a capillary tuft called the glomerulus enclosed by Bowman’s capsule. The filtrate then passes through:
Bowman’s capsule → Proximal convoluted tubule (PCT) → Loop of Henle → Distal convoluted tubule (DCT) → Collecting duct
Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. Capillaries around the tubule allow exchange between the filtrate and blood.
Three Steps of Urine Formation
1. Glomerular Filtration
Blood pressure pushes water and many small dissolved substances through the filtration barrier into Bowman’s capsule. Blood cells and most proteins normally remain in the blood.
The volume of filtrate formed per minute is the glomerular filtration rate (GFR). NCERT gives an approximate textbook value of 125 mL per minute for a healthy adult.
2. Tubular Reabsorption
The kidney returns needed substances from the filtrate to the blood. These include much of the water, as well as useful ions, glucose and amino acids. Most reabsorption occurs in the PCT.
3. Tubular Secretion
Tubule cells move selected substances, including hydrogen ions, potassium ions and ammonia, from blood into the filtrate. Secretion helps maintain ionic and acid–base balance.
Functions of Nephron Parts
| Part | Key function |
|---|---|
| PCT | Reabsorbs most useful nutrients and much water and salts |
| Descending limb of Henle’s loop | Allows water to leave the filtrate |
| Ascending limb of Henle’s loop | Moves salts out; is largely impermeable to water |
| DCT | Adjusts reabsorption and secretion according to the body’s needs |
| Collecting duct | Reabsorbs water and helps set the final urine concentration |
The loop of Henle and nearby vasa recta help maintain a concentration gradient in the kidney medulla. This counter-current mechanism allows water to leave the collecting duct and helps the kidney produce concentrated urine.
Regulation of Kidney Function
The kidneys adjust water and salt balance through several linked mechanisms.
- ADH (antidiuretic hormone): Increases water reabsorption in later parts of the nephron, helping the body conserve water.
- Renin–angiotensin–aldosterone system: Responds to reduced kidney blood flow and helps increase blood pressure and sodium retention.
- Atrial natriuretic factor (ANF): Released in response to increased blood volume and helps counter mechanisms that raise blood pressure.
Reduced ADH secretion or action can cause diabetes insipidus, in which large amounts of dilute urine are produced. This is different from diabetes mellitus.
Micturition
Micturition is the release of urine from the urinary bladder. As the bladder fills, stretch receptors signal the nervous system. Bladder muscle contraction and relaxation of the urethral sphincter allow urine to leave the body.
Other Organs That Help in Excretion
- Lungs remove carbon dioxide and some water vapour.
- Liver helps eliminate certain substances through bile.
- Skin releases small amounts of water, salts and other substances in sweat.
Excretory Disorders and Treatment
- Uremia: Accumulation of urea and other wastes in blood when kidney function is severely impaired.
- Renal calculi: Solid deposits, commonly called kidney stones, that form in the urinary system.
- Nephritis: Inflammation affecting kidney tissue; glomerulonephritis affects the glomeruli.
- Kidney failure: Serious loss of kidney function.
Dialysis can remove wastes and excess fluid from blood when the kidneys cannot do so adequately. A kidney transplant may be an option for some people with advanced kidney failure. These are treatments selected through medical assessment.
Quick Revision
- Nephron: Functional unit of the kidney.
- Glomerulus: Capillary tuft where filtration begins.
- PCT: Major site of reabsorption.
- Loop of Henle: Helps establish the medullary concentration gradient.
- ADH: Helps conserve water.
- GFR: Amount of filtrate formed by kidneys per minute.
- Micturition: Release of urine from the bladder.
Check Your Understanding
- What is the main nitrogenous waste excreted by humans?
- Name the three main processes of urine formation.
- What structures form the renal corpuscle?
- Which hormone increases water reabsorption in the later nephron?
- Why is the loop of Henle important?
Answers: (1) Urea. (2) Filtration, reabsorption and secretion. (3) Glomerulus and Bowman’s capsule. (4) ADH. (5) It helps create the concentration gradient needed to conserve water and concentrate urine.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is excretion?
It is the removal of metabolic wastes and excess substances from the body.
What are ammonotelic animals?
They excrete most nitrogenous waste as ammonia, usually with substantial water.
What are ureotelic animals?
They excrete most nitrogenous waste as urea; adult humans are ureotelic.
What are uricotelic animals?
They excrete nitrogenous waste mainly as uric acid, conserving water.
What are the main parts of the human urinary system?
Two kidneys, two ureters, a urinary bladder and a urethra.
What is a nephron?
It is the kidney’s functional unit, consisting of a renal corpuscle and tubule.
What makes up the renal corpuscle?
The glomerulus and Bowman’s capsule.
What are the three main steps in urine formation?
Glomerular filtration, tubular reabsorption and tubular secretion.
What happens during glomerular filtration?
Water and small solutes move from glomerular blood into Bowman’s capsule to form filtrate.
What is tubular reabsorption?
Useful substances and much of the water move from the tubule back into the blood.
What is tubular secretion?
Selected substances move from blood into the tubule for removal or regulation.
What is the role of the loop of Henle?
It contributes to the medullary concentration gradient used to concentrate urine.
How does ADH affect urine?
ADH increases water reabsorption, generally producing a smaller volume of more concentrated urine.
What is dialysis?
It is a treatment that removes wastes and excess fluid from blood when kidneys cannot do so adequately.
What are renal calculi?
They are kidney stones formed from crystallised substances in the urinary tract.