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Science Grade XI Biology

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Chapter 16: Excretory Products and their Elimination

Grade 11 Science | Chapter 16

Excretory Products and their Elimination

The body must remove the wastes made by its cells. This chapter develops excretion, the urinary system, the nephron, and how urine is formed.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Removing Waste

The Urinary System

The Nephron

How Urine Is Formed

Other Excretory Organs

Keeping Water in Balance

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Removing Waste

As cells work, they make wastes that would be harmful if they built up. Excretion is the removal of these wastes from the body. The main waste from breaking down proteins is removed by the kidneys, which filter the blood and make urine. This chapter follows how the body cleans its blood and keeps its fluids in balance.

Core idea

Excretion removes harmful wastes. The kidneys filter the blood and make urine, which passes through the ureters to the bladder and out through the urethra.

2. The Urinary System

The urinary system does most of the body’s excretion. Two bean shaped kidneys filter the blood and make urine. The urine drains down two tubes, the ureters, into a stretchy bag, the bladder, where it is stored. When the bladder is full, urine leaves the body through a tube called the urethra.

Diagram 1 – The Urinary System

The urinary system showing kidneys, ureters, bladder and urethra

Fig 1. The kidneys make urine, which drains through the ureters to the bladder and out by the urethra.

3. The Nephron

Each kidney contains about a million tiny filtering units called nephrons. Blood enters a tight knot of capillaries, the glomerulus, sitting in a cup called the Bowman capsule, where the blood is filtered. The filtered liquid then flows along a long, looping tubule and into a collecting duct. The nephron is where urine is actually made.

Diagram 2 – A Nephron

A nephron with glomerulus, Bowman capsule, tubule and collecting duct

Fig 2. Blood is filtered at the glomerulus into the Bowman capsule, then processed along the tubule.

4. How Urine Is Formed

Urine is made in three steps. First, in filtration, the blood is filtered at the glomerulus, and water, salts, glucose and wastes pass into the tubule. Next, in reabsorption, the useful materials such as glucose, much of the water and needed salts are taken back into the blood. What remains, mostly water with dissolved wastes, becomes urine and is removed.

Diagram 3 – How Urine Is Formed

The three steps of urine formation: filtration, reabsorption and urine

Fig 3. Blood is filtered, useful materials are reabsorbed, and the remaining waste leaves as urine.

5. Other Excretory Organs

The kidneys are not the only organs that excrete. The lungs remove carbon dioxide and some water as we breathe out. The skin removes water, salts and a little waste in sweat, which also cools the body. The liver processes wastes and turns harmful substances into safer ones for the kidneys to remove. Together these share the work of excretion.

6. Keeping Water in Balance

Besides removing waste, the kidneys keep the body’s water and salts in balance, a job called osmoregulation. If the body has too much water, the kidneys make more dilute urine; if it is short of water, they make less, more concentrated urine to save water. In this way the kidneys keep the internal conditions of the body steady.

7. Key Reasoning (Principles)

Principle 1: Wastes must be removed

Cell activity makes wastes that would harm the body if they built up, so excretion removes them and keeps the blood clean.

Principle 2: Filter, then reclaim the useful

The kidney first filters out a large amount of liquid, then reabsorbs the useful materials, so only true waste is lost as urine.

Principle 3: The kidney keeps the body steady

By adjusting how much water and salt it removes, the kidney keeps the body’s fluids in balance whatever a person drinks.

8. Worked Examples

Example 1

Q: What is excretion?

▶ Show Solution

The removal of harmful wastes from the body.

Answer: Removing harmful wastes.

Example 2

Q: Which organs make urine?

▶ Show Solution

The kidneys.

Answer: The kidneys.

Example 3

Q: Name the tube that carries urine from a kidney to the bladder.

▶ Show Solution

The ureter.

Answer: The ureter.

Example 4

Q: Where is urine stored?

▶ Show Solution

In the bladder.

Answer: In the bladder.

Example 5

Q: What is the filtering unit of the kidney called?

▶ Show Solution

The nephron.

Answer: The nephron.

Example 6

Q: Where is the blood filtered in a nephron?

▶ Show Solution

At the glomerulus, into the Bowman capsule.

Answer: At the glomerulus.

Example 7

Q: Name the three steps of urine formation.

▶ Show Solution

Filtration, reabsorption and the removal of urine.

Answer: Filtration, reabsorption, urine.

Example 8

Q: What is taken back into the blood during reabsorption?

▶ Show Solution

Useful materials such as glucose, water and needed salts.

Answer: Useful materials.

Example 9

Q: Name two organs other than the kidneys that excrete.

▶ Show Solution

The lungs and the skin (also the liver).

Answer: Lungs and skin.

Example 10

Q: What do the kidneys do when the body has too little water?

▶ Show Solution

They make less, more concentrated urine to save water.

Answer: Make concentrated urine.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. Urine is made by the: (a) liver (b) kidneys (c) lungs (d) skin

2. Urine is stored in the: (a) ureter (b) bladder (c) urethra (d) kidney

3. The filtering unit of the kidney is the: (a) alveolus (b) nephron (c) neuron (d) villus

4. Blood is filtered at the: (a) tubule (b) glomerulus (c) bladder (d) urethra

5. Useful materials are returned during: (a) filtration (b) reabsorption (c) breathing (d) clotting

▶ Reveal Answers

1. (b) kidneys.

2. (b) bladder.

3. (b) nephron.

4. (b) glomerulus.

5. (b) reabsorption.

Set B – Short Answer (Understanding)

1. Define excretion.

2. Trace the path of urine out of the body.

3. Name the parts of a nephron.

4. Describe the three steps of urine formation.

5. Name three other excretory organs and what each removes.

▶ Reveal Answers

1. The removal of harmful wastes from the body.

2. From the kidneys through the ureters to the bladder and out by the urethra.

3. Glomerulus, Bowman capsule, tubule and collecting duct.

4. Filtration of the blood, reabsorption of useful materials, and removal of the remaining urine.

5. Lungs remove carbon dioxide, skin removes sweat, and the liver processes wastes.

Set C – Application and Reasoning

1. Why must the body remove wastes?

2. Why is glucose reabsorbed rather than lost in urine?

3. On a hot day with much sweating, what kind of urine forms?

4. Why does each kidney have about a million nephrons?

5. Why is the bladder stretchy?

▶ Reveal Answers

1. Because wastes would build up and harm the body if not removed.

2. Because glucose is a useful fuel, so the body reclaims it instead of wasting it.

3. Less, more concentrated urine, to save water lost in sweat.

4. So a large volume of blood can be filtered and processed.

5. So it can store urine until it is convenient to remove it.

Set D – Higher Order (Challenge)

1. Explain why the kidney filters first and then reabsorbs, rather than picking out only the waste.

2. Explain how the kidneys keep the body’s water in balance.

3. Explain why a person drinking a lot of water passes more urine.

4. Explain how the liver and kidneys work together in excretion.

5. Explain why excretion is essential to keeping the internal conditions steady.

▶ Reveal Answers

1. Filtering everything and then reclaiming the useful materials is simpler and surer than trying to select only the wastes from the blood.

2. By adjusting how much water they remove, making dilute urine when water is plentiful and concentrated urine when it is scarce.

3. Because the kidneys remove the extra water as more, dilute urine to keep the balance.

4. The liver turns harmful substances into safer wastes, and the kidneys then filter those wastes out of the blood.

5. Because removing wastes and balancing water and salts keeps the blood and body fluids within the limits the cells need.

Chapter Summary

Excretion

Removal of harmful wastes from the body.

Urinary System

Kidneys, ureters, bladder, urethra.

Nephron

Filtering unit: glomerulus, Bowman capsule, tubule, duct.

Urine Formation

Filtration, reabsorption, then urine.

Other Organs

Lungs, skin and liver also excrete.

Water Balance

Kidneys adjust urine to keep fluids steady.

Quantity

Unit

Symbol

Urine maker

kidneys

–

Filtering unit

nephron

–

Storage

bladder

–

8-Point Exam Quick-Check

Excretion removes harmful wastes from the body.

Urinary system: kidneys, ureters, bladder, urethra.

Each kidney has about a million nephrons.

Blood is filtered at the glomerulus into the Bowman capsule.

Urine forms by filtration, then reabsorption of the useful.

Lungs, skin and liver also help with excretion.

Kidneys adjust urine to keep water and salts in balance.

Concentrated urine when short of water; dilute when plenty.

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Class 11 Biology Chapter 16: Excretory Products and their Elimination, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Biology syllabus and develops excretion, covering the need to remove wastes, the urinary system of kidneys, ureters, bladder and urethra, the nephron with its glomerulus, Bowman capsule and tubule, the formation of urine by filtration and reabsorption, the role of the lungs, skin and liver, and how the kidneys keep water in balance, with three diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.