Introduction

Every cell in your body needs a constant supply of oxygen to release energy from food through cellular respiration. At the same time, cells produce carbon dioxide as a waste product that must be removed from the body. The respiratory system performs both of these vital tasks by bringing oxygen into the body and removing carbon dioxide.

The respiratory system works closely with the circulatory system, which transports oxygen from the lungs to body cells and carries carbon dioxide back to the lungs to be exhaled. Without this continuous exchange of gases, body cells could not produce the energy needed for life.

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What Is the Respiratory System?

The respiratory system is the organ system responsible for:

• Taking oxygen into the body.
• Removing carbon dioxide.
• Supplying oxygen for cellular respiration.
• Helping maintain homeostasis.

Every breath supports the survival of every cell in the body.

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Figure 1. The respiratory system transports oxygen into the body and removes carbon dioxide.

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Major Organs of the Respiratory System

The respiratory system includes:

• Nose (nasal cavity)
• Pharynx (throat)
• Larynx (voice box)
• Trachea (windpipe)
• Bronchi
• Bronchioles
• Lungs
• Alveoli
• Diaphragm

Each structure plays a specialised role in moving air and exchanging gases.

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The Pathway of Air

Air follows a specific route through the respiratory system.

Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli (in the lungs)

At the alveoli, oxygen enters the blood while carbon dioxide leaves the blood.

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Figure 2. Air passes through several structures before reaching the gas-exchange surfaces of the lungs.

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The Nose

The nose is the main entry point for air.

Its functions include:

• Filtering dust and microorganisms using hairs and mucus.
• Warming incoming air.
• Moistening inhaled air.

These processes help protect the delicate tissues of the lungs.

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The Trachea

The trachea, or windpipe, carries air between the throat and the lungs.

It contains:

• Rings of cartilage that keep it open.
• Tiny hair-like structures called cilia.
• Mucus that traps dust and microorganisms.

The cilia move the mucus upward toward the throat, helping keep the lungs clean.

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The Bronchi and Bronchioles

The trachea divides into two bronchi.

One bronchus enters each lung.

Inside the lungs:

• The bronchi divide into smaller bronchioles.
• Bronchioles branch repeatedly until they reach the alveoli.

This branching system distributes air throughout the lungs.

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Figure 3. The trachea branches into bronchi and bronchioles, allowing air to reach all parts of the lungs.

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The Lungs

The lungs are the main organs of the respiratory system.

Their functions include:

• Receiving oxygen from inhaled air.
• Removing carbon dioxide from the blood.
• Providing a large surface area for gas exchange.

The lungs contain millions of alveoli, where gas exchange occurs.

The right lung has three lobes, while the left lung has two lobes to make room for the heart.

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Breathing

Breathing is the movement of air into and out of the lungs.

There are two stages.

Inhalation (Breathing In)

During inhalation:

• The diaphragm contracts and moves downward.
• The rib muscles contract.
• The rib cage moves upward and outward.
• The volume of the chest cavity increases.
• Air flows into the lungs.

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Exhalation (Breathing Out)

During exhalation:

• The diaphragm relaxes and moves upward.
• The rib muscles relax.
• The chest cavity becomes smaller.
• Air is forced out of the lungs.

Breathing continually supplies fresh oxygen and removes carbon dioxide.

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Figure 4. The diaphragm and rib muscles work together to move air into and out of the lungs.

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Why the Respiratory System Is Essential

The respiratory system allows the body to:

• Supply oxygen for cellular respiration.
• Remove carbon dioxide.
• Help maintain blood pH.
• Support physical activity.
• Maintain homeostasis.

Without oxygen, most body cells would begin to die within minutes.

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Working with the Circulatory System

The respiratory and circulatory systems work together.

The respiratory system:

• Brings oxygen into the lungs.
• Removes carbon dioxide.

The circulatory system:

• Carries oxygen from the lungs to body cells.
• Returns carbon dioxide from body cells to the lungs.

Together, these systems ensure that every cell receives oxygen and can remove waste gases.

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Figure 5. The respiratory and circulatory systems cooperate to transport oxygen and carbon dioxide throughout the body.

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Worked Example

Question

Place the following structures in the correct order that air travels through the respiratory system.

• Bronchi
• Nose
• Trachea
• Bronchioles
• Alveoli
• Pharynx
• Larynx

Solution

Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

This pathway carries inhaled air to the gas-exchange surfaces of the lungs.

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Real-World Connection

When you exercise, your muscles require more oxygen to produce energy. Your breathing rate increases, allowing more air to enter the lungs. At the same time, your heart beats faster to transport oxygen-rich blood to your muscles and carry carbon dioxide back to the lungs. This close cooperation between the respiratory and circulatory systems helps your body meet the increased demands of physical activity.

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Did You Know?

An average adult breathes about 12–20 times per minute while resting. That adds up to around 20,000 breaths every day! During this time, your lungs move thousands of litres of air, supplying oxygen to trillions of cells throughout your body.

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Key Terms

Alveoli – Tiny air sacs in the lungs where oxygen and carbon dioxide are exchanged.

Bronchi – The two main air passages that carry air from the trachea into the lungs.

Bronchioles – Smaller branches of the bronchi that lead to the alveoli.

Cilia – Tiny hair-like structures that move mucus and trapped particles out of the airways.

Diaphragm – A dome-shaped muscle beneath the lungs that plays a major role in breathing.

Exhalation – The process of breathing air out of the lungs.

Inhalation – The process of breathing air into the lungs.

Respiratory system – The organ system responsible for gas exchange between the body and the environment.

Trachea – The windpipe that carries air from the larynx to the bronchi.

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Key Takeaways

• The respiratory system supplies oxygen to the body and removes carbon dioxide.
• Air follows the pathway: nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
• The nose filters, warms, and moistens incoming air.
• The trachea, bronchi, and bronchioles transport air to the lungs.
• Breathing is driven mainly by the movement of the diaphragm and rib muscles.
• The respiratory system works closely with the circulatory system to deliver oxygen to cells and remove carbon dioxide, making it essential for survival.