The Respiratory System Revision Notes for Junior Cert Science

The Respiratory System

Why do we need a respiratory system?

Your body needs energy to function, and that energy comes from a process called respiration. Respiration takes place in every cell of your body. During respiration, glucose (a type of sugar from food) reacts with oxygen to release energy:

$\text{glucose} + \text{oxygen} \rightarrow \text{energy} + \text{carbon dioxide} + \text{water}$

Most respiration in your body requires oxygen to release this energy. The waste products from respiration are carbon dioxide and water, which your body must remove.

This is where the respiratory system comes in. Its main job is to:

Bring oxygen into your body so it can reach all your cells
Remove carbon dioxide and water vapour produced by respiration

Understanding respiration and breathing

chatImportant

Respiration and breathing are not the same thing:

Respiration is the release of energy from food. It happens inside the cells of your body.
Breathing is taking air in and out of your lungs. It brings oxygen into your body for respiration and removes carbon dioxide and water vapour.

The organs of the respiratory system and their functions

Your respiratory system is made up of several organs that work together to help you breathe. Each organ has an important job to do.

The nose

Your nose is the main entrance for air coming into your body. Small hairs inside your nose act as filters, trapping dust and dirt particles before they can enter your lungs. This helps keep your airways clean and protects your lungs from harmful particles.

The trachea

The trachea (windpipe) is a tube that carries air from your nose and mouth down into your lungs. It has rings of cartilage around it to keep it open and prevent it from collapsing.

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A special flap of tissue called the epiglottis sits at the top of the trachea. When you swallow food, the epiglottis closes over the trachea to prevent food from going down the wrong way into your lungs.

The bronchi and bronchioles

When the trachea reaches your lungs, it splits into two branches called bronchi (singular: bronchus). One bronchus goes to each lung.

Inside the lungs, the bronchi divide into smaller and smaller tubes called bronchioles. These are very narrow tubes that carry air deep into the lungs, right to the alveoli.

The lungs

You have two lungs in your chest. They are the main organs of the respiratory system. The lungs are protected by your ribs, which form a cage around them.

Inside your lungs are millions of tiny air sacs called alveoli. This is where the important job of gas exchange happens.

The alveoli

Alveoli (singular: alveolus) are tiny air sacs found at the end of the bronchioles. They have very thin walls and are surrounded by a network of blood vessels called capillaries.

chatImportant

The alveoli are specially designed for gas exchange:

Their walls are extremely thin (only one cell thick)
They have a very large surface area
They are surrounded by many tiny blood vessels

These features allow oxygen to pass from the air in the alveoli into the blood, and carbon dioxide to pass from the blood into the alveoli.

The ribs and intercostal muscles

Your ribs are bones that form a protective cage around your lungs and heart. Between your ribs are muscles called intercostal muscles. These muscles play a crucial role in breathing by moving your chest up and down.

The diaphragm

The diaphragm is a large, dome-shaped muscle that sits below your lungs. It separates your chest cavity from your abdomen. The diaphragm is one of the most important muscles for breathing.

Breathing

Breathing is the process of moving air into and out of your lungs. This happens because of the actions of the intercostal muscles and the diaphragm.

How breathing works

Breathing involves two main actions:

Inhalation (breathing in) - taking air into your lungs
Exhalation (breathing out) - pushing air out of your lungs

These actions happen because of changes in the volume and pressure inside your chest.

Inhalation (breathing in)

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Worked Example: The Inhalation Process

When you breathe in, several things happen:

The intercostal muscles contract. This makes the ribs move upward and outward.
The diaphragm contracts. This causes it to flatten and move downward.
These movements make the chest cavity larger, increasing the volume of space inside your chest.
As the volume increases, the pressure inside your lungs decreases.
Air from outside (which is at higher pressure) rushes into your lungs to fill the space.

Exhalation (breathing out)

When you breathe out, the opposite happens:

The intercostal muscles relax. This allows the ribs to move downward and inward.
The diaphragm relaxes. It moves back up to its dome shape.
The chest cavity gets smaller, decreasing the volume inside your chest.
This increases the pressure on the lungs.
Air is forced out of your lungs to the outside.

infoNote

Remember: Contract to expand - when muscles contract, the chest expands (breathing in). Relax to reduce - when muscles relax, the chest reduces in size (breathing out).

The bell jar model

Scientists use a model called a bell jar to demonstrate how breathing works. This model helps us understand the role of the diaphragm.

The model consists of:

A glass bottle representing the ribcage
A Y-shaped tube representing the bronchi
Two balloons representing the lungs
A rubber sheet at the bottom representing the diaphragm

When you pull the rubber sheet down (like the diaphragm contracting), the balloons inflate. When you push it up (like the diaphragm relaxing), the balloons deflate. This shows how the movement of the diaphragm changes the pressure and causes air to move in and out.

Gaseous exchange

Gaseous exchange is the process of swapping oxygen and carbon dioxide between the air in your lungs and your blood. This happens in the alveoli.

What is diffusion?

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Diffusion is the movement of a substance from where there is a high concentration to where there is a low concentration. In other words, particles naturally spread out from crowded areas to less crowded areas.

Diffusion is the process that allows gases to move between the alveoli and the blood.

Gas exchange at the alveoli

The alveoli are specially designed for efficient gas exchange. Here's what happens:

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Worked Example: Gas Exchange Process

Air rich in oxygen enters the alveoli from the bronchioles.
The oxygen concentration is high in the alveoli but low in the blood.
Oxygen diffuses through the thin walls of the alveolus and the capillary into the blood.
At the same time, carbon dioxide concentration is high in the blood but low in the alveoli.
Carbon dioxide diffuses from the blood into the alveoli.
The air in the alveoli now has less oxygen and more carbon dioxide.
This air is breathed out.

The alveoli are perfect for this job because:

They have very thin walls (gases can pass through easily)
There are millions of them (huge surface area for gas exchange)
They are surrounded by many blood vessels (quick transfer of gases)

Comparing inhaled and exhaled air

The air you breathe in is different from the air you breathe out. The table below shows the percentage of different gases in inhaled and exhaled air:

Gas	Percentage in inhaled air	Percentage in exhaled air
Oxygen	$21$	$16$
Carbon dioxide	$0.04$	$4$
Nitrogen	$78$	$78$
Water vapour	Varies	Increased

Key differences:

Oxygen decreases from $21\%$ to $16\%$ because your body uses it for respiration
Carbon dioxide increases from $0.04\%$ to $4\%$ because it's a waste product of respiration
Nitrogen stays the same at $78\%$ because your body doesn't use it
Water vapour increases because it's produced during respiration

How the organs work together

The organs of the respiratory system don't work alone. They interact with each other to make breathing efficient and safe.

The nose and lungs

The nose filters the air before it reaches the lungs. The tiny hairs in your nose trap dust and particles, preventing them from entering your lungs. This is important because your lungs work better with clean air.

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Gas exchange is much more efficient in the lungs when air passes through moist membranes. The nose also warms and moistens the air before it reaches the lungs.

The intercostal muscles and diaphragm

The intercostal muscles and the diaphragm work together in a coordinated way:

During inhalation:

Both the intercostal muscles and the diaphragm contract at the same time
This increases the chest volume and draws air into the lungs

During exhalation:

Both the intercostal muscles and the diaphragm relax at the same time
This decreases the chest volume and pushes air out of the lungs

This teamwork ensures smooth and efficient breathing.

Investigating carbon dioxide in air

You can test whether there is more carbon dioxide in exhaled air than inhaled air using a simple experiment with limewater.

The limewater test

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Worked Example: Testing for Carbon Dioxide

What you need:

Two test tubes (A and B) containing limewater
Tubes for breathing in and out

What to do:

Breathe in through tube X (connected to test tube A)
Breathe out through tube Y (connected to test tube B)
The air you breathe in goes through limewater in tube A
The air you breathe out goes through limewater in tube B

What happens:

The limewater in tube A (inhaled air) stays clear or turns only slightly milky
The limewater in tube B (exhaled air) turns much more milky

Why this happens: Limewater turns milky when carbon dioxide passes through it. Since exhaled air contains much more carbon dioxide than inhaled air ( $4\%$ compared to $0.04\%$ ), the limewater in tube B turns much more milky. This proves that exhaled air contains more carbon dioxide than inhaled air.

How the respiratory system interacts with other systems

Your respiratory system doesn't work alone. It interacts closely with other systems in your body, particularly the digestive and circulatory systems.

Interaction with the digestive system

The trachea (part of the respiratory system) and the oesophagus (part of the digestive system) are very close together in your throat. Both pass through the same area at the back of your mouth.

chatImportant

To prevent food from entering your trachea when you swallow, you have a special flap of tissue called the epiglottis. When you swallow:

The epiglottis closes over the entrance to the trachea
This prevents food from going down the wrong way
Food travels down the oesophagus to your stomach instead

If food does accidentally enter your trachea, you automatically cough to clear it out. This is an important safety mechanism.

Interaction with the circulatory system

The respiratory and circulatory systems work very closely together to transport oxygen and carbon dioxide around your body.

How they work together:

The heart pumps deoxygenated blood (blood low in oxygen and high in carbon dioxide) to the lungs through the pulmonary artery.
This blood travels to the capillaries surrounding the alveoli.
Gas exchange happens:
- Oxygen from the air diffuses into the blood
- Carbon dioxide from the blood diffuses into the alveoli
The now oxygenated blood (rich in oxygen, low in carbon dioxide) travels back to the heart through the pulmonary vein.
The heart pumps this oxygenated blood to all the cells in your body.
At the cells, oxygen moves from the blood into the cells for respiration, and carbon dioxide moves from the cells into the blood.
This deoxygenated blood returns to the heart and the cycle continues.

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Without the circulatory system, the oxygen you breathe in couldn't reach your cells. Without the respiratory system, your blood couldn't pick up the oxygen your cells need.

Remember!

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Key Points to Remember:

Respiration happens in your cells and releases energy from glucose. Breathing is moving air in and out of your lungs. They are not the same thing!
The main organs of the respiratory system are: nose, trachea, bronchi, bronchioles, lungs, and alveoli. The intercostal muscles and diaphragm help with breathing.
Breathing in: intercostal muscles and diaphragm contract → chest expands → pressure decreases → air rushes in. Breathing out: muscles relax → chest shrinks → pressure increases → air is pushed out.
Gas exchange happens at the alveoli through diffusion. Oxygen moves from the air into the blood. Carbon dioxide moves from the blood into the air.
Exhaled air contains less oxygen ( $16\%$ vs $21\%$ ) and more carbon dioxide ( $4\%$ vs $0.04\%$ ) than inhaled air. You can test this using limewater, which turns milky with carbon dioxide.
The respiratory system works with the digestive system (epiglottis prevents choking) and the circulatory system (blood transports gases to and from cells).

The Respiratory System (Junior Cert Science): Revision Notes