The process of breathing

 

The gas exchange system

Breathing is also called ventilation. It has two phases: inhaling and exhaling.

 

When you inhale, your diaphragm and intercostal muscles contract and move down and out. This makes your lungs bigger, which lowers the pressure inside them and draws air in. The air has a lot of oxygen that goes into the gas exchange system and then the body.

 

When you exhale, your diaphragm and intercostal muscles relax and move up and in. This makes your lungs smaller, which pushes air out. The air has more carbon dioxide that leaves the body.

The gas exchange system has these parts and functions:

 

• The trachea or windpipe is a tube that runs from the mouth to the lungs. It has cartilage rings that keep it open.
• The bronchus (plural: bronchi) is a branch of the trachea that leads to a lung. There are two bronchi, one for each lung.
• The bronchiole is a small tube that branches from the bronchus and takes air deeper into the lung. There are thousands of bronchioles in each lung.
• The alveolus (plural: alveoli) is a tiny air sac at the end of a bronchiole. It is where oxygen and carbon dioxide exchange between the air and the blood.
• The intercostal muscles are muscles between the ribs that form the chest wall. They help with breathing by moving the ribs up and down.
• The diaphragm is a flat, dome-shaped muscle under the lungs. It helps with breathing by moving up and down.

 

The human gas exchange system

 

Alveoli

The alveoli are small air sacs in the lungs where gas exchange happens. They take in oxygen from the air and give out carbon dioxide to the air. The oxygen is used for respiration, which is a process that releases energy from glucose that comes from food. Carbon dioxide is a waste product of respiration and needs to be removed.

Some water vapour also leaves the alveoli and goes into the lungs. We can see this water vapour when we breathe out on cold days.

Note: Deoxygenated blood appears blue in this diagram for explanation purposes only.

Alveoli have the following key features to maximise gas exchange:

 

• A huge combined surface area
• Moist, thin walls to maximise diffusion.
• Millions of tiny blood vessels called capillaries just behind these walls.

 

 

🔬 Knowledge Check: Breathing and Gas Exchange

Test your knowledge of the human respiratory system and how we breathe.

1. Which structure is also known as the windpipe?

• Bronchiole
• Alveoli
• Trachea

2. What are the tiny air sacs where gas exchange occurs called?

• Alveoli
• Bronchi
• Intercostal muscles

3. When you inhale, what happens to the ribs?

• They move downwards and inwards
• They move upwards and outwards
• They stay completely still

4. Which muscle sits at the bottom of the chest and helps move air into the lungs?

• The intercostal muscle
• The heart
• The diaphragm

5. During gas exchange, which gas moves from the alveoli into the blood?

• Carbon dioxide
• Oxygen
• Nitrogen

Click to Reveal Answers

1. Trachea (The main tube connecting the throat to the lungs).
2. Alveoli (Tiny sacs that provide a large surface area for gas exchange).
3. Upwards and outwards (This increases the volume of the chest cavity).
4. The diaphragm (It contracts and flattens to pull air in).
5. Oxygen (Oxygen is absorbed into the blood while carbon dioxide is removed).

 

Summary:

 

• Air has three main gases: oxygen, nitrogen and carbon dioxide.
• Gas exchange swaps oxygen and carbon dioxide between blood and air.
• Diffusion moves oxygen from air sacs into the blood.