The respiratory system

The respiratory system includes all the body parts involved in breathing, which is the process of taking in oxygen and removing carbon dioxide. This system is located in the chest cavity, the space inside the chest protected by the ribcage.

Key components of the respiratory system

The respiratory system consists of a pathway that allows air to move in and out of the body, delivering oxygen to where it is needed and removing waste gases.

• Nose and mouth - Entry points for air into the body.
• Trachea - A large tube that carries air from the nose or mouth towards the lungs; also known as the windpipe.
• Bronchi - Two tubes that branch off from the trachea, with one bronchus going to each lung.
• Bronchioles - Smaller tubes that branch from the bronchi and get progressively narrower as they spread throughout the lungs.
• Alveoli - Tiny air sacs at the end of the bronchioles where gas exchange occurs; each one is called an alveolus.

Pathway of air through the respiratory system

1. Air enters through the nose or mouth and travels down the trachea.
2. The trachea divides into two bronchi, one for each lung.
3. Each bronchus splits into smaller bronchioles.
4. The bronchioles end in clusters of alveoli, where gases are exchanged with the blood.

The role of respiratory muscles in breathing

Breathing involves the movement of air into and out of the lungs, controlled by specific muscles that change the size of the chest cavity.

Key respiratory muscles

• Diaphragm - A dome-shaped muscle at the base of the chest cavity that flattens during inhalation to increase space in the lungs.
• External intercostal muscles - Muscles between the ribs that help lift the ribcage during breathing.

The process of inhalation (breathing in)

Inhalation occurs when the respiratory muscles contract to expand the chest cavity, creating a lower pressure inside the lungs compared to outside. This draws air in.

1. The diaphragm contracts and moves downwards.
2. The external intercostal muscles contract, pulling the ribcage upwards and outwards.
3. The chest cavity expands, reducing pressure in the lungs.
4. Air flows into the lungs to equalise the pressure.

The process of exhalation (breathing out)

Exhalation happens when the respiratory muscles relax, shrinking the chest cavity and increasing pressure in the lungs. This forces air out.

1. The diaphragm relaxes and moves upwards.
2. The external intercostal muscles relax, allowing the ribcage to move downwards and inwards.
3. The chest cavity shrinks, increasing pressure in the lungs.
4. Air is pushed out of the lungs.

Gas exchange in the alveoli

Gas exchange is the process where oxygen enters the blood and carbon dioxide is removed from it. This occurs in the alveoli, which are specialised for efficient transfer of gases.

Adaptations of alveoli for gas exchange

• Large surface area - Millions of alveoli in the lungs provide a vast area for gases to pass through.
• Thin walls - The walls of alveoli are only one cell thick, allowing gases to diffuse easily.
• Moist lining - A thin layer of moisture helps gases dissolve and move across the walls.
• Rich blood supply - Alveoli are surrounded by a network of capillaries, small blood vessels that carry blood close to the alveolar walls for rapid exchange.

How diffusion enables gas exchange

Diffusion is the movement of gases from an area of higher concentration to an area of lower concentration, without needing energy. In the alveoli, this process allows oxygen to enter the blood and carbon dioxide to leave it.

Concentration gradients in gas exchange

• Oxygen diffuses from the alveolus (high concentration) into the capillary (low concentration).
• Carbon dioxide diffuses from the capillary (high concentration) into the alveolus (low concentration).

This exchange ensures that blood leaving the lungs is rich in oxygen and low in carbon dioxide.

Interaction between the respiratory and cardiovascular systems

The respiratory and cardiovascular systems work together to transport oxygen to the muscles and remove carbon dioxide from them. The cardiovascular system involves the heart, blood vessels, and blood, which circulate gases around the body.

The gas transport process

1. Oxygenated blood, which is blood rich in oxygen, leaves the heart and circulates around the body, delivering oxygen to tissues like muscles and collecting carbon dioxide.
2. Deoxygenated blood, which is blood low in oxygen and high in carbon dioxide, returns to the heart.
3. The heart pumps deoxygenated blood to the lungs, where it flows through capillaries surrounding the alveoli.
4. In the lungs, carbon dioxide diffuses from the blood in capillaries into the alveoli to be exhaled.
5. Oxygen diffuses from the alveoli into the red blood cells in the capillaries.
6. Oxygenated blood returns to the heart and is pumped out to the body again.

Red blood cells are specialised cells in the blood that carry oxygen attached to a protein called haemoglobin, ensuring efficient delivery to where it is needed, such as working muscles during exercise.