Respiratory system structure and function

Cross-section of the respiratory system. The nasal cavity, mouth, trachea, alveoli, bronchiole, bronchus, diaphragm and lungs are all shown.

Passage of air into the lungs

  1. Air enters the body and is warmed as it travels through the mouth and nose.
  2. It then enters the trachea.
  3. The trachea divides into two bronchi. One bronchus enters each lung.
  4. Each bronchus branches out into smaller tubes called bronchioles. Air travels through these bronchioles.
  5. At the end of the bronchioles, the air enters one of the many millions of alveoli where gaseous exchange takes place.


Breathing is the term given to the process of taking air into and out of the lungs.

When air is inhaled, the rib cage expands as the rib muscles contract, and the diaphragm contracts. When air is exhaled, the rib cage gets smaller as the rib muscles relax, and the diaphragm relaxes.The process of inhalation and exhalation

Two important structures for breathing are the diaphragm and intercostal muscles.

The diaphragm is a sheet of muscle that separates the chest (or thoracic) cavity from the rest of the body.

The intercostal muscles are found between the ribs and they control rib movement.

Inspiration (breathing in)

The diaphragm contracts and moves downwards. The intercostal muscles contract and move the ribs upwards and outwards. This increases the size of the chest and decreases the air pressure inside it which sucks air into the lungs.

Expiration (breathing out)

The diaphragm relaxes and moves back to its domed shape. The intercostal muscles relax so the ribs move inwards and downwards under their own weight. This decreases the size of the chest and increases the air pressure in the chest so air is forced out of the lungs.

Composition of the air we breathe

Breathed inBreathed out
21% oxygen16% oxygen
0.03% carbon dioxide4% carbon dioxide

The human body is designed to take in oxygen and to remove carbon dioxide. The respiratory system, in combination with the cardiovascular system, is responsible for providing this function.

Gas exchange

Gas exchange occurs at the alveoli in the lungs and takes place by diffusion. The alveoli are surrounded by capillaries so oxygen and carbon dioxide diffuse between the air in the alveoli and the blood in the capillaries.

Diffusion is the movement of gas from an area of high concentration to an area of low concentration.

There is a high concentration of oxygen in the alveoli and a low concentration of oxygen in the blood, so oxygen diffuses from the alveoli into the blood.

There is a high concentration of carbon dioxide in the blood and a low concentration in the alveoli, so carbon dioxide diffuses from the blood into the alveoli.

Both oxygen and carbon dioxide are capable of combining with an iron-rich protein in the blood called haemoglobin. Haemoglobin carries oxygen to be exchanged at the working muscle and carbon dioxide to be exchanged at the lung.

Dexoygenated red blood cells enter the capillary. Carbon dioxide is absorbed into the alveoli and oxygen passes from the alveoli into the blood cells. When the blood cells leave they are oxygenated.As the blood moves through the capillaries in the alveoli, oxygen diffuses into it and carbon dioxide diffuses out of it

Capillaries surround the alveoli in the lungs. Both the capillaries and alveoli walls are very thin - just one cell thick. They are made of semi-permeable membranes which allow oxygen and carbon dioxide to pass through them.


Describe the process of gas exchange at the muscles.

In the muscle there is a high concentration of carbon dioxide and in the bloodstream there is a high concentration of oxygen.

Oxygen diffuses from the bloodstream into the muscles and carbon dioxide diffuses into the blood from the muscles.