The Breathing System

Mammals have two large lungs in their thorax. There are spongy in texture and each have

two lobes. They are positioned behind the heart and they

are protected by the rib cage.

Inspired Air / %

Expired Air / %

Nitrogen 78.6 74.9

Oxygen 20.9 15.3

Carbon Dioxide

0.03 3.6

Water Vapour 0.5 3.2

Air enters a mammal through the nose or mouth. The air then moves to the trachea, through the epiglottis (the opening at the back of the mouth) and into the trachea.

The trachea is kept open by Rings of Cartilage. At the top of the trachea is the voice box or

larynx. The trachea splits into two bronchi (bronchus) at

the base. When the bronchi enter the lungs they split

repeatedly into bronchioles. Eventually each bronchiole ends up at an air sac

or alveolus. It is at the alveoli that gases are exchanged i.e.

oxygen taken in and CO2 given out.

The alveoli are completely covered in blood vessels (capillaries).

The walls of the capillaries are extremely thin and the distance between air and blood is only 0.3μm.

This means that O2 has no distance to travel to get into the blood.

The alveoli increase the surface area hugely.

Humans have 300 million alveoli in both lungs giving a total surface area of 80 m2 (The skin is 2m2).

Breathing is controlled by the respiratory centre of the hindbrain.

This area controls the movement of the diaphragm, a muscular sheet which seals the thorax.

When the diaphragm contracts, it pulls the lungs downwards and air is forced into the trachea, bronchi and bronchioles.

When the diaphragm relaxes, the air is forced out of the lungs and the cycle repeats itself.