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 Oxygen Carbon Dioxide Water Vapour0.53.2

 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 CO 2 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 O 2 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 m 2 (The skin is 2m 2 ).

 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. 