The lungs and associated structures form an extremely efficient system with minimal water loss and maximum gas exchange. Air reaches the lungs through the nose where it is moistened and dust and germs are caught.   From here the air passes through the throat or pharynx and then into the trachea (wind pipe).  The trachea, which is kept open by C - shaped rings of cartilage,  splits into two bronchi which lead directly into the right and left lungs. Bronchi then split into many bronchiole which all then eventually end in alveoli.

The alveoli

The alveolar epithelium is covered with a thin layer of detergent fluid called surfactant, into which oxygen dissolves.  Due to the ball like structure of the 700 million alveoli they create a total surface area of  70 square meters.    Tiny blood capillaries surround the alveolus with only two cells width between them and a distance of about 0.3 micrometers,  so as to  maximise the amount of gaseous exchange.  An alveolus is comprised of three epithelial cells which form its boundaries and secrete surfactant. 

The breathing cycle

The rate at which we breath is dependant on our activities and need for oxygen.   During rest we breath using only the tidal volume of our lungs which amount to about half a litre of air.  If we breath in deeply we can take in about 3 extra litres of air, called inspiratory reserve volume.  If we breathe out more that usual we exhale the expiratory reserve volume of about a litre.    Thus the total volume is the sum of the two reserve volumes and the tidal air, which adds up to between 4 and 6 litres.  In addition there is also a residual volume of air (1,5 litres) that is never expelled which ensures that there is always air which waste can diffuse into, and oxygen can diffuse out off.

How we breathe

Air is sucked into our lungs by a lower pressure which is induced by expanding the volume of the lungs.  The lungs are enlarged by contracting the intercostal muscles, between the ribs, and by contracting (straightening) the diaphragm, which is underneath the lungs.  This causes inspiration.  Expiration on the other hand is cause by relaxing the above groups of muscles causing the chest area to decrease in size and force the air out.  These actions are all controlled by the brain.

Homeostasis and gaseous exchange

The rate of breathing is controlled by chemoreceptors in the neck, they test the blood's pH level.  When carbon dioxide is in the blood it forms carbonic acid and thus by checking the pH of the blood the amount of carbon dioxide can accurately be ascertained.  If there is too much carbon dioxide in the blood the chemoreceptors inform the brain which intern causes the rate of breathing to be stepped up.