The lungs are the largest organ of the respiratory system. The respiratory system supplies the oxygen needed by the cells of the body and takes away the waste carbon dioxide.
Air is taken in through the nose and mouth and down the airway (the trachea or windpipe) into the lungs where oxygen is exchanged for carbon dioxide from the body’s tissues. The carbon dioxide is then exhaled out of the lungs and out of the mouth or nose.
The lungs sit in the chest cavity on top of the diaphragm. The left and right lungs are slightly different: the left lung has 2 lobes and the right lung has 3 lobes. The left lung is smaller than the right, because it is slightly displaced by the heart which takes up more space on the left of the chest cavity.
When you breathe in, air first passes through the nose and is warmed to body temperature. It is also moistened as it passes over the damp mucous membrane of the inside of the nose.
Then the air passes down your throat into the voice box — the larynx. From the larynx, air passes down the trachea which then branches into the left bronchus and the right bronchus (collectively known as bronchi). A flap of tissue called the epiglottis covers your larynx when you swallow food — to prevent food from going down into the trachea and the lungs.
The airways of the lungs resemble the structure of a tree viewed upside down. In fact, the airways are often referred to as the bronchial tree. The left and right bronchi are the 2 main branches of this tree, and they supply the left and right lung respectively. From these 2 main bronchi, many smaller bronchi branch off, dividing into smaller and smaller branches which end in very fine bronchioles.
The terminal bronchioles are less than a millimetre in diameter. At the end of each terminal bronchiole are the respiratory bronchioles. These are the narrowest of the airways and at their ends are tiny clusters of air chambers called alveoli. The alveoli look like bunches of grapes on the end of the respiratory bronchioles.
It is here, in the alveoli, that the final exchange of oxygen and carbon dioxide occurs. All of the alveoli are encased in a network of very fine blood vessels called capillaries. The blood feeding into this fine network of capillaries is carried there by the pulmonary arteries and carried away by the pulmonary veins.
Oxygen from the alveoli passes through the thin wall of the alveoli into the blood capillaries and from there into the bloodstream to supply the body’s tissues. Carbon dioxide passes in the opposite direction — from the blood capillaries through the thin wall into the alveoli. From there, it is exhaled up the bronchial tree and out of the lungs into the trachea and then out of the nose or mouth. This the only physiological circumstance in which arteries carry the de-oxygenated blood to an organ and veins carry newly oxygenated blood away.
Last Reviewed: 18 June 2009