Lung is the chief breathing organ of mammals, birds, reptiles, and most adult amphibians.  The main job of the lungs is to exchange gases.  As blood flows through the lungs, it picks up oxygen from the air and releases carbon dioxide.  The body needs oxygen to burn food for energy, and it produces carbon dioxide as a waste product.  This article discusses the human lungs, but the lungs of other animal’s function in a similar way.  

Parts of the lungs.  Human beings have two lungs--a left lung and a right lung--, which fill up most of the chest cavity.  A lung has a spongy texture and may be thought of as an elastic bag filled with millions of tiny air chambers, or sacs.  If the walls of the air sacs could be spread out flat, they would cover about half a tennis court.  The somewhat bullet-shaped lungs are suspended within the ribcage.  They extend from just above the first rib down to the diaphragm, a muscular sheet that separates the chest cavity from the abdomen.  A thin, tough membrane called the visceral pleura covers the outer surface of the lungs.  The heart, large blood vessels, and oesophagus (the tube connecting the mouth and stomach) lie between the two lungs.  

The lungs are designed to receive air, which enters the body through the mouth or nose.  The air passes through the pharynx (back of the nose and mouth) and the larynx (voice box) and enters the airways are a system of tubes that leads into the lungs.  The largest of these tubes is the trachea (windpipe), which divides into two smaller tubes called bronchi.  Each bronchus enters one lung, about a third of the way from the top to the bottom of the lung.  Within the lung, the bronchus divides into smaller and smaller tubes, much as a tree limb divides into branches and twigs.  The final "twigs" are tiny tubes called bronchioles.  The smallest bronchioles, called terminal bronchioles, lead to the respiratory units of the lung.  The respiratory units are made up of many alveolar sacs.  Each sac contains about 20 tiny air spaces called alveoli.  The walls of each alveolus contain networks of extremely small blood vessels called pulmonary capillaries.  It is here that gas exchange takes place.  

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Three to five terminal bronchioles and the alveoli that they supply with air form a lobule.  Many lobules unite to form the major subdivisions of the lung, called lobes.  The left lung has two lobes, and the right lung has three.  Each lobe has its own branches of bronchi and blood vessels, so a diseased lobe may be removed without sacrificing the usefulness of the other lobes.  

Blood reaches the lung through two routes.  Almost all of the blood comes through the pulmonary circulation.  This blood has already circulated through the body tissues, where it has given ...

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