Lungs are organs that surround the heart in the chest cavity (the thorax) and consist of spongy tissue with a rich blood supply. Air passes from the nose and mouth into the trachea (windpipe) and into each lung through two airways called the bronchi. These divide into smaller airways called bronchioles which divide again and end in smaller air spaces called alveoli. These are air sacs with walls just one cell thick and it is here that oxygen filters into the blood and carbon dioxide filters out.
Attribution: BUPA