Mike Stack, age 13, of Tacoma, Wash., for his question:
HOW LARGE ARE HUMAN LUNGS?
Lungs are part of the human respiration system. Each of an adult's two lungs is about 10 to 12 inches in length and is the place where oxygen from the air is substituted for carbon dioxide contained in the blood that flows through the lung.
Inside the pyramid shaped lungs are millions of tiny air sacs. If the walls of all the air sacs could be spread out flat and placed side by side, they would cover an area of almost 1,000 square feet, which would be about like the floor space in a room that measures 30 feet long and 30 feet wide.
The lungs are in the chest cavity and extend upward behind the collarbone. They rest on the muscular diaphragm that separates the chest and the abdominal cavities. Between the two lungs lie the esophagus, the heart and the large blood vessels.
A thin, continuous elastic membrane that covers the outside of the lungs and the inner walls of the chest cavity is called the pleura. Ordinarily there isn't any space between the part of the pleura that covers the lungs and the part that covers the inside of the chest cavity.
Air enters the lungs by way of the nose, pharynx, larynx, trachea, bronchi and bronchioles. At the place where the trachea enters the chest cavity, it divides into a right and left bronchial tube, or right and left bronchus. Each bronchus enters a lung midway between the top and bottom.
Inside each lung the bronchus divides into smaller and smaller tubes, much like a tree limb divides into first branches and then twigs. Finally the bronchi divide into countless tiny tubes called bronchioles.
The bronchioles open into tiny cup shaped hollows known as alveoli or air sacs. Scientists estimate that there are more than 600 million of these air sacs in the lungs.
The walls of each alveolus hold a meshlike network of tiny blood vessels called capillaries. All of the blood in each lung flows through these capillaries. The blood vessels, lymph vessels and nerves of the lungs are bound together by elastic tissue. The lungs stretch when filled with air.
Because the lungs are made up of a countless number of air cells, they weigh very little.
The lungs give the blood its oxygen supply. They also remove carbon dioxide from the blood. As the blood circulates, carrying nourishment to the tissues, it gives up oxygen and absorbs carbon dioxide. It then returns to the lungs for more oxygen.
Oxygen enters the lungs when you take a breath and carbon dioxide is expelled when you breathe out.
To maintain good health, it is necessary to breathe fresh, clean air. Medical authorities urge all to avoid smoking tobacco. Not only does tobacco smoke prevent the lungs from properly handling its work, the smoke also increases a person's chances of developing lung cancer.
Smoking can also cause emphysema and chronic bronchitis.