David Lieberman, age 10, of Las Cruces, N.M., for his question:
WHAT DO THE LUNGS DO?
The lungs are the internal organs of breathing. You'll find them in humans and many animals. In addition, the lungs act as an exchange point where oxygen from the air is substituted for carbon dioxide contained in the blood that flows though the lungs.
Blood carries oxygen from the lungs to all parts of the body and brings carbon dioxide. Assisting in this job is the large and complex network of air sacs in the lungs.
If the walls of the air sacs in a person's lungs could be spread out flat and placed side by side, they would cover from about 600 to 1,000 square feet. This is about the same floor space that you'll find in a room measuring about 30 feet wide and 30 feet long.
As the air we breathe enters the chest area from the nose, pharynx and trachea, it divides into a right and left bronchus, or bronchial tube. Each bronchus enters a lung about midway between the top and bottom.
Within each lung, the bronchus divides into smaller and smaller tubes, much as a tree limb divides into branches and 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 there are more than 600 million alveoli in the lungs. The walls of each alveolus contains a meshlike network of capilaries, or tiny blood vessels. All of the blood in each lung flows through these capillaries.
The alveoli form clusters like bunches of grapes around the bronchioles. These clusters are called lobules. They unite to form larger groups called lobes. Two lobes make up the left lung and three make up the right lung.
The bronchial tubes, together with the blood vessels, lymph vessels and nerves of the lungs are bound together by elastic tissue. The lungs stretch like baloons when filled with air.
As the blood circulates, carrying nourishment to the body's tissues, it gives up oxygen that was received in the lungs and it absorbs carbon dioxide.
Oxygen enters the lungs when a person takes a breath, and carbon dioxide is expelled when the person breathes out.
The plura is a continuous, thin, elastic membrane that covers the outside of the lungs and the inner walls of the chest cavity. Normally, there is no actual space between the portion of the pleura that covers the lungs and the portion that covers the inside of the chest cavity.
These membranes are in constant contact. But in certain diseases, blood, fluid or air may collect between the layers of the plura. When this happens, a space develops between the membranes that compress the lungs. The most dangerous diseases of the lungs are pneumonia, tuberculosis and cancer.