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Preksha Dhyana :
much like an inverted tree. Each tiny bronchiole terminates: in a cluster of minute air sacs, the alveoli which look like a miniature bunch of grapes. An extensive network of capillaries surrounds each alveolus. Gases diffuse back and forth between the alveoli and capillaries network, and the actual exchange of oxygen and carbon dioxide occurs. here. The Chest
The lungs, the heart and major blood vessels are protected by the bony rib cage called chest or thoracic cavity. It is closed at the top by the muscles of the neck; enclosed on the sides, back and front, by the ribs, the vertebrae, and the sternum; is bounded below by the diaphgram, a large sheet of muscle that assumes a dome shape at rest. It is constructed so as to expand and contract readily as the lungs inflate and deflate. The major portion of the chest is taken up by the two lungs. In the centre between the two lungs is situated the heart and major blood vessels. The Lungs and the Pleura
A pair of human lungs contains about 300 million alveoli covering a total surface area more than 90 sq. mtrs., enough to carpet a tennis court. This enormous surface area provides for an efficient exchange of gases. The two lungsare cone-shaped with the base resting on the diaphragm and the apex into the root of the neck. They are freely movable except at the roots. The right lung is larger and broader than the left, but is slightly shorter. The right lung is divided into 3 lobes and the left one is divided into 2. They are light, porous and spongy. Their internal structure is a mass of branching tubes and air sacs. The alveolus is a roughly globular structure about 100 micrones in diameter. In has an extremely thin wall and is surrounded by a network of equally thin-walled capillaries. The total surface area of the capillary network is about the same as that of the alveoli. Each lung is enclosed and protected by a double-walled membrane, the pleura, which also lines the inner surface of the chest wall. A small amount of fluid between two layers
1. Tho estimate of the number of alveoli in an adult human body varies between 250 millions and 600 millions and area 70 to 90 sq.m.
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