The lungs have a structure that looks like the branches of a tree turned upside down. The windpipe (trachea) is like a tree trunk that divides into several branches called bronchi. These bronchi further subdivide into smaller branches called bronchioles. At the end of the bronchioles are several sacs filled with air. These sacs are called alveoli, which look like a bunch of tiny grapes. The alveoli are surrounded by several small blood vessels - the capillaries. There is about seven million alveoli in the lungs, each the size of a pinhead. The total surface area of the alveoli is very large. It is estimated that if all the alveoli of an adult male were to be laid out flat, they would cover a tennis court.
The lungs are protected by the rib cage surrounding it. Below the lungs is a muscular layer called the diaphragm. The process of breathing uses muscles of the rib contract, they pull the ribs upwards and outwards. At the same time, the diaphragm contracts downwards. Contraction of the muscles of the ribcage and the diaphragm makes the chest and lung cavity bigger. The air is therefore sucked inside to fill the additional space created. When the muscles of the rib cage and diaphragm relax, the lungs return to their original size and therefore empty out the air collected in it.
When you breathe in, the air containing oxygen enters the lungs and reaches the alveoli. It passes through their walls and enters the blood vessels surrounding them. The oxygen then combines with the red blood cells which transport it to every cell of the body. Oxygen is an essential fuel for metabolism. Metabolism is the term used for all the chemical reactions that take place in every cell of the body. These reactions generate growth, Millions of chemical reaction goes on in the body every minute and oxygen is an essential fuel for all these reactions.
During metabolism, oxygen combines with carbon and forms the waste carbon dioxide. The blood carries the waste carbon dioxide back to the lungs where it crosses the walls of the alveoli and enters them. When you breathe out, the lungs expel the carbon dioxide.
The brain automatically controls the rate and depth of your breathing depending on the oxygen requirement of the body. During exercise or other strenuous activities, the oxygen demand of various organs increases. Normally, you breathe in about half a liters of air at time, but in order to meet this additional demand you can breathe in about two litters of air at a time.
Sources : http://www.herbalcureindia.com/
Read more on Herbal Supplements and Lung Problem Supplements and Hair Loss Supplements