Human respiratory system begins with nose. The air then goes into nasal passage. The nasal passages is lined with fine hairs. When air passes through the nasal passage the dust particles and other impurities in the air got traped in the nasal hair and mucus. Only clean air goes in, and the part of the throat between the mouth and windpipe is called pharynx. From the nasal passage air enters into pharynx and in and then goes into the wind pipe. Trachea does not collapse even when there is no air in it because it is supported by the rings of soft bone called cartilage. Trachea runs down the neck and divide into two smaller tubes called bronchi at its lower ends. The bronchi are connected to the two lungs. The lungs in the chest cavity thoracic cavity which is separated from abdominal cavity by a muscular partition call diagram. Each bronchi divide in the lungs to form a large number of, still smaller tubes called bronchioles. The pouch like air sacs at the ends of the smallest from bronchioles are called alveoli. The walls of alveoli are very thin and they are surrounded by very thin blood capillaries.
When we breathe and we left our trip and flatten diaphragm and chest cavity become larger as a result. Because of this air is sucked into the lungs and fills the expanded alveoli. The alveoli are surrounded by thin blood vessels called capillaries, carrying blood in them so the oxygen or air diffusers out from the alveolar walls into the blood. Oxygen is carried by blood to all the parts of the body. As the blood passes through the tissues of the blood body the oxygen present in a diffuses into the cells. The oxygen combined with the digested food present in the cells to release energy. Carbon dioxide gas is produced as a waste product during respiration in the cells of the body tissues. This carbon dioxide diffuses into the blood. Blood carries the carbon dioxide back to the lungs where it diffuses into the alveoli. Then we breathe out air. The diaphragm and muscles attached to the ribs relax due to which a chest cavity contract and become smaller. The contraction movement of the chest pushes the carbon dioxide from the alveoli of lungs into the trachea then nostrils and then out of the body into the air.