It consists of paired lungs and air passage ways. Lungs lie in thoracic cavity walls of which are formed of intercostals muscles attached with bony cage formed by 12 pairs of ribs, vertebral column and sternum bone. Thoracic cavity is separated from abdomen by a muscular partition called diaphragm.
Air enters into lungs by a pair of openings called external nares or nostrils which lead into pharynx. Pharynx leads air into larynx by opening called glottis guarded by flap of tissue called epiglottis. Larynx leads air into trachea. Trachea leads into two bronchi. Each bronchus leads into each lung and subdivides into bronchioles. Each bronchiole ends into alveoli or air sacs.
Alveoli are respiratory surfaces of lungs. In alveoli exchange of gases takes place.
Breathing:
It is the process of taking in (inspiration or inhalation) air and giving out air (expiration or exhalation) from atmosphere.
Inspiration (inhalation): Inspiration or process of taking in of air, in which volume of thoracic cavity is increased due to concentration of intercostal muscles and diaphragm. Thoracic cavity in larges and negative pressure is developed inside the thoracic cavity and in lungs. So the air through respiratory passage rushes into the lungs up to alveoli where exchange of gases occurs.
Expiration (exhalation): Expiration is giving out of air. It is passive process which takes due to increased pressure in thoracic cavity as well as lungs. It is caused by relaxation of external intercostal muscles and the contraction of internal intercostal muscles. Which move ribs as well as sternum inward and downward. Diaphragm also relaxes which makes it dome shaped thus reducing the volume of thoracic cavity. As a consequence lungs are compressed as the air along with water vapour is exhaled outside through respiratory passage.
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