Respiratory System

Important Respiratory Organs:

The human respiratory system consists of external nares or nistrils, nasal cavity, nasopharnyx, larynx, trachea, bronchi, bronchiole and lungs.
The air enters into the nasal cavity through the nostrils.
Larynx is a small box and it forms the connection between pharynx and the windpipe or trachea. Larynx is also known as voice box.
Larynx is covered by pieces of cartilage forming a box. In males, the adam's apple is the protuding cartilage of the voice box.
Trachea is a tubular structure of about 12 cm in length and 2.5 cm in diameter.
Lungs are paired structure present in the thoracic or pleural cavity.
During inhalation, diaphragm contracts and lungs increases while during exhalation, the diaphragm relaxes and the lungs are compressed.
The cycle of breathing (ventilation of lungs) consists of three phases:

Inspiration is an active process. It is the result of muscle contraction.
Expiration is an passive process. It is the result of muscle relaxation.
The main muscles of respiration in normal breathing are intercostal muscles and diaphragm.
Larynx does not contribute to the breathing movements.
Breathing occurs involuntarily but its rate is controlled by the respiratory centre of brain. Under normal conditions rate of breathing is 15-18 times per minute.