Respiratory system

The function of respiratory system is to breathe in oxygen for respiration (producing energy from food), and to breathe out carbon dioxide produced by respiration.

The major organs of respiratory system in human beings are: (i) Nose (ii) Nasal Passage (iii) Trachea (iv) Bronchi (v) Lungs and (vi) diaphragm.

In human beings, air is taken into the body through the nostrils, is filtered by fine hairs that line the passage. When air passes through the nasal passage, the dust particles and other impurities present in it are trapped by nasal hair and mucus so that clean air goes into the lungs. From here, the air passes through the throat and into the lungs. Trachea does not collapse even when there is no air in it because it is supported by rings of soft bones called cartilage.
Within the lungs, the passage divides into smaller and smaller tubes which finally terminate in balloon-like structures which are called alveoli. The alveoli provide a surface where the exchange of gases can take place. The walls of the alveoli contain an extensive network of blood-vessels. When we breathe in, the ribs are lift up and the diaphragm flattens which increases the size of the chest cavity. Because of this, air is sucked into the lungs and fills the expanded alveoli. The blood brings carbon dioxide from the rest of the body for release into the alveoli, and the oxygen in the alveolar air is taken up by blood in the alveolar blood vessels to be transported to all the cells in the body. During the breathing cycle, when air is taken in and let out, the lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released.