Answer:
The
human respiratory system begins from nostrils.
The air enters through nostrils into the pharynx and then trachea (or
wind pipe). The trachea runs down and divides into two tube-like structure
called bronchi. Each bronchi then enters into a lung where it divides into
small bronchioles. They finally terminate into balloon-like structure called
alveoli where gaseous exchange takes place. (1)
Breathing in human involves three steps
(i) Inspiration When we breathe in, we lift our ribs and
flatten our diaphragm and the chest cavity becomes larger. Air is sucked into
the lungs and fills the expanded alveoli.
(ii) Gaseous Exchange Haemoglobin binds with oxygen and
carries it along the body in blood. As blood passes through the tissues of the
body, the oxygen from the blood diffuses into the cell, whereas carbon dioxide
which is produced during respiration diffuses into the blood and is carried to
the lungs. (1)
(iii) Expiration Ribs move down and diaphragm becomes
dome-shaped decreasing the chest cavity and air comes out. (1)
The diagram showing human respiratory system is given below
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