Human beings like other land animals breathe though their nostrils in
noses and with the help of lungs. A pair of lungs are located in the
airtight thoracic cavity that is bounded by a convex muscular and
elastic sheet called diaphragm.
Functionally, the lungs are elastic bags resembling rubber balloons.
They lack any muscle, which may allow them to expand or contract by
In normal breathing, through the nose, air travels through the nasal
passages that are lined with ciliated mucous epithelium. Here, it is
cleaned and warmed. Sensory cells detect odours. As air continues
through the pharynx or throat, it crosses the path of food. This is
why we can breathe through the mouth. Then, air passes the epiglottis,
enters the larynx or voice box, and goes down the trachea or windpipe.
A bronchus runs to each lung, divides in a tree like manner to give
smaller bronchioles and finally deposits the air in the microscopic
thin walled air sacs or alveoli (singular alveolus). A group of
alveoli appears like a cluster of grapes and gives the lungs, a sponge
like structure. There are about 150 million alveoli in each lung and
altogether they cover a very large surface area (approximately 70
The alveoli are lined by a layer of moist flat epithelial cells and
surrounded by networks of blood capillaries. The blood, which flows to
the lungs by pulmonary artery, contains little oxygen and much carbon
dioxide. On the other hand, the air in the alveoli has a high
concentration of oxygen and relatively less carbon dioxide. Thus a
2-way diffusion takes place through the cells of the capillaries.
Oxygen enters the blood and CO2 leaves it. Since enormous breathing
surface of lungs is exposed to the external environment the exchange
of gases is computed within a few seconds.