alveoli, the combined area of which is about 100
square meters, i.e., about a hundred times the area of skin on the
human body. Thus a vast surface is exposed for exchange of gases
in the lungs.
A transverse section of a lung shows a number of
bunches of grape-like structures called alveoli lined by
flat epithelial cells showing connective tissues and blood
capillaries in between. It also shows a section of terminal
bronchioles lined by smooth muscle fibers. Respiratory bronchioles
and blood capillaries may also be visible.
Figure 17.5 Transverse Section of lung
Mechanism of breathing
The mechanism of breathing in man involves two
events: inspiration or breathing in of fresh air and expiration
or breathing out of foul air,. Humans breathe about 16 to 18 times
per minute.
