Cambridge International A Level Biology Revision Guide

Chapter 9: Gas exchange and smoking
a
trachea
bronchus
heart
intercostal
muscles
terminal
bronchiole
rib
left lung
rib
c × 60
pleural
membranes
deoxygenated
blood
oxygenated
blood
diaphragm
b × 20
capillaries
alveolus
187
respiratory
bronchiole
terminal
bronchiole
alveolar
duct
alveoli
Figure 9.2 The human lungs. Air passes through a the trachea and bronchi to supply many branching bronchioles
b which terminate in alveoli c where gas exchange occurs. A gas exchange surface of around 70 m 2 fits into the
thoracic cavity, which has a capacity of about 5 dm 3 .
Trachea, bronchi and
bronchioles
The lungs are ventilated with air that passes through a
branching system of airways (Table 9.1). Leading from
the throat to the lungs is the trachea. At the base of the
trachea are two bronchi (singular: bronchus), which
subdivide and branch extensively forming a bronchial
‘tree’ in each lung. Each bronchus divides many times to
form smaller bronchioles. Terminal bronchioles divide to
form even narrower respiratory bronchioles that supply
the alveoli with air.
Cartilage in the trachea and bronchi keeps these
airways open and air resistance low, and prevents them
from collapsing or bursting as the air pressure changes
during breathing. In the trachea, there is a regular
arrangement of C-shaped rings of cartilage; in the bronchi,
there are irregular blocks of cartilage instead (Figure 9.3).