Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Examining the airways
a
The surgeon in Figure 9.1a is using an endoscope to
examine the airways looking for any blockages or for
possible signs of lung cancer. Endoscopes are flexible
tubes with a light and camera at one end. They are
used for many routine examinations of body cavities.
The type of endoscope used to examine the airways
and lungs is a bronchoscope.
The surgeon inserts the bronchoscope through the
nose or mouth. It passes through the vocal cords in
the larynx and then goes down the trachea (windpipe)
and into one of the bronchi. If the bronchoscope has
a video camera it is possible for others, including
the patient, to view any blockages or damage to the
airways on a monitor screen.
In this case, the white area at the base of the
trachea is inflamed (Figure 9.1b). If the patient is a
smoker, the surgeon may suspect lung cancer. He
can use the bronchoscope to carry out a biopsy by
removing a small amount of tissue from this area and
sending it to the lab for analysis.
b
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Figure 9.1 a A surgeon using a bronchoscope
to view the interior of the airways. b A view
through a bronchoscope of the base of the
trachea where it branches into the two
bronchi. The white area is inflammation of the
tissues that line the trachea.
Gas exchange
The human gas exchange system links the circulatory
system (Chapter 8) with the atmosphere. It is adapted to:
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clean and warm the air that enters during breathing
maximise the surface area for diffusion of oxygen and
carbon dioxide between the blood and atmosphere
minimise the distance for this diffusion
maintain adequate gradients for this diffusion.
Most organisms need a supply of oxygen for respiration.
In single-celled organisms, the oxygen simply diffuses
from the fluid outside the cell, through the cell surface
membrane and into the cytoplasm. In a multicellular
organism such as a human, however, most of the cells are a
considerable distance away from the external environment
from which the oxygen is obtained. Multicellular
organisms therefore usually have a specialised gas
exchange surface where oxygen from the external
environment can diffuse into the body, and carbon dioxide
can diffuse out.
In humans, the gas exchange surface is the alveoli
(singular: alveolus) in the lungs. Figure 9.2 shows the
distribution of alveoli in the lungs and their structure.
Although each individual alveolus is tiny, the alveoli
collectively have a huge surface area, probably totalling
around 70 m 2 in an adult. This means that a large number
of oxygen and carbon dioxide molecules can diffuse
through the surface at any one moment to give us a high
rate of gas exchange.
Lungs
The lungs are in the thoracic (chest) cavity surrounded by
the pleural membranes, which enclose an airtight space.
This space contains a small quantity of fluid to allow
friction-free movement as the lungs are ventilated by the
movement of the diaphragm and ribs.