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Gas Exchange in Mammals
Aim – to understand the structure and function of the lungs.
Objectives- by the end of this lesson you should be able to
• Name and identify parts of the gas exchange system.
• Explain how the alveoli are adapted for efficient gas
exchange.
• Describe the structure and function of ciliated epithelium,
goblet cells, cartilage, smooth muscle and elastic fibres
• Explain how the lungs are ventilated.
The Human Gas Exchange System
The human gas exchange system
consists of the nasal passages, the
pharynx or throat, the larynx or
voice box, the trachea, the right
and left bronchus and the lungs
Larynx
Trachea
(with rings of cartilage)
Left lung
Bronchioles
Right
bronchus
Ribs
Section through
ribs
Intercostal
muscles
Diaphragm
(a powerful sheet of muscle
separating the thorax from the abdomen)
The trachea or
windpipe is about
10 cm long and is
supported by
C-shaped rings
of cartilage to
prevent the tube
from collapsing
during breathing
The Trachea
The trachea
subdivides to give
rise to the right
and left bronchus –
these tubes are
also strengthened
by cartilage
The two bronchi
subdivide to form
an extensive
network of
bronchioles that
deliver air to
the gas exchange
surfaces – the
alveoli
Trachea
Right and Left
bronchus
Air enters the body
through the nasal
passages and mouth,
and passes via the
pharynx and larynx
to the trachea
Air is delivered to
the alveoli as the
trachea branches
into bronchi and
bronchioles
Bronchioles
This photomicrograph of a transverse section through the trachea shows
the C-shaped ring of cartilage
C-shaped
cartilage ring
Magnify
This magnified view of the wall of the trachea shows the cartilage cells together
with the cells that line the lumen of the trachea – ciliated epithelium
Ciliated
epithelium
Cartilage
cells
This highly magnified view of the lining of the trachea shows the cilia and
mucus-secreting goblet cells that make up the epithelium
Lumen of
trachea
Goblet cell that secretes
mucus to trap dust and
other foreign material
that may enter the
respiratory system
The wafting of these cilia
removes the mucus and
trapped foreign material
from the respiratory
system
The Gas Exchange Surface
Move the cursor over the area of lung (yellow circle) to show the alveoli...
The Gas Exchange Surface
Thorax
Section of lung
A Single alveolus
Respiratory
bronchiole
Alveolar
duct
The bronchioles divide
many times forming
respiratory bronchioles,
which in turn divide to
form alveolar ducts that
terminate in groups
of sacs – the alveoli
Each alveolus is a
hollow, thin-walled sac
that is surrounded by a
dense network of
capillaries and is the
site of gas exchange
in the lungs
The Gas Exchange Surface
As deoxygenated blood from the body tissues flows through the network of
capillaries surrounding each alveolus, oxygen diffuses into the blood and carbon
dioxide diffuses from the blood into the alveolus; oxygenated blood travels from
the lungs to the left of the heart for delivery to the body tissues
Gases are exchanged across the alveoli by diffusion
According to Fick’s Law...
Rate of diffusion =
surface area x difference in concentration
thickness of exchange surface
Maximum rate of diffusion of respiratory gases is achieved by:
• the large surface area presented by the alveoli (there are about 350 million
alveoli in the two lungs presenting an enormous surface area of
approximately 90 square metres – about the area of a tennis court)
• the large differences in concentration of metabolites between the alveoli
and the blood capillaries
• the thinness of the diffusion barrier (alveolar and capillary walls provide
a total thickness of only 0.005 mm)
Alveolar Tissue
A photomicrograph of a section of alveolar tissue
showing the delicate nature of the lungs and the
'one cell thick' walls of the alveoli which facilitate
diffusion of respiratory gases.
The Mechanics of Breathing
Breathing in (inspiration) and
breathing out (expiration) are
mechanical processes involving
the ribs, intercostal muscles
and the diaphragm
Two sets of antagonistic
muscles are located between
the ribs; these are the external
and internal intercostal muscles
The intercostal muscles
are antagonistic in the
sense that contraction of
the external muscles raises
the rib cage, whereas
contraction of the
internal muscles
lowers the rib cage
External intercostal
muscles
Internal intercostal
muscles
The diaphragm is a
powerful sheet of
muscle that
separates the
thorax from the
abdomen; it is
dome-shaped when
relaxed and flattens
on contraction
Diaphragm
Inspiration - Breathing In
The volume of the thorax
increases, lowering the air
pressure in the chest cavity
to less than that of the
atmosphere outside
During inspiration, the
external intercostal muscles
contract and raise the rib
cage upwards and
outwards; the diaphragm
also contracts and flattens
A pressure gradient is
created between the
atmosphere and the
lungs, and air rushes
in via the trachea to
equalise the pressure
difference
Air moves from a higher to a lower pressure region and
inflates the lungs as inspiration takes place
Expiration - Breathing Out
During an expiration, the external intercostal
muscles relax and lower the rib cage; the
diaphragm relaxes and becomes dome-shaped
The volume of the thorax decreases, raising the
air pressure in the chest cavity to above that of the
atmosphere outside
A pressure gradient is created between the lungs
and the atmosphere, and air rushes out via the
trachea to equalise the pressure difference
Expiration is assisted by the elastic recoil of the lungs following the stretching of elastic
fibres during the process of inspiration
Air moves from a higher to a lower pressure region and deflates the lungs as expiration
takes place
The mechanism described is that for breathing at rest
At rest, inspiration is an active process involving contraction of the muscles of breathing
Expiration is a purely passive process involving relaxation of the muscles of breathing
together with elastic recoil of the lungs
During forced breathing, as in exercise, expiration becomes an active process
Summary
Inspiration
External intercostal muscles contract and
raise the ribs upwards and outwards
The diaphragm
muscle contracts
and flattens
The volume of the thorax increases
The air pressure in the thoracic cavity
falls below that of the atmospheric air
Air rushes into the lungs along a
pressure gradient
Expiration
External intercostal muscles relax and
the ribs move downwards and inwards
The diaphragm
muscle relaxes and
becomes domeshaped
The volume of the thorax decreases
The air pressure in the thoracic cavity
rises above that of the atmospheric air
Air rushes out of the lungs along a
pressure gradient