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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
Bronchioles
Larynx
Trachea
(with rings of cartilage)
Left lung
Ribs
Right
bronchus
Section through
ribs
Intercostal
muscles
Diaphragm
(a powerful sheet of muscle
separating the thorax from the abdomen)
The lungs are contained
within the thoracic cavity,
the sides of which are
bounded by 12 pairs
of ribs that articulate
(join) with the
vertebrae towards
the back of the body,
and the sternum or
breast bone, towards
the front
The portions of the ribs
that articulate with the
breastbone are composed
of cartilage rather than bone
Cartilage
Cartilage is softer
and more pliable then
bone and thus assists
the movements of
the rib cage during
breathing
Two sets of
antagonistic muscles
are located between
the ribs – these
are the intercostal
muscles
This chest X-ray shows the gas-filled lungs within the thoracic cavity
Ribs
Air-filled
lungs
Position
of
the heart
Position of the diaphragm
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
The bronchioles divide
many times forming
respiratory bronchioles,
which in turn divide to
to form alveolar ducts
that terminate in groups
of sacs – the alveoli
A single alveolus
Alveolar
duct
Respiratory
bronchioles
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)
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
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
Internal intercostal
muscles
Diaphragm
The Mechanics of Breathing
The external and internal
intercostal muscles are
responsible for the movements
of the rib cage during
breathing
Internal intercostal
muscles
During periods of increased
activity such as during
exercise, the internal
intercostal muscles contract
to bring out more
forceful expirations
Rib
External intercostal
muscles
Contraction of the
external intercostal
muscles moves the ribs
upwards and outwards
during inspiration
Relaxation of the
external intercostal
muscles causes the ribs
to move downwards
and inwards
during expiration
at rest
Expiration at rest is a passive process; expiration during periods of exercise
is an active process involving contraction of the internal intercostal muscles
Inspiration - Breathing In
Sternum
(breastbone)
Vertebral
column
Rib
During an inspiration, the
external intercostal muscles
contract and raise the rib
cage upwards and
outwards; the diaphragm
also contracts and flattens
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
Sternum
(breastbone)
Rib
During an 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
Diaphragm
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
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
Air moves from a higher to a lower pressure region and deflates the
lungs as expiration takes place
During periods of increased activity such as exercise,
the rate and depth of breathing increases
The more forceful,
downward and inward
movements of the rib cage
during expiration of exercise
are achieved through the
contraction of the internal
intercostal muscles
At the same time,
contraction of the abdominal
muscles just below the
thorax, push the diaphragm
into a more domed position
As the diaphragm
pushes further into
the thorax, the
volume of the chest
cavity decreases
more significantly
and the increased
thoracic pressure
aids expiration
During periods of exercise, expiration becomes an active process
Inspiration
Summary
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
dome-shaped
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
Pressure Changes During the Breathing Cycle
The lungs are sealed in an
airtight, fluid-filled, doublemembrane sac called the pleura
Two pleural membranes
surround the lungs and
the cavity between them
is filled with pleural fluid
to protect the lungs from
the bony ribs
The pressure within
the pleural cavity is
known as the
intrapleural pressure
and this pressure is
always below that of
the atmosphere
(sub-atmospheric)
The pressure within
the airways of the
respiratory system is
known as the
intrapulmonary
pressure
Intrapulmonary
pressure within
the airways
The magnitude of these pressures varies during the breathing cycle
The pressure and volume changes occurring
during the breathing cycle can be represented as a
graph where zero, on the pressure axis, represents
atmospheric pressure
inspiration
expiration
+2
i
n
t
r
a
p
u
l
m
o
n
a
r
y
p
r
e
s
s
u
r
e
As inspiration begins and the ribs move upwards and
outwards, the pressure within the airways (the intrapulmonary pressure) falls below that of atmospheric
air (shown as 0 mm Hg)
Air rushes into the lungs to equalise the pressures and
intrapulmonary pressure increases to that of
the atmosphere
The intrapleural pressure falls even more below that of
the atmosphere as the pleural cavity expands on inspiration
+1
Atmospheric
pressure
0
pressure
(mm Hg)
-1
-2
in
t
r
a
p
le
u
r
a
l
p
r
e
s
s
u
r
e
-3
-4
As expiration begins and the ribs move downwards and
inwards, the pressure within the airways (the intrapulmonary pressure) rises above that of atmospheric
air (shown as 0 mm Hg)
-5
-6
Air rushes out of the lungs to equalise the pressures and
intrapulmonary pressure falls to that of
the atmosphere
v
o
lu
m
e
o
f
b
r
e
a
t
h
The intrapleural pressure rises as the pleural cavity
decreases in size on expiration
0.6
0.4
volume
3
0.2
The volume of air inspired and expired during one
breathing cycle is shown in the lower part of the graph
0
1
2
3
time (s)
0
4
(dm )
The Collapsed Lung
The pressure within
the pleural cavity
(intrapleural pressure)
is always below that of
the atmosphere
(sub-atmospheric)
If a lung is pierced, then
air rushes into the pleural
cavity along a pressure gradient
Atmospheric air pressure is
GREATER than the fluid
pressure within the
pleural cavity
As air rushes into the pleural cavity, the pressure difference across the lung
wall is eliminated, and the stretched lung collapses
Air enters the fluid-filled pleural cavity and this condition
is known as a pneumothorax
Acknowledgements
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