Lung Structure
Lung function & Structure
You should know
 The gross structure of the respiratory system
 The essential features of the alveolar epithelium as a surface
over which gas exchange occurs
 The exchange of gases in the lungs
 The mechanism of breathing
 Pulmonary ventilation as the product of tidal volume and
ventilation rate
Lung Structure
Lungs are the interface for the exchange of gases and their function
is affected by both pathogens and lifestyle.
All aerobic organisms require a constant supply of oxygen in order
to release energy in the form of ATP during respiration. The
waste gas product carbon dioxide also has to be removed because
the build up can be harmful.
Why have lungs?
 We need lungs to enable oxygenation of our blood system in order to
circulate to the relatively large volume of living cells. Mammals have to
maintain a high body temperature and therefore have high metabolic and
respiratory rates.
 Air enters via the trachea and then into the left and right bronchus
(plural bronchi). The bronchi lead into a pair of lobed structures called
the lungs. Lungs consist of a series of highly branched tubules called
bronchioles which end up in tiny air sacs called alveoli.
Why are a mammals lungs located within
the body?
 Air is not dense enough to support and protect delicate structures
 They would cause loss of a great deal of water and dry out
 Because lungs are located internally, we need to have a means of
moving the external medium (air) over the surface of our lungs. This
movement is called ventilation.
Rib cage
 Rib cage is moved by
muscles between ribs
(intercostal muscles covered
later)
 Causes ventilation by a
tidal stream
 Means constant
replenishment of air
 Trachea
 Flexible and supported with cartilage
 Cartilage prevents trachea collapsing when air
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pressure falls
Tracheal walls consist of muscle
Lined with ciliated epithelium and goblet cells
Goblet cells produce mucus to trap dirt
particles and bacteria
Cilia move mucus + dirt/bacteria up the throat
into oesophagus to stomach
Bronchi
 Two divisions of the trachea –
each leading to one lung
 Bronchi (plural) and left and
right bronchus (singular)
 Similar cell structure as
trachea – produce mucus and
have cilia
 Larger bronchi have cartilage
– cartilage reduced as
bronchi get smaller
Bronchioles
 Series of branching
subdivisions of the bronchi
 Walls made of muscle lined
with epithelial cells
 Muscle permits constriction
to control the flow of air in
and out of the alveoli
Alveoli (plural);
alveolus (singular)
 Alveoli are minute air sacs (about 100-300 μm) at the end of the
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bronchioles
Contain collagen and elastic fibres
Elastic fibres allow alveoli to stretch when breathing in
When breath exits, elastic fibres spring/recoil back to aid expulsion
of carbon dioxide
Lined with squamous epithelium
Alveoli membrane is the gas-exchange surface of the lungs
Blood vessels line the alveoli to diffuse gases
List key structures of air travelling from
the nostril to the blood vessels
surrounding the alveoli?
Nostril → nasal cavity → trachea → bronchi →bronchioles → alveoli →
blood vessels
Mechanisms of Breathing
Remember in Biology we DO NOT call the process of intake of
air ‘Respiration’ (that is the process of ENERGY production),
we call this process Ventilation.
We must remember the
for particle movement
HIGH → LOW
This time we are talking about air pressure.
rule again
Breathing
 Inspiration
 Expiration
 External intercostal muscles
 Internal intercostals muscles
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pull ribs up and out.
Diaphragm muscles flatten
diaphragm
Volume of thorax increases
Air pressure within thorax
drops
Air enters lungs
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contract so ribs fall
Diaphragm muscles relax so
it becomes dome-shaped
Volume of thorax decreases
Air pressure within thorax
increases
Air leaves lungs
ACTIVE PROCESS – uses ENERY
Largely PASSIVE PROCESS –
requires little energy
Diaphragm
 A sheet of muscle between
the thorax and abdomen
 The diaphragm is curved
upward (domed position)
when relaxed
 When diaphragm muscles
contract, it flattens and moves
down
 Causing an increase in thorax
volume
recoil of the
elastin within alveoli walls of
the lungs is the MAIN cause of air being forced out
 During NORMAL quiet breathing, the
(similar to balloons). Muscles become most important during
strenuous condition such as exercise.
What is Pulmonary Ventilation?
 The total
To calculate this we need multiply two factors:
volume of air TIDAL VOLUME = volume of air normally taken
that is moved
in at EACH breath when body is at REST
into the lungs (usually 0.5dm3)
during one
VENTILATION (Breathing) RATE
minute
= the number of breaths taken in one minute.
(usually 12-20 breaths)
Pulmonary ventilation = tidal volume x ventilation rate
(dm3 min-1)
(dm3)
(min-1)
Pulmonary ventilation is the product of
tidal volume and ventilation rate.
Exchange of Gases in the Lungs
Ventilation of gases within lungs is essential to provide a
constant supply of oxygen to create a diffusion gradient
within the alveolar surface.
What is the role of the alveoli in gas
exchange?
We have about 300 million alveoli in EACH human lung.
Their total surface area is 70m2 (half a tennis court)
Each alveoli is lined with squamous epithelial cells (0.05μm to
0.3μm thick)
AROUND each alveoli is a network of pulmonary capillaries
Very narrow (7-10μm thick) so that red blood cells are flattened
against the walls
Also have a single very thin wall (0.04μm – 0.2μm)
Complete the following table identifying
essential features of gas exchange
Characteristic
Benefit to gas exchange
surfaces:
Large surface area to volume ratio
Squamous epithelial cells
Speeds up gas exchange
Very thin to keep diffusion distance
small
Partially permeable
Selective materials permitted across
Movement of environmental medium
Maintain diffusion gradient (O2 in &
(air)
CO2 out)
Movement of internal medium (blood)
Moisture
Maintain diffusion gradient (O2 in &
CO2 out)
Aids optimal gas diffusion
Fick`s Law
Rate of Diffusion is proportional to:
Surface Area of exchange x Concentration Gradient
Diffusion Distance
Fill in missing gaps with additional key
words:
Label the above diagram showing the key features with the appropriate number(s):
Summary Questions
How does each of the following features contribute to gas exchange
efficiency?
 The wall of each alveolus is not more than 0.3μm thick?
Diffusion distance small – thus rapid movement
 There are 300 million alveoli in each lung?
Collectively, very large surface area
The surfaces of the alveoli are moist?
Permits optimal gas exchange; gases are exchanged in solution.
Each alveolus is covered by a dense network of pulmonary blood
capillaries?
Collectively, provide a VERY large surface interface with blood
 Each pulmonary capillary is very narrow?
Enables slowing down of blood (thus diffusion occurs faster)
and blood cells pushed against capillary way to make
diffusion distance smaller also
Explain the differences in air
composition between the three
samples.
Plot to show the relative difference in gas samples during
breathing.
25
% composition of air
20
15
Oxygen
Carbon dioxide
10
5
0
Inhaled Air
Alveolar Air
Exhaled Air
Explain the
differences in air
composition
 Exhaled air has been mixed with residual air
between the three
within the air passages (bronchioles,
samples.
Alveolar air has
oxygen removed
and carbon
dioxide added.
bronchi and trachea) so is closer to
atmospheric air in composition than that in
the alveolar lumen.
Summary
 Lung function
 The gross structure of the human gas exchange system
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limited to the alveoli, bronchioles, bronchi, trachea and
lungs.
The essential features of the alveolar epithelium as a surface
over which gas exchange takes place.
The mechanism of breathing.
Pulmonary ventilation as the product of tidal volume and
ventilation rate.
The exchange of gases in the lungs.