Transcript The Breathing System

4.3 Exchange of gases in
the lungs
Learning outcomes
Students should understand the following:
 The essential features of the alveolar epithelium as a
surface over which gas exchange takes place.
 The exchange of gases in the lungs.
Alveoli
 The site of gas exchange in mammals is the epithelium
of the alveoli.
 Minute air-sacs 100 – 300mm in diameter
 There are about 300 alveoli in each human lung.
 Each alveoli is lined with
epithelial cells 0.05 mm to
0.3 mm thick.
 Each alveolus is surrounded
by a network of pulmonary
capillaries.
Inside an alveolus
Oxygen makes its way to special air sacs.
Actually, each air sac is
found to be a bundle of
air sacs. Together, they
are known as an
alveolus.
The outside of the
alveolus is covered with
tiny blood vessels.
We can look inside the alveolus to get some
idea of why they are shaped the way they are.
A cross-section of an alveolus
oxygen (O2)
gas passes
through here
This O2 is then
able to dissolve in
a small moist
lining.
epithelial
lining of the
alveolus
The moist lining also
stops the alveolus
from drying and
cracking. It lubricates
the insides of the air
bag.
Label the alveolus
Diffusion at work
Oxygen diffusion into red blood cells
After the oxygen dissolves it also diffuses.
O2
O2
O2
O2
D IFF USI ON
cell lining of
alveolus
cell lining of
capillary
blood
The oxygen molecules must diffuse through both the
lining of the alveolus and the lining of the blood capillary.
They are eventually picked up by red blood cells.
Blood leaving the lungs
The blood now carries
this oxygen to the cells
of the body.
Right
Lung
Left
Lung
blood vessel
O2
O2
blood
body cells
Laws of diffusion
The movement of the oxygen
from the blood to the cells also
follows the law of diffusion.
blood coming
from the lungs
high
concentration
It is highly concentrated
within the blood.
Meanwhile the concentration
is low within the cell.
Therefore the oxygen passes
into the body cells.
low
concentration
body cell
Alveoli adaptations
Alveoli have the following adaptations which make them a
really good surface for gas exchange:
 Large surface area to volume ratio – speeds up the rate of
exchange
 Thin exchange surfaces – short diffusion pathway
 Partially permeable – to allow selected materials to diffuse
easily
 Steep concentration gradient maintained by breathing and
blood flow
Alveoli adaptations
 The walls of the alveoli are moist and oxygen diffuses
into this liquid before diffusing across the alveolar wall
and into the blood.
 This liquid contains a surfactant which reduces surface
tension and prevents the air-sacs collapsing.
 It is produced in babies from 7 months onwards which
is why those born before this time may have breathing
difficulties.
 Artificial surfactants are now being used which can
help prevent brain damage due to lack of oxygen in the
first few weeks.
Learning outcomes
Students should understand the following:
 The essential features of the alveolar epithelium as a
surface over which gas exchange takes place.
 The exchange of gases in the lungs.