1. © SSER Ltd.

2. 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)
Bronchioles
Left lung
Ribs
Right
bronchus
Section through
ribs
Intercostal
muscles
Diaphragm
(a powerful sheet of muscle
separating the thorax from the abdomen)

3. 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

4. The Trachea Trachea 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
Air enters the body
through the nasal
passages and mouth,
and passes via the
pharynx and larynx
to the trachea
Trachea
The trachea
subdivides to give
rise to the right
and left bronchus –
these tubes are
also strengthened
by cartilage
Air is delivered to
the alveoli as the
trachea branches
into bronchi and
bronchioles
The two bronchi
subdivide to form
an extensive
network of
bronchioles that
deliver air to
the gas exchange
surfaces – the
alveoli
Bronchioles
Right and Left
bronchus

5. Lumen of
trachea
This highly magnified view of the lining of the trachea shows the cilia and
mucus-secreting goblet cells that make up the epithelium
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

6. The Gas Exchange Surface
A single alveolus
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 bronchioles divide
many times forming
respiratory bronchioles,
which in turn divide to
to form alveolar ducts
that terminate in groups
of sacs – the alveoli
Alveolar
duct
Respiratory
bronchioles
Alveoli

7. 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 mm)

8. 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

9. Summary Inspiration Expiration
External intercostal muscles contract and
raise the ribs upwards and outwards
External intercostal muscles relax and
the ribs move downwards and inwards
The diaphragm
muscle contracts
and flattens
The diaphragm
muscle relaxes
and becomes
dome-shaped
The volume of the thorax increases
The volume of the thorax decreases
The air pressure in the thoracic cavity
falls below that of the atmospheric air
The air pressure in the thoracic cavity
rises above that of the atmospheric air
Air rushes into the lungs along a
pressure gradient
Air rushes out of the lungs along a
pressure gradient

10. Pressure Changes During the Breathing Cycle
The lungs are sealed in an
airtight, fluid-filled, double-
membrane sac called the pleura
The pressure within the airways of the respiratory system is known as the intrapulmonary pressure
Two pleural membranes
surround the lungs and the cavity between them
is filled with pleural fluid to protect the lungs from the bony ribs
Intrapulmonary
pressure within
the airways
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 magnitude of these pressures varies during the breathing cycle

11. 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
Atmospheric
pressure
As inspiration begins and the ribs move upwards and
outwards, the pressure within the airways (the intra-
pulmonary 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
As expiration begins and the ribs move downwards and
inwards, the pressure within the airways (the intra-
pulmonary pressure) rises above that of atmospheric
air (shown as 0 mm Hg)
Air rushes out of the lungs to equalise the pressures and
intrapulmonary pressure falls to that of
the atmosphere
The intrapleural pressure rises as the pleural cavity
decreases in size on expiration
The volume of air inspired and expired during one
breathing cycle is shown in the lower part of the graph

12. 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