4.2 The mechanism of
breathing
Learning outcomes
Students should understand the following:
 The mechanism of breathing.
 Pulmonary ventilation as the product of tidal volume
and ventilation rate.
The 2 breathing processes
The body separates the procedure of breathing in and
breathing out.
Breathing in is one
process and is known as…
Breathing out is a separate
process and is known as…
Inhalation
(When we breathe in
we inhale)
Exhalation
(When we breathe out we
exhale)
By separating these two processes, the body can concentrate
on the two tasks in turn.
Breathtaking features
The breathing system does not have a fixed shape.
trachea
ribs
rib muscles
right bronchus
diaphragm
Right Lung
Left Lung
It has the ability to move, whilst remaining enclosed within
the protection of the ribcage.
A mobile ribcage?
This means that the rib cage must also be able to
change position.
Take your hands and place them flat on your
chest just above your hips on each side of your
body. Now breathe in and out very deeply.
Whilst you do this, watch to see what happens
to your hands.
You should notice the following things…..
Take a breath
When you breathe in (inhale), your hands move up and outwards.
When you breathe out (exhale), your hands move down and inwards.
Inhaling: chest expansion
When we inhale, our lungs fill with air.
As they fill, they become enlarged.
The ribs must then move upwards and
outwards to make more room in the thorax.
The overall effect of this is that our chest
expands.
Pressure changes
The pressure changes in the lungs are brought about by
the movement of 2 sets of muscles:
 The diaphragm – a sheet of muscle that separates the
thorax and the abdomen.
 The intercostal muscles:
 Internal intercostal muscles – contraction leads to
expiration
 External intercostal muscles – contraction leads to
inspiration
The diaphragm
Your diaphragm is located beneath the lungs, which means
that it separates the thorax from the abdomen.
It is a sheet of muscle that
spans the width of the body.
Before we inhale, it is found in a dome shape.
As we inhale, it contracts and flattens.
The result of this change in shape is a
change in the volume of the thorax.
inhaling
Pressure regulation
As the volume of the thorax increases, the internal air
pressure drops.
This means that the air pressure outside the lungs is
greater than the air pressure inside the lungs.
High
High air pressure outside
Low air pressure inside
Low
Diaphragm flattens
Thorax volume increases
Air pressure drops
Air diffuses into the lungs
During exercise
Inspiration is always an active process. When the body is
at rest expiration is passive ( the diaphragm muscles
relax and the elastic recoil of the lungs and chest wall
return the thorax to it’s original shape.)
During exercise expiration is boosted by contraction of
the internal intercostal muscles
Key definitions
 Tidal Volume - The volume of air inhaled and exhaled at
each breath, this is normally equivalent to only 10% of the
total 5L volume of the trachea and lungs.
 Residual Volume - the amount of air still remaining in the
lungs after the most forceful expiration.
 Vital Capacity - the maximum amount of air that can be
exhaled after a maximum inhalation.
 Training can increase the strength of the diaphragm and
intercostal muscles and can thus raise the vital capacity
and hence improve performance.
Pulmonary ventilation
This is the total volume of air that is moved into the lungs
during one minute. To calculate it we can multiply 2
factors together:
 Tidal volume – the volume of air normally taken in in
each breath when the body is at rest (normally 0.5dm3).
 Ventilation rate – the number of breaths taken in one
minute (normally 12-20 breaths).
Pulmonary Ventilation = Tidal Volume x Ventilation Rate
(dm3min-1)
(dm3)
(min-1)
Wordsearch
E
D
I
X
O
I
D
N
O
B
R
A
C
N
S
V
T
Y
S
Z
F
E
T
B
V
X
R
E
D
M
V
U
E
U
R
M
G
W
Q
C
Q
E
L
R
N
X
W
X
N
O
S
T
R
I
L
A
S
L
O
N
W
D
Y
D
G
E
F
A
H
E
L
P
S
N
K
R
I
B
M
U
S
C
L
E
S
V
I
M
L
P
Z
A
L
V
E
O
L
I
H
O
E
R
B
A
O
R
P
D
B
I
C
B
U
H
X
O
A
R
E
O
I
H
A
E
D
T
F
T
N
Y
L
T
O
H
Q
B
R
O
N
C
H
U
S
J
G
U
I
N
C
P
C
A
G
E
A
O
I
K
I
E
S
O
C
A
P
A
G
C
R
M
R
U
J
L
N
M
N
H
R
R
G
M
H
G
N
A
S
A
L
C
A
V
I
T
Y
E
P
L
Y
K
X
N
Y
R
A
H
P
S
J
T
Learning outcomes
Students should understand the following:
 The mechanism of breathing.
 Pulmonary ventilation as the product of tidal volume
and ventilation rate.