Today we are covering from the
specification:
Key words & Definitions
Ventilation - Scientific term for “breathing” – where
air is constantly moving in and out of the lungs.
Inspiration - Also known as “inhalation”, occurs when
air pressure in the atmosphere is greater than that of
the lungs; forcing air into the alveoli.
Expiration - Also known as “exhalation”, occurs when
air pressure in the lungs is greater than that in the
atmosphere; forcing air out of the alveoli.
How does ventilation happen?
• Ventilation requires
pressure changes to occur
in the lungs, so that air can
be forced in and out of the
alveoli.
• The pressure changes are
brought about by the
movement of two sets of
muscles....
How does ventilation happen?
1. The diaphragm muscle,
which is a sheet of tissue
that separates the thorax
(chest) from the abdomen.
2. The intercostal muscles
which lie between the ribs.
There are two sets of
intercostal muscles....
The intercostal muscles
– an antagonistic pair
•
•
The external intercostal
muscles, whose contraction
leads to inspiration.
The internal intercostal
muscles, whose contraction
leads to expiration (N.B.
these are not in use during
normal breathing).
The pleural cavity
Each of the lungs is enclosed in a double membrane known
as the pleural membrane. The space between the two
membranes is called the pleural cavity, and is filled with a
small amount of pleural fluid.
lung
This fluid lubricates the
lungs. It also adheres to
the outer walls of the
lungs to the thoracic
(chest) cavity by water
cohesion, so that the
lungs expand with the
chest while breathing.
pleural membranes
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The mechanism of ventilation
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Ventilation
Ventilation
Task – 15 minutes
• Draw a table like the one below....
Inspiration
Expiration
External intercostal muscles
Internal intercostal muscles
Diaphragm
Air pressure in lungs
Air movement along
pressure gradient
Lung volume
• Fill in what happens to the things on the left (i.e
diaphragm, air pressure in lungs...) during inspiration and
expiration.
Answers, please!
Inspiration
External intercostal muscles
Internal intercostal muscles
Diaphragm
Air pressure in lungs
Air movement along
pressure gradient
Lung volume
Expiration
Answers, please!
Inspiration
Expiration
External intercostal muscles
Contract – pulling ribs
upwards and outwards.
Relax – permitting rib cage
to move downwards and
outwards.
Internal intercostal muscles
Are relaxed.
Contract – moving the ribs
downwards and decreasing
the volume of the thorax
(forced expiration only).
Diaphragm
Contracts – moves
downwards from domed
position.
Relaxes – elasticity returns it
to domed position.
Air pressure in lungs
Decreases
Increases
Air movement along
pressure gradient
Into lungs
Out of lungs
Lung volume
Increases
Decreases
Maintaining the structure of the alveoli
During inhalation, the chest cavity increases in volume,
lowering the pressure in the lungs to draw in fresh air.
This decrease in pressure leads to a tendency for the lungs
to collapse. Cartilage keeps the trachea and bronchi open,
but the alveoli lack this structural support.
Lung surfactant is a
phospholipid that coats the
surfaces of the lungs.
Without it, the watery lining
of the alveoli would create a
surface tension, which would
cause them to collapse.
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alveoli
surfactant
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Composition of inhaled/exhaled air
composition (%)
In one breathing cycle, the air in the lungs loses only some
of its oxygen content. This is why mouth-to-mouth
resuscitation can be effective.
90 78% 78%
80
70
60
50
40
30
20
10
0
N2
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inhaled air
exhaled air
21%
15%
0.04% 4%
O2
CO2
<1% 3%
<1% <1%
H 2O
other
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Spirometry
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Spirometric terms
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Describe the trace
Say what’s happening and why (what must the
person be doing at each time?).
But how do we measure tidal volume?
We can use a spirometer
Pulmonary Ventilation
• Pulmonary ventilation is the total volume of
air that is moved into the lungs during one
minute (dm3min-1). To calculate it we multiply
together two factors....
Pulmonary Ventilation
1. Tidal volume – the volume of air normal
taken at each breath when the body is at
rest. This is usually around 0.5dm3 (half a
litre).
2. Ventilation (breathing) rate – i.e the number
of breaths taken in one minute. This is
normally 12-20 in a healthy adult.
Pulmonary Ventilation
– to summarise
Pulmonary
ventilation
(dm3min-1)
= tidal volume x ventilation rate
(dm3)
(breaths min-1)