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Ventilation
6
...
11 Before birth, the foetus
obtains oxygen from the mother’s
blood in the placenta and excretes
carbon dioxide by the same route
(k 12
...
3)
...
7
...
intrapleural pressure
754 mmHg
inspired air
Ventilation (breathing) is a regular and mainly automatic process in which
air is alternately inhaled into the lungs (inspiration) and then exhaled
(expiration)
...
With each breath:
• the ribcage enlarges and the diaphragm flattens, allowing the lungs to
expand, which lowers the pressure within the lungs and air rushes in to
fill up the space (k Fig
...
7)
• gaseous exchange takes place in the alveoli where oxygen moves from the
air into the blood and carbon dioxide moves in the opposite direction
• the ribcage reduces in size, the diaphragm relaxes, and air is forced out of
the lungs (k Box 6
...
alveolar (intrapulmonary)
pressure 758 mmHg
intrapleural pressure
756 mmHg
expired air
alveolar (intrapulmonary)
pressure 762 mmHg
atmospheric pressure
is approximately
760 mmHg at sea level
INSPIRATION
• diaphragm and external intercostal muscles contract
• ribs move upwards and outwards
• diaphragm flattens
• lung volume increases
• alveolar (intrapulmonary) pressure falls
• air flows into lungs
EXPIRATION
• diaphragm and external intercostal muscles relax
• ribs move in and down
• diaphragm becomes domed
• lung volume decreases
• alveolar (intrapulmonary) pressure increases
• air flows out of lungs
Fig
...
7 Inspiration and expiration
...
12 A number of factors
can affect breathing, e
...
exercise,
emotions, allergens and smoking
...
Although breathing usually involves the intercostal and abdominal muscles,
the extent to which each is used varies with individuals, with training, and
with the demands made upon the lungs at any particular time:
• thoracic breathing uses the intercostal muscles – the chest can be seen to
rise and fall
• abdominal breathing uses the diaphragm – as the diaphragm moves
upwards and downwards, the abdomen moves in and out
• eupnea (quiet breathing) is mainly abdominal, with most of the work
being done by the diaphragm muscle (k Box 6
...
6
...
1 Accessory muscles
Accessory muscles are those not normally used for breathing, but only when
there is a high demand for gaseous exchange, e
...
:
• during exercise, forceful breathing can be assisted by contraction of
abdominal muscles which push the abdominal organs upwards to help
reduce the volume of air in the thoracic cavity
• when there is dysfunction of the respiratory system such as an asthma
attack or in people with emphysema, the muscles in the neck (scalene
and sternocleidomastoid (k Fig
...
20) can be recruited to assist with
raising the ribcage to increase the flow of air into the lungs (k Box 6
...
6
...
2 Respiration rate
The respiration rate is the number of breaths taken in one minute
...
The
number of respirations increases when undertaking physical activity, due
to fever, or when feeling anxious or frightened
...
14)
...
5
...
13 Use of accessory muscles
during quiet breathing is usually
taken as a sign of respiratory
distress
...
14 Assessing the respiratory
rate (RR or ‘Resps’) and the depth,
rhythm and sounds of a patient’s
breathing is an essential skill for all
healthcare practitioners (k 17
...
1)
...
g
...
Lung function describes how well a person is breathing, which is affected by
gender, age and height
...
Lung function can be measured
using a spirometer – a device for measuring the air capacity of the lungs
(k Fig
...
8a)
...
A peak flow meter (k Fig
...
8b) is a device which
measures peak expiratory flow rate (PEFR) and is used in the management
of asthma and to monitor the effectiveness of treatment (k Fig
...
9)
...
6
...
Fig
...
9 Normal range of values of peak flow expiratory flow rate (PEFR) in adults
aged up to 85 years
...
15 People with a greater lung
capacity are likely to be male, tall,
physically fit, live at high altitudes,
not smoke, and not be obese
...
Box 6
...
6
...
4 Lung capacity
Lung capacity describes the volume of air that an individual can hold in
the lungs (k Fig
...
10)
...
15)
...
1)
...
It can be:
• anatomical dead space (conducting zone) – the area that includes
mouth, nose and all of the airways where air is present but no gaseous
exchange is taking place
• physiological dead space represents alveolar sacs where gaseous
exchange is reduced because:
• ventilation (air flow to the alveoli) is restricted, e
...
in pneumonia
where fluid is present in the air sacs, or in asthma when
bronchoconstriction and mucus obstruct the bronchioles, and
• perfusion (blood flow to the lungs) is restricted, e
...
when a clot blocks
the supply of blood (perfusion) of pulmonary capillaries in pulmonary
embolism (k Box 6
...
6000
millilitres (ml)
5000
inspiratory
reserve volume
3100 ml
4000
3000
VITAL CAPACITY
4800 ml
tidal volume 500 ml
TOTAL LUNG
CAPACITY 6000 ml
expiratory
reserve volume
1200 ml
2000
1000
residual volume
1200 ml
0
20 seconds
Fig
...
10 Lung volume and capacities in an adult
...
1 Average lung volumes of an 80 kg man
Description
Volume
Tidal volume (TV)
Amount of air that moves into and out of the lungs during normal, relaxed breathing
500 cm3
Inspiratory reserve volume (IRV)
Additional air that can be forcibly inhaled following inspiration
3100 cm3
Expiratory reserve volume (ERV)
Additional air that can be exhaled during a forced breath
1200 cm3
Residual volume (RV)
Amount of air still remaining in the lungs after the expiratory reserve volume is exhaled
1200 cm3
Vital capacity (VC)
Total amount of air that can be forcibly exhaled after fully inhaling
4800 cm3
Total lung capacity (TLC)
Maximum amount of air that can fill the lungs, calculated by adding vital capacity
(VC) and reserve volume (RV)
6000 cm3
Functional residual capacity (FRC)
Amount of air remaining in the lungs after a normal expiration
2400 cm3
Dead space
The volume of air in the bronchi and bronchioles which does not participate
in gas exchange
150 cm3