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Gas Exchange in Humans
Exchange System
The gas exchange system in humans and other mammals consists essentially of a
conducting system for the conduction of inspired and expired gases, and an interface
for the exchange of gases between air and blood
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The lungs are situated inside a protective bony case, the thorax
or the rib cage, the walls of which are formed by the ribs and intercostal muscles,
and the floor by a muscular sheet, the diaphragm, which separates the thorax from
the abdomen
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The inner visceral membrane is in contact with the lungs and the outer
parietal membrane lines the walls of the thorax and diaphragm
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The conducting system begins with the nasal passages and continues as the wind
pipe or trachea
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The walls of the nasal passage are lined with
ciliated epithelium that also contains goblet cells secreting
mucus
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It also moistens the
incoming air which also gets warmed by the superficial
blood vessels
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Here, odours in the air are detected
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Air from the larynx enters the
trachea that lies directly in front of the oesophagus
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The open section of the C is applied against the oesophagus
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The trachea is lined internally with pseudostratified ciliated
columnar epithelium
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The trachea divides to form the left and right primary or main bronchi
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The left primary bronchus divides into two secondary bronchi that
enter the two lobes of the left lung
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The tertiary bronchi branch into
numerous bronchioles
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These branch into many short tubes of equal diameter called
alveolar ducts, which end in tiny hollow bags called air sacs
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An alveolus is the functional unit
of the lung and forms the gas exchange surface of a human
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Pulmonary Ventilation
Two sets of muscles are mainly involved in pulmonary ventilation
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The external intercostal
muscles slant forwards and downwards while the internal intercostal muscles slant
backwards and downwards across the space between each pair of neighbouring ribs
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During inspiration, the external intercostal muscles contract and the antagonistic
internal intercostal muscles relax
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Simultaneously, the muscles in the
diaphragm contract and cause the diaphragm to flatten, increasing the length of the
thoracic cavity
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4 kPa below atmospheric pressure during quiet breathing
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Inspiration is an active process
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During expiration, these fibres recoil to their relaxed length, causing the
lungs to deflate
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This causes the ribs to move downwards and
inwards, decreasing the diameter of the thorax
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As the overall volume of the thorax decreases, the
intrapulmonary pressure rises to about 0
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Thus, a pressure
gradient is established and air from the lungs is forced out, deflating the alveoli until
the intrapulmonary pressure is equal to the atmospheric pressure outside
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However, during forced
expiration and inspiration, many other muscles are used, including the abdominal
muscles, which contract during forced expiration and cause more active upward
movement of the diaphragm
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Each alveolus consists of three tissue
components : epithelium, connective tissue and blood vessels
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Type I
pneumocytes are large, extremely flattened cells, with the central nucleus forming a
bump in the surface and adjacent cells joined by strands of cytoplasm
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Type II pneumocytes secrete a
detergent like lipoprotein called a surfactant, which helps to reduce the surface
tension of the alveoli and hence the effort needed to breathe
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Without this surfactant, the alveoli collapse and lung
tissue loses its elastic recoil, making it very difficult to inflate the lungs, resulting in
respiratory distress syndrome
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These contribute to the elasticity of the lung
tissue
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The structure of the alveoli, with the surrounding capillaries is well adapted for the
purpose of gas exchange
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As such, it offers
minimum resistance to diffusion of gases and thus increases the efficiency of the
diffusion process
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These provide a large surface area over which gaseous
exchange can occur, thus increasing the rate of diffusion
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iv) Each alveolus is surrounded by a dense network of blood capillaries
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This results in a larger surface area of the erythrocytes being
exposed for exchange of gases
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v) The oxygen absorbed into the blood is immediately combined with haemoglobin
(Hb) of the erythrocytes to form oxyhaemoglobin (HbO2), removing oxygen from
solution and thus allowing for more oxygen to be absorbed
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Gas Exchange
The capillaries that surround the alveoli originate from the pulmonary artery and
finally drain into the pulmonary vein
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Thus, there is a concentration gradient of these two gases, in opposite directions,
between the alveolar air and the blood
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hydrogencarbonate

Carbon

ions

dioxide

(HCO3-)
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Carbon dioxide then
diffuses across the capillary wall, epithelial lining of the alveoli and ultimately
accommodates in the alveolar air from where it is exhaled
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Control of Breathing
Breathing occurs due the rhythmic activity of the motor nerves which send impulses
to the respiratory muscles
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The basic regular rhythm of breathing is under the influence of the medullary
rhythmicity centre, situated in the medulla oblongata
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Nerve
impulses from the inspiratory centre stimulate inspiration and impulses from the
expiratory centre inhibit inspiration
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The breathing centre responds to both chemical and non-chemical stimuli
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3 kPa causes reflex
stimulation of the inspiratory centre to increase the rate of breathing so that
more oxygen can be absorbed into the blood
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ii) CARBON DIOXIDE : The normal range of arterial carbon dioxide is about 5
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3 kPa
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The
inspiratory centre then sends impulses via the phrenic and thoracic nerves to the
diaphragm and to the intercostal muscles causing them to increase the rate at
which they contract
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A fall
in the level of carbon dioxide in blood below normal results in the inhibition of the
medullary rhythmicity centre and slower breathing
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iv) NON-CHEMICAL STIMULI : As the alveoli inflate during inspiration, stretch
receptors in the alveoli inflate during inspiration, stretch receptors in the alveoli
and in the bronchial tree send inhibitory impulses to the inspiratory centre via the
vagus nerve to the ventilation centre
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After
expiration has taken place, the alveoli are no longer stretched and the stretch
receptors no longer stimulated
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This cycle is repeated rhythmically
throughout life
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VOLUNTARY CONTROL : Within limits, the rate and depth of breathing are under
voluntary control
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The voluntary system is used to regulate
breathing during activities such as speaking or playing a wind instrument
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This increase in ventilation is brought about by the following factors
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Stretch receptors in the exercising muscles and tendons send impulses to the
medullary rhythmicity centre to increase the tidal volume and the rate of
breathing
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The excess of carbon dioxide in blood is detected by chemoreceptors
that send impulses to the ventilation centre to increase the tidal volume and the
rate of breathing
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The fall in pH stimulates inspiration
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At the end
of the exercise, breathing remains much above the resting value for a prolonged
period, to bring back the level of oxygen and carbon dioxide to normal and to oxidize
the lactic acid
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Blood supply to muscles is increased and the cardiac output is also increased
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As fats have a much larger energy value, more fats than carbohydrates are
broken down for respiration
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Lung Capacities
A spirometer is a device which is used to measure and record the volumes of air
inspired and expired
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The volume of air breathed in or out during quiet breathing is known as the tidal
volume (TV)
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The maximum volume of air that can be forcibly expired after a tidal expiration is
known as the expiratory reserve volume (ERV)
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The amount of air which always remains in the lungs and cannot be removed is
known as the residual volume (RV)
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This also allows gas exchange to
occur continually, even when inspiration is not occurring
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The total volume of air that can be accommodated into the lungs when they are fully
inflated, that is, the sum of inspiratory reserve volume, tidal volume, expiratory
reserve volume and residual volume, is known as the total lung capacity
(=IRV+TV+ERV+RV)
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Pulmonary
ventilation is the product of respiratory rate and tidal volume
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This air remains in the trachea
and bronchial tubes, collectively known as the dead space
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AV = respiratory rate × (TV – dead space)

Gas Exchange in Amoeba
In Amoeba, the surface area:volume ratio is large, so gaseous exchange can occur
rapidly by diffusion
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As the Amoeba grows is size, its surface area:volume ratio falls
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About Surface Area and Volume
Large multicellular organisms have a low surface area:volume ratio and exchange
of gases through the body surface may not be enough to meet the metabolic needs
of the organisms
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Increase in size also results in an increase in the distance of respiring cells from the
respiratory surface, slowing the rate of diffusion
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A ventilation mechanism brings fresh supplies of air or water in
contact with the respiratory surface and maintains a high concentration of oxygen
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The oxygen carrying capacity of the blood is
increased by the presence of a respiratory pigment, such as haemoglobin, present in
specialised blood cells – the erythrocytes
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Manarat Dhaka International College
Dhaka, Bangladesh

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