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Gaseous Exchange
This is the exchange of respiratory gases between cells and the environment
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The area where gaseous exchange takes place is called respiratory
surface
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Organisms which have a large SA:V gaseous
exchange occurs by simple diffusion
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Fick’s Law:
Rate of diffusion α SA x difference of concentration on either side
Thickness of respiratory surface
Respiratory surface:
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Permeable and thin

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Large SA

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Good blood supply (steep diffusion gradient)

Respiratory pigments: increase the efficiency of the blood’s oxygen carrying capacity
since it permits for greater amount of O2 to be taken up and transported
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g
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Every cell is surrounded by an extracellular
fluid, therefore the slow diffusion of oxygen in water affects air-breathing organisms,
and therefore every cell in the body is close to a blood vessel
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Oxygen availability and altitude
The amount of O2 in the atmosphere decreases with altitude
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Gasesous exchange in insects
A system of tubes – tracheal system, this allows O2 to diffuse from the outside air
directly to the tissues without the need for transportation by blood
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They
also have hairs which prevent foreign body from entering and water loss
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It branches into tracheoles
among the tissues (no chitin)
At rest tracheoles are filled with fluid – low diffusion
During excercise, increases metabolic rate and lactic acid and makes Ψ negative
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The size of the aperture is adjusted according to the level of CO2 in the body
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Ventilation movements are also triggered by high CO2 in large insects
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the tissue between the slits – bronchial arches / gill arches
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Each lamella has a rich supply of blood capillaries
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Afferent blood vessels – blood to gills
Efferent blood vessels – blood away from gills
Operculum – protects the gills
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It controls the movement of water in and out of the opercular
cavity like a valve
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Buccal cavity expands, pressure decreases, water drawn into the mouth
2
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During expiration:
1
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Increase in pressure forces open the posterior end of
the operculum
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Mammalian Lungs
Found in the thoracic cavity with 12 ribs – rib cage and intercostals muscles
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The space between the
membranes is called the pleural cavity which is filled with fluid, this lubricates the
membranes
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Mucus escalator:
Particles are trapped and the beating of cilia sends it back to the buccal cavity
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Cystic fibrosis – disease affecting mucus: thick
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Their volume is anatomical dead space
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They have a dense
network of blood capillaries
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Special cells in their walls secrete surfactant (Detergent)
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Lowers the surface tension

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Speeds up transport

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Kills bacteria

Gaseous exchange at the alveolus
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By diffusion:
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Large SA

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Short distance

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Steep diffusion gradient

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Surfactant

Capillary are very thin:
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Diffusion distance is small

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Exposes more SA of RBC

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Increases the time available for gas exchange

Mechanism of ventilation
Air is passed in and out by the intercostals and diaphragm muscles
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Inspiration:
1
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Diaphragm contracts (flattens), volume of thorax increases and the pressure
decreases and becomes less than the atmospheric pressure
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External relax and internal contract (rib cage drops)
2
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The pressure
increases and becomes more than the atmospheric pressure
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It consists of a chamber filled with oxygen, suspended in water, connected to
the subject
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A nose clip is worn to have a
closed system
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The result is called the kymograph
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Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve
volume
Residual volume: keeps the alveoli full of air so they do not collapse, this is the
remaining air that cannot be forced out
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Blood transport of respiratory gases
Haemoglobin transports oxygen around the body
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An
iron atom is attached to each haem group and each of these can combine with one
molecule of oxygen
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This occurs when oxygen
concentration is high, when the concentration is low, the bonds holding O2 become
unstable and O2 is released
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Dissociation Curves
The amount of O2 that can combine with haemoglobin is determined by the oxygen
concentration of partial pressure (amount of pressure exerted on the walls of the
container by a mixture of gases)
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It provides a reserve of oxygen for
muscles for times when metabolic demands are high and blood flow is interrupted
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Foetal haemoglobin has a higher affinity for oxygen than normal haemoglobin
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The Bohr Effect
In regions with an increased partial pressure of CO2, the curve shifts to the right
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The increase in CO2, causes the haemoglobin to release its
O2
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In solution: transported in physical solution

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2
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3
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Some of the carbonic acid dissociates into hydrogen and hydrogencarbonate
ions
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This forms
haemoglobinic acid, acting as a buffer, it enables large quantities of CO2 to
be carried to the lungs without any major change in blood pH
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This is
balanced by Cl- diffusing into the RBC
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Carbon monoxide and haemoglobin
Haemoglobin combines with carbon monoxide to forms carboxyhaemoglobin
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Control of the rate and depth of breathing
Breathing is under involuntary control, this is carried out by a breathing centre in the
medulla oblongata
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The breathing centre communicates to the intercostals muscles by the intercostals
nerves and the diaphragm by the phrenic nerves
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During inspiration, the inspiratory centre sends nerve impulses for the intercostals
muscles to contract and to the diaphragm
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This inhibits inspiration
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Expiratory centre becomes
inactive since the stretch receptors are no longer stimulated, and inspiration occurs
once again
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When
CO2 conc is high, chemoreceptors in the carotid arteries and aorta are stimulated
and send nerve impulses to the inspiratory centre
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Gaseous exchange in flowering plants
Plants have a lower metabolic rate and so they need less energy and so they rely on
diffusion for gas exchange
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Leaves are thin and have a large SA, inside the leaf there is a spongy mesophyll with
large air spaces which allow efficient diffusion
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These
enable oxygen to reach the intercellular spaces of the interior tissues and CO2 to be
released to the atmosphere
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