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Thursday, 3 January 2019

Gaseous Exchange, Digestion and Absorption

Size and Surface Area
Cells need to take in oxygen and nutrients, as well as excrete waste products such
as carbon dioxide and urea
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Single-celled organisms have a large surface area : volume ratio
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Smaller animals have higher surface area : volume ratios, therefore they lose a large
amount of heat relative to their mass
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Larger animals have lower surface area : volume ratios
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Gaseous Exchange In Humans
As you breathe in, air enters the trachea, which is supported by incomplete rings of
cartilage that prevent it collapsing during the pressure changes which occur in
ventilation
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Each bronchus then divides into smaller tubes called bronchioles, which
end in air sacs called alveoli
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The alveoli are adapted as a gaseous exchange surface as:

• their shape and number provide a large surface area

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Thursday, 3 January 2019
• they have a short diffusion pathway by being only two cell layers


- a single layer of flattened epithelial cells (alveolar wall)

- a single layer of endothelial cells (capillary wall)

An extensive network of blood capillaries surrounds each alveolus, providing a large
surface area for absorbing oxygen and releasing carbon dioxide
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It then combines with haemoglobin in the red
blood cells to form oxyhemoglobin
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Blood flow ensures deoxygenated blood continually reaches the alveoli so that
oxygenated blood is removed, maintaining a high diffusion gradient for gaseous
exchange
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Ventilation Mechanism
A ventilation mechanism maintains a high diffusion gradient by continually
providing air with a high oxygen concentration and removing air with a high carbon
dioxide concentration
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This cause the rib cage to move upwards and outwards, and the diaphragm to
flatten, which increases the volume of the thoracic cavity

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Thursday, 3 January 2019
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This cause the rib cage to move downwards and inwards, and the diaphragm to
move up towards its dome shape, which decreases the volume of the thoracic
cavity

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Elastic recoil of the lung tissue helps to force air out of the lungs during
expiration


Pulmonary Ventilation
Pulmonary ventilation is the

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