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3.1 Exchange Surfaces

In small organisms, exchange takes place over the surface of the body as they do not require a
specialised exchange system.
In larger organisms the body's surface is no longer sufficient so specialised exchange surfaces are

Factors that affect the need for exchange surfaces:
Size ­…

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A: Inspiration
Diaphragm & external intercostal muscles contract

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Rib cage raised (upwards and outwards)
Diaphragm lowered (becomes flatter)
Volume of chest cavity increases
Pressure in chest cavity drops to below atmospheric pressure to 758 mmHg
Air moves into lungs from atmosphere
Active process
B: Expiration
Diaphragm & external intercostal muscles relax
Rib cage lowered
Diaphragm raised (dome shape)…

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Moist exchange surface ­ allows gases to dissolve
Water in the alveoli contains a surfactant (phospholipid) ­ reduces surface tension ­
prevents collapse of alveoli
Alveoli contain phagocytic cells for defence against airborne pathogens
Tissues in the gas exchange system

Include the trachea, bronchi and bronchioles
Must be large…

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Wall is one cell thick
100300m diameter
Good blood supply

Ciliated Epithelial
Simple columnar epithelial cells
Fine hairlike outgrowths
Rapid, rhythmic, wavelike beatings
Movement of mucus
Usually found in the air passages like the nose, uterus and fallopian tubes

Smooth muscle
Can contract to restrict airway
Prevents harmful substances from…

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Oxygen uptake ­ the volume of oxygen absorbed by the lungs in one minute
Tidal volume ­ the volume of air inhaled or exhaled in one breath usually measured at rest

Spirometer ­ a device that can measure the movement of air into and out of the lungs

Vital capacity…

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Equipment should be checked for faults
Constant temperature must be maintained so the gas volume stays the same

Volume Definition
Tidal Volume (TV) Volume of air moved into and out of lungs with each breath at rest.
0.5 L Provides body with enough O 2 for its resting needs while…

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5. The blood flows through the capillaries in the opposite direction to the flow of water
which creates a countercurrent flow that absorbs the maximum amount of O2 from the


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a. Water is kept flowing over the gills using a
buccalopercular pump
b. The buccal cavity changes volume by moving
the floor of the mouth up and down
c. As water is pushed from the buccal cavity, the
operculum moves outwards
d. This movement reduces pressure of the


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