3.1 Gas Exchange

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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
necessary.
Factors that affect the need for exchange surfaces:
Size ­ small organisms are supplied with nutrients and oxygen by diffusion however
multicellular organisms have a longer diffusion pathway through multiple layers of cells
and diffusion is too slow to enable a sufficient supply to the innermost cells
Surface area to volume ratio ­ surface area has to be large enough, in relation to the
volume, to provide all cells with oxygen and nutrients
2 3
Radius (r) (mm) Surface area (SA = 4r ) Volume (V = 4/3r ) SA:V
1 12.568 4.189 3:1
2 50.272 33.515 1.5:1
5 314.200 523.667 0.6:1
Level of activity metabolic activity uses energy from food and oxygen in aerobic
respiration, cells in a more active organism need more oxygen to supply the energy for
movement and in animals that need to keep themselves warm
Features of good exchange surfaces
Large surface area for molecules to pass through which is often achieved by folding
Thin barrier to diffusion which reduces the diffusion distance
Good blood supply to bring fresh supplies of molecules to the supply side and keep the
concentration gradient high

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Ventilation
A: Inspiration
Diaphragm & external intercostal muscles contract…read more

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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) due to push from abdominal organs
Volume of chest cavity decreases
Pressure in chest cavity increases to above atmospheric pressure to763 mmHg
Air forced out of lungs into atmosphere
Aided by elastic recoil and abdominal…read more

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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
Airways
Include the trachea, bronchi and bronchioles
Must be large enough to allow sufficient air flow
Must be supported (prevents collapse when air
pressure is low for inspiration)
Must be flexible to allow movement
Are lined with by ciliated epithelium to keep the
contribute to…read more

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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 reaching the alveoli
Elastic fibres
Reverses the effect of the smooth muscle
When the smooth muscle constricts it deforms the elastic fibres.…read more

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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 ­ the greatest volume of air that can be expelled from the lungs after taking the
deepest possible breath
Spirometry is useful:
to evaluate respiratory impairment
to aid diagnosis by identification of particular disease
to…read more

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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.…read more

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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
water.…read more

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Ventilation
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.…read more

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