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Properties of exchange surfaces
· A large surface area, the larger the area across which a
substance can diffuse, the more substance can cross the
surface in a given time.
· The exchange surface should be thin, the shorter the
distance across which a substance has to diffuse, the less
time it takes.
· There must be a DIFFUSION GRADIENT, the concentration
of the substance on one side of the surface must be different
from the concentration on the other side so the substance
can diffuse down the gradient.
· Must be protected from drying out, if `wet' cells are
exposed to dry air, water vapour will diffuse out of them and
into the air. If too much water is lost the plasma membrane
will lose its structure.…read more

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The mammalian gas exchange system
Pleural membranes which
secrete pleural fluid The pleural fluid
Thorax provides an airtight,
slippery covering
that allows the
Lungs lungs to inflate
easily.…read more

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AIR
TRACHEA
Bronchus Bronchus
Bronchioles Bronchioles
Capillaries…read more

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The lungs
· The lungs have no muscles, so movement is made by muscles in
the diaphragm and by the muscles between the ribs, which are
the intercostal muscles.
· The cells that line the path down to the lungs are adapted to
remove particles from the air.
· There are two main types of cells that make up the epithelial
tissue that lines the trachea and bronchi. These are called ciliated
cells and goblet cells.
· All of these cells sit on a basement membrane. This contains fibres
made from proteins the cells underneath them have secreted.
· There are many cilium on each ciliated cell. Each cilium contains
microtubules which help it to move.
· Goblet cells produce mucus. Mucus contains glycoproteins. These
have long sugar molecules attached to them. This makes the
mucus sticky. Mucus lines the epithelium and traps bacteria as
well as stops the cells from drying out.…read more

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Other tissues in the airway
· Cartilage- a tough tissue that helps support the walls of the
trachea and bronchi. It is very strong and flexible. The rings
of cartilage in the walls of the trachea help to keep it open.
In the trachea it is arranged in c shape rings, in the bronchi
it is less regular.
· Smooth muscle- found in the walls of the trachea, bronchi
and bronchioles. It contracts slowly and steadily and can
remain contracted for a long period of time. It is an
involuntary muscle, so we have no conscious control over
it. When it contracts it narrows the airway.
· Elastic fibres- found in the walls of all of the airways, even
in the smallest ones. During breathing in, the alveoli
expand, stretching the elastic fibres. During breathing out
the fibres recoil, helping to decrease the volume inside the
lungs forcing air out.…read more

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