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excess salt by transporting Cl- ions
across special "chloride cells" in the gills. Na+
ions follow. They already
lose water through osmosis, so they only excrete a
small amount as
urine.
b.
Freshwater
Freshwater fish have the opposite
problem. Water comes in continually
through
osmosisand is removed by
excreting large amouts of dilute urine. A
trout excretes a volume of
urine equivalent to its total blood volume every few
hours.
To
reduce the loss of solutes, they
reclaimmost of the
salt andglucose in the
kidney and take up
saltby active transport
acrosschloride
cellsin the
gills.
2.
Salt-excreting glands44.9
Marine birds and reptiles that
stay at sea for months have to drink sea water. They get rid
of the excess salt through salt-excreting
glandsin their heads. The
tubules of these glands are
lined with special transport cells that pump salt out of the
blood. Thefluid
in the glands flows opposite to the
bloodin the
surrounding
capillaries.
Q. Why
is this important?
- counter-current flow favors exchange, even though active
transport is involved.
3.
Metanephridia in
earthworms44.16
Although we think of earthworms
as terrestrial, they take up O2through
the skin and are covered by a
thin film of water. Because ysis
greater inside the worm than
outside, water is constantly coming in that the worm has to
deal with.
Like
many animals, the earthworm has a segmented body plan and
each segment contains a pair of metanephridia. These are
tubules that collect body fluids. As the fluid moves through
the tubules, the epithelium pumps essential ions across and
they are reabsorbed by the blood. Dilute urine containing
wastes is excreted through the nephrostome (up to 60% of the
body weight/day).
4.
Sweat glands
Your sweat
glandsprimarily function
in thermoregulation, but they are also
involved in the water
balance. When you sweat, water and dissolved solutes
move into the gland from the
blood. As the fluid moves down the duct, salt
is
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