Freshwater organisms tend to gain water across membrane surfaces and must
actively get rid of it. Land and marine organisms tend to lose water;
they must retain water and excrete salt. Vertebrate kidneys filter the blood
first, then retrieve (resorb) useful molecules.
- Osmotic pressure measures the level
of dissolved ions in solution.
- Hypotonic solutions (low osmotic pressure, few dissolved ions):
Cells swell (or may burst) because water diffuses in. Freshwater organisms
always gain water from hypotonic surroundings; they void lots of dilute
urine and may actively take up some ions.
- Hypertonic solutions (high osmotic pressure, many ions):
water diffuses out; cells shrink. Marine and land animals lose water across
membranes; they excrete concentrated urine or salt-rich fluids.
- Isotonic solutions: Cells have the same concentration of dissolved ions.
Water enters and leaves at the same rate, so cells stay the same size.
- Simple excretory systems: Some freshwater protists pump water out
by contractile vacuoles. Many small aquatic animals allow wastes
to diffuse out. Flatworms have single-celled excretory tubules called
flame cells.
- Nephridial systems: Tubules (nephridia) drain coelomic
fluid from the body cavity and exchange ions with small blood vessels nearby.
- Vertebrate kidneys: Cortex (outer layer) contains mostly
glomeruli and convoluted tubules; medulla (inner layer) is made of
several medullary pyramids, which contain Henle's loops.
- Kidney tubules: Blood plasma is filtered from a series of
thin-walled blood vessels (the glomerulus) into Bowman's capsule.
In the proximal convoluted tubule, the blood resorbs glucose and
some ions. In mammals, Henle's loop resorbs water. In the distal
convoluted tubule, more ions return to the blood. Collecting tubules
finally concentrate the urine and drain into the renal pelvis, which
drains into the ureter.
- Nitrogen wastes: In mammals, the principal nitrogen waste is urea,
some organisms excrete ammonium salts or uric acid instead.
- Other organs of excretion: Lungs and gills get rid of CO2.
Animals excrete salt and nitrogen wastes through the skin.
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