Invertebrates Excretory Systems
• Protonephridia
• Metanephridia
• Malpighian Tubules
Protonephridia
• freshwater flatworms
• network of blind-ended tubes opening only
to the exterior
• tubes branch through the body, ending in
flame bulbs
• tuft of cilia that beat, forcing fluids through tubes
• urine empties via a nephridiopore
Metanephridia
• most annelids
• each segment contains a pair of metanephridia
– tubules bathed in coelomic fluid and encircled by
capillaries
– nephrostome collects fluid from coelom ( ultra
filtration) in the first filterate is isosomotic
– transport epithelia in lumen of tubules resorb and
secrete molecules
– urine exits nephridiopore
Osmoregulation in insects
 Osmoregulatory system of insects
 The main organs involved in solute and
water balance are:
1) Malpighian tubules (MTs)
• Form primary urine
2) Lower MTs and hindgut (ileum, colon, rectum)
• Reabsorption of water and ions
Malpighian Tubules
• insects and other terrestrial arthropods
• remove wastes from hemolymph and
osmoregulate
• open in digestive tract, tips immersed in
hemolymph
• transport epithelia line tubules
– solutes are secreted into tubules and some are
reabsorbed by the rectum
– causes the precipitation of uric acid
Osmoregulation in insects
 Malpighian tubules
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•
•
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•
MTs empty into the alimentary canal between the
midgut and hindgut
The number of MTs varies from 4-200 depending on
the species
2-100 mm in length and 30-100 μm in diameter
Walls of the MTs consist of a single layer of epithelial
cells
Process ECF at high rates to regulate composition and
volume of ECF
MTs are not innervated and fluid secretion is
controlled by the action of hormones
Osmoregulatory system of insects
(Eckert, Fig. 14-42)
Osmoregulation in insects

Malpighian tubules
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MTs lie free in hemocoel and are not supplied with blood
vessels
Insect circulatory system is at relatively low pressure,
therefore urine is formed entirely by secretion
NaCl and KCl are transported from the hemolymph into the
lumen of the MT
MTs secrete K+ in herbivorous insects and Na+ in bloodfeeders
NaCl and KCl are returned to the hemolymph across the
rectal wall
Osmoregulatory system of an insect
Na+, K+, Cl-
-reabsorption of
water and ions
Hyperosmotic
or isosmotic
urine/excreta
K+, Cl-
(Eckert, Fig. 14-42)
Na+, K+, Cl& water
-formation of
primary urine
Osmoregulation in insects
Hormonal control of fluid secretion
 Diuretic hormones (DHs)
• Substances that increase tubule secretion
and/or inhibits fluid reabsorption in the
hindgut
 Antidiuretic hormones (ADHs)
• Substances that inhibit tubule secretion
and/or promotes reabsorption of ions and
water in the hindgut
Ramsay Assay for Measuring Fluid Secretion
(or synthetic peptides,
neurotransmitters)
Liquid paraffin
Osmoregulation in insects
 Hormonal control of fluid secretion
in Rhodnius prolixus

Types of DHs in Rhodnius :
i.
ii.
Serotonin (5-hydroxytryptamine, 5-HT)
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•
•
Also a cuticular plasticizing factor
Signals through cAMP pathway
Widely distributed in the nervous system and released
from abdominal nerves into the hemolymph after
feeding
Corticotropin-releasing factor (CRF)-like peptides
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At least 15 different CRF-like peptides identified
30-47 aa residues
Signal through a cAMP pathway
Present in the brain and mesothoracic ganglionic mass
(MTGM) and released from abdominal nerves into the
hemolymph after feeding
Central nervous system of Rhodnius
-contain CRF-like
peptides
subesophageal ganglion
prothoracic ganglion
Abdominal nerves
Posterior lateral
neurosecretory cells
mesothoracic
ganglionic mass
-source of CRF&
other unidentified
diuretic peptides
Osmoregulation in insects
Hormonal control of fluid secretion in
Rhodnius prolixus
 Rhodnius consumes >10 times its body weight during a
single blood meal
 The excess fluid gained after feeding severely
restricts mobility, therefore excess fluid load (salt
and water) must be voided rapidly
 Minutes after a blood meal, the MTs increase fluid
secretion 1000-fold
 Rapid elimination of Na+ and water requires
coordinated synergistic action of diuretic hormones
Unfed Rhodnius prolixus
Blood-fed Rhodnius prolixus
Osmoregulation in insects
Hormonal control of fluid secretion in
Rhodnius prolixus
• H+-ATPase on the apical membrane creates EC
gradient
• H+ is returned to the cytoplasm in exchange for
either Na+ or K+
• Na+-K+-2Cl- cotransporter on basolateral side
• Cl- diffuses out on the apical side, some K+ recycled
on the basolateral side
• Extracts of MTGM (CRF +other peptide DHs) and 5HT act synergistically to promote diuresis
Osmoregulation in insects
Hormonal control of fluid secretion in
Rhodnius prolixus
 Cessation of urine production must also be
tightly controlled to avoid dehydration and
excessive loss of NaCl
 Cardioaccelatory peptide 2b (CAP2b) functions
as an antidiuretic hormone
 CAP2b activates a cGMP second messenger
pathway to increase a cAMP phosphodiesterase
thereby inhibiting cAMP-mediated diuresis
II. Osmoregulation in aquatic
environments
Marine mammals
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Do not have salt glands and do not drink seawater
Obtain water from food and metabolism
Highly efficient kidneys produce a hypertonic urine
Nursing females produce milk with high fat but low
water content
 Some juvenile animals can use water derived from the
oxidation of body fat
 Modifications in nasal passages to reduce water loss
 Ability to lower metabolic rate
Water-salt relations in a marine mammal
-obtain water from food and metabolism
-conserves water by producing a hypertonic urine