1. Invertebrates Excretory Systems
Protonephridia
Metanephridia
Malpighian Tubules

2. 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

4. 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

6. Osmoregulation in insects
Osmoregulatory system of insects
The main organs involved in solute and water balance are:
Malpighian tubules (MTs)
Form primary urine
Lower MTs and hindgut (ileum, colon, rectum)
Reabsorption of water and ions

7. 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

8. Osmoregulation in insects
Malpighian tubules
MTs empty into the alimentary canal between the midgut and hindgut
The number of MTs varies from depending on the species
2-100 mm in length and 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

9. Osmoregulatory system of insects
(Eckert, Fig )

11. Osmoregulation in insects
Malpighian tubules
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 blood-feeders
NaCl and KCl are returned to the hemolymph across the rectal wall

12. Osmoregulatory system of an insect
Na+, K+, Cl-
-reabsorption of
water and ions
Hyperosmotic
or isosmotic
urine/excreta
K+, Cl-
Na+, K+, Cl-
& water
-formation of
primary urine
(Eckert, Fig )

13. 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

14. Ramsay Assay for Measuring Fluid Secretion
Liquid paraffin
(or synthetic peptides,
neurotransmitters)

15. Osmoregulation in insects
Hormonal control of fluid secretion in Rhodnius prolixus
Types of DHs in Rhodnius :
Serotonin (5-hydroxytryptamine, 5-HT)
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
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

16. Central nervous system of Rhodnius
-contain CRF-like
peptides
subesophageal ganglion
prothoracic ganglion
Abdominal nerves
mesothoracic
ganglionic mass
-source of CRF&
other unidentified
diuretic peptides
Posterior lateral
neurosecretory cells

17. 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

18. Unfed Rhodnius prolixus
Blood-fed Rhodnius prolixus

19. 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 5-HT act synergistically to promote diuresis

20. 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

26. II. Osmoregulation in aquatic environments
Marine mammals
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

27. Water-salt relations in a marine mammal
-obtain water from food and metabolism
-conserves water by producing a hypertonic urine