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• 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

(Eckert, Fig. 14-42)

Osmoregulatory system of insects

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

Osmoregulatory system of an insect

(Eckert, Fig. 14-42)

-reabsorption of water and ions

Hyperosmoticor isosmotic urine/excreta

-formation of primary urine

Na+, K+, Cl-

K+, Cl- Na+, K+, Cl-

& water

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

Liquid paraffin

(or synthetic peptides,neurotransmitters)

Osmoregulation in insects

Hormonal control of fluid secretion in Rhodnius prolixus

Types of DHs in Rhodnius :i. 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

ii. 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

subesophageal ganglion

prothoracic ganglion

Abdominal nerves

mesothoracicganglionic mass-source of CRF& other unidentified diuretic peptides

Posterior lateral neurosecretory cells

Central nervous system of Rhodnius

-contain CRF-like 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

5-HT 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 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