Excretory Systems

Kidneys and their ducts

• Homeostatic hormones– Renin* – aids in controlling blood pressure– Erythropoietin* –

• stimulates production of RBCs• Regulates O2 carrying capacity of blood

– Vitamin D** – regulates Calcium balance

• Ridding of Nitrogenous wastes– Metabolism

• Osmoregulation– Balance between solutes (salts) and water– Fresh water systems versus salt water systems– In what sort of environment did the craniate kidney evolve?

* synthesized in kidney** activated in kidney

• Fresh water:– Kidney – enormous

capacity to filter water– Perceived fossil record

• Salt water– Kidney – enormous

capacity to reabsorb water (weaker argument)

– Updated fossil record• Old thoughts – deposits

– fresh HOH• New thoughts – salt

– Hagfish are marinegut

anus

Malpigian tubuleshemolymph

HOH, solutes, wastes

wastesHOH, solutes

Regardless generally conservative with highly adaptable segments

Basic nephros“Archinephros”

Renal corpuscleDevelopment of corpuscle

conservative

variable

Above line: dorsal body wall retroperitonealBelow line:• No tube, celomic cavity – “external glomerulus”• Tube with connection to coelom and collecting duct – “internal glomerulus, open

nephrostome”• Tube with no connection to coelom but to collecting duct – “i.g., closed nephrostome”

Additional Apparatus

• Regulating Anatomy:– Convoluted tubules – ciliated – move filtrate – Loop of henle – solute regulation– Size of glomerulus – hi = lots of HOH in filtrate– Collecting ducts – long = lots of absorption (with

proper hormones)– Podocytes (tiny filtering structures)

– Gills– Drinking– Mucus membranes on skin

Comparative anatomy of nephrons and their blood supply.

Environmental challenges and solutions

• Freshwater– Excessive water uptake (must excrete)– Soln: Waste excreted as cheap ammonia

• Problem: Toxic• Soln: Water used as solvent

– Additonal cost – must actively transport solutes for retention

– Examples:• Freshwater teleosts, aq. amphibians

• Salt Water– Prob: Excessive water loss (must conserve)– Prob: Excessive salt uptake (must excrete)

– Soln 1: Become isosmotic (Hagfish, marine teleosts)• Lose glomerulus decrease water loss

– Soln 2: Become hyperosmotic (some elasmos)• Retain urea – somewhat costly, minimal toxicity (soluable in water)• Increase HOH uptake• Cost – retain glomerulus (increased metabolism) – excrete water

– Soln 3: Develop special salt excretion glands (some elasmos, teleosts)• Rectal gland• Salt glands on gills

• Terrestrial– Prob: Dry environment

• Water and salts rare must retain both– Soln?: Return (secondarily) to water

• Prob: reintroduction to above problems

– Soln: N excreted in three forms• NH4, Urea, Uric Acid• Balance the costs-benefits

– Soln: Reduce glomerulus (arid reptiles, anurans)– Soln: Solute recovery (develop loop of henle) (birds, mamms) – Soln: Salt glands (marine taxa)

NH4 – cheap but toxic, hi HOHUrea – Int cost – low toxicityU. Acid – hi cost – not toxic, lo HOH