Bio 205

Urinary System

Function

• excretion of nitrogenous wastes
• osmoregularion

Comparative development, morphology, and evolution

Structure of nephron

• renal corpuscle
  • glomerulus
  • renal (Bowman's) capsule
• tubules
  • proximal
  • intermediate
  • distal
• collecting tubule
• excretory duct

Development

• mesomere
• nephric ridge
• nephrotome
• nephrocoel
• peritoneal funnel
• nephric tubule
• (archi)nephric duct

Tripartitle organization

• pronephros
  • archinephric duct that grows posteriorly and empties into the cloaca
• mesonephros
  • gap between pro and meso
  • secondary and tertiary tubules develop so segmental organization is obscured
• opisthonephros
  • anamniotes - development of posterior tubules combined with mesonephros forms opisthonephros
• metanephros
  • metanephric duct (ureter): ureteric bud branches off of caudal archinephric duct and grows toward posterior nephric ridge, where it branches greatly and induces formation of renal tubules in caudal nephric ridge. The ureteric bud forms the collecting tubules and ureter. The mesonephros stops functioning as a kidney.

Phylogeny

• internal vs. external glomeruli
• holonephros (is it primitive?)
• pronephros
  • occurs in all vertebrates
  • coelomic funnels in anamniotes
  • not functional in elasmobranchs and amniotes, except that it does form the archinephric duct
  • hagfishes and some telosts retain the pronephros as adults where it is called the head kidney
• mesonephros
  • function embryologically in all vertebrates
• opisthonephros
  • all anamniotes but the variation is great. Often the anterior part ceases to function in its urinary role.
• metanephros
  • in reptiles, the ureters separate from the archinephric duct and independently enter the cloaca.
  • in mammals, the ureters enter the urinary bladder

Urinary bladder - functions to retain urine

fishes - swellings in archinephric ducts may be called urinary bladders or urogenital sinuses - function is unknown since urine is continuously discharged

amphibians - large bilobed urinary bladders that evaginate from the ventral part of the cloaca. Urine flows from dorsal to ventral part of cloaca into bladder. Some water is reabsorbed in bladder

amniotes - the cloacal evagination of amphibians occurs in amniotes but becomes extraembryonic and is known as the allantois, which is one of the extraembyonic membranes of the amniote. The extraembryonic part is lost but the intraembryonic part becomes the urinary bladder and the urethra. Ureters typically open into the dorsal wall of cloaca and must cross cloaca to get to bladder. In marsupials and eutherians, the ureters empty directly into the bladder.

Excretion and Osmoregulation

• Nitrogen from Amino acids and some nucleic acids is bound up in Ammonia, which is made in the liver, is toxic, and must be either converted to something less toxic or flushed from the body. Ammonia is soluble so can be flushed with high water turnover.
• Ammonia my be converted to urea which is less toxic, so does not need to be flushed out rapidly but still takes water to get rid of it.
• Ammonia may also be converted to uric acid, which has low solubility in water and forms a pasty goop that is non-toxic and takes very little water to get rid of it.

Freshwater fish

• tissues are hyperosmotic so water moves into body, which must be flushed out
• This is convenient for flushing Ammonia
• Most ammonia excreted at the gills
• while the kidney is essentially for osmoregulation
• urine continually discharded to maintain water balance

Marine teleosts

• tissues are hyposomotic so water moves out of body
• less water is moved through kidney so urine production is less than in FW teleosts
• marine teleosts drink water
• again, most ammonia excreted at gills

Marine elasmobranchs

• convert ammonia to urea
• some urea eliminated in urine which gets rid of excess nitrogen
• urea is also reabsorbed in kidney and cannot permeate gills, hence urea concentration in tissues is high
• high urea concentration makes tissues nearly isoosmotic with sea water (hence elasmobranchs are osmoconformers and not osmoregulaters).

amphibians

• larval and aquatic amphibians excrete ammonia through gills and have excess urine to get rid of water.
• terrestrial amphibians convert ammonia to urea in order to conserve water. Nevertheless alot of water is lost by the kidney and through the skin, hence most amphibians have to live in moist environments.

reptiles

• ammonia converted to uric acid, which requires little water for its excretion
• aquatic reptiles tend to have more dilute urine and can eliminate ammonia directly

mammals

convert ammonia to urea but have evolved a system that concentrates the urea in the urine so that most of the water can be reabsorbed.