Excretion

contractile vacuoles

• paramecium

protonephridia (flame cells)

• flatworms and rotifers

• powered by cilia (that flick like a flame)

• removal of waste/excess water through excretory pores

metanephridia

• annelids

• opens inside and outside body cavity

• multi-cellular (versus unicellular protonephridia)

• cilia move interstitial fluid

• capillaries reabsorb ions

antennal (green) glands

• crustaceans

• fluid filters in from the hemocoel

malpighian tubules

• insects

  • 1. potassium ions are pumped into tubules (active transport)

  • 2. water then diffuses into tubules due to ion gradient (osmosis)

  • 3. soluble wastes such as urate ions enter with water (diffusion)

  • 4. aqueous solution gets flushed towards the rectum

  • 5. in the rectum, the waste solution is acidified

  • 6. soluble urate ions are converted to insoluble uric acid and precipitate

  • 7. water is reabsorbed through epithelial tissue

coxal glands

• arachnids

• muscles promote fluid flow through dilation

kidneys (vertebrates)

• amphibians

  • kidneys function similarly to freshwater fish 

    • in aqueous environments, water diffuses in (saltwater fish hardly urinate to prevent dehydration)

    • kidney helps excrete excess water and nitrogenous wastes 

  • when on land, amphibians are able to use the bladder to aid water conservation: 

    • when in water, frog’s bladder quickly fills up with hypotonic urine

      • on land, this water is reabsorbed

• amniotes

  • nephrons (>million)

  • glomerular capsule

    • filtration unit

• reptiles and birds

  • many reptiles live in dry environments

    • they convert nitrogenous wastes into uric acid

      • only need a small amount of water

  • this allows them to forgo drinking water

  • mammals

    • all mammals use urea as their chief nitrogenous waste

      • requires much more water to be excreted than does uric acid

    • mammals produce large amounts of nephric filtrate but are able to reabsorb most of this in the tubules

      • humans use several hundred mL each day to flush urea

    • some mammals have more efficient kidneys than humans

      • the kangaroo rat can produce a urine 17 times more concentrated than its blood (the best we can do is 3-4 times as concentrated)

      • this allows them to survive without drinking water 

        • they simply depend on the water content of their food and that produced by cellular respiration

marine teleosts (bony fish) 

• water is naturally hyperosmotic

  • water tends to diffuse out of the fish

• strategies:

  • drink water to counteract the loss

    • gills actively pump out ions

  • reduce water loss

    • sending little water to the kidneys

freshwater teleosts

• water is hypoosmotic

  • water tends to enter (bloating)

  • ions tend to diffuse out

• water regulation:

  • minimize drinking; consume food only

  • coat body with mucus 

  • excrete lots of urine