Outline for Excretion

1. Basic Mechanisms
   1. Ultrafiltration followed by active secretion and absorption to form final urine
   2. Active pumping of ions, metabolites, etc., to remove water
2. Organs of excretion
   1. Contractile Vacuoles (protozoans and sponges): active pumping (of Na in, K out) to form hypoosmotic vesicle.
   2. Nephridia (most invertebrates except insects): ultrafiltration
      1. Protonephridia: blind ended. Primarily acoelomates. Solenocytes and flame cells.
      2. Metanephridia: open ended. Coelomate invertebrates.
   3. Malpighian tubules: insects. Active pumping (of K)
      1. Special case: cryptonephridia (e.g., Tenebrio)
   4. Nephrons: (vertebrate kidneys): ultrafiltration.
3. Functioning of the Vertebrate Kidney
   1. Anatomy of the nephron: five divisions, each specialized in function.
      1. Glomerulus: Malpighian body (Bowman's capsule) and glomerular capillaries. Ultrafiltration and colloid osmotic pressure.
      2. Proximal Convoluted Tubule: primary site of water resorption by active resorption of salts and metabolites (esp. glucose). Also active secretion.
      3. Intermediate segment: no resorption. See mammalian kidneys below.
      4. Distal Convoluted Tubule: some passive water resorption due to active pumping (resorption) of ions (not K).
      5. Collecting Duct: in mammals and birds, primary regulator of urine osmolarity. Regulation by permeability of duct walls (ADH regulation) as duct descends through medullary osmotic gradient
4. Regulation of mammalian kidney
   1. ADH: collecting duct permeability
   2. Aldosterone: (renin-angiotensin system): Na resorption, mostly in distal convoluted tubule.
   3. Natriuretic Factor: (from atria): dilation of renal vasculature, inhibition of Na resorption.
5. Some specializations:
   1. Fish:
      1. Marine teleosts: aglomerular nephrons
      2. Elasmobranchs: urea resorption
   2. Birds and mammals: Intermediate segment becomes loop of Henle, specialized for countercurrent multiplication, allows hyperosmotic urine.
6. Nitrogen Excretion
   1. Source of nitrogen for elimination: protein and nucleic acid metabolism
   2. Nitrogenous waste products
      1. Ammonia: (ammonotelic animals): very soluble, but toxic. Mostly aquatic organisms.
      2. Urea: (ureotelic animals): fairly soluble, less toxic.
      3. Uric acid: (uricotelic animals): quite insoluble. Good for animals on tight water budgets.
      4. Other nitrogenous compounds: creatinine, guanine, allantoin
   3. Biochemistry:
      1. Urea: ornithine cycle
      2. Uric acid: from adenine and guanine, or amino nitrogen. Breakdown to allantoin, allantoic acid, urea, or ammonia, especially in invertebrates.