Excretion

• Excretion


  To maintain a constant internal environment, any material an organism takes in must be balanced by an equal amount removed
• Major functions of excretory organs


  • Maintenance of proper concentration of solutes
  • Maintenance of proper body volume (water content)
  • Removal of metabolic end products
  • Removal of foreign substances or their metabolic products
• CO2 removed primarily by respiratory organs; most other metabolic end products are removed by the excretory organs
• Foreign substances may be removed either unchanged or after modifications that renders them harmless (detoxification) or more easily excreted
• Ultrafiltration


  Pressure forces a fluid through a semi-permeable membrane that retains proteins & similar large molecules but allows water and small molecular solutes, such as salts, sugars, & amino acids, to pass
• Active transport


  Movement of solute against its electrochemical gradient by processes requiring the expenditure of metabolic energy
• Active secretion


  Transport from animal into lumen of excretory organ (or organelle)
• Active reabsorption


  Transport from the lumen back into the animal
• Vertebrate kidneys


  • Function on the filtration-reabsorption principle with tubular secretion added

  
• A few teleost fish differ from this general pattern; they lack the ultrafiltration mechanism & depend entirely on a secretory type kidney
• Ultrafiltration results in a filtrate. Many filtered compounds are valuable; thus reabsorption mechanisms are present to conserve glucose, amino acids & vitamins
• More than 99% of the filtered volume is reabsorbed & less than 1% is excreted as urine
• Active secretion is the third part of the vertebrate kidney system

  
• All vertebrates can produce urine that is isotonic with or hypotonic to the blood, but only birds & mammals can produce urine more concentrated than the body fluids (hypertonic urine)
• Structure of vertebrate kidneys

  • Large number of nephrons, empty into collecting ducts
  • Each nephron begins with a Malpighian body (glomerulus and Bowman's capsule)
  • Fluid modified by tubular reabsorption and tubular secretion to form the final urine

  
• Proximal tubule


  Many solutes (salt, glucose) & water are reabsorbed
• Distal tubule


  Continues the process of changing the tubular fluid into urine
• Loop of Henle


  Hairpin like segment separating proximal and distal tubules
• Birds & mammals have Loop of Henle which is responsible for the production of hypertonic urine
• Fish, amphibians & reptiles lack the Loop of Henle, and can produce urine no more concentrated than the blood plasma
• Mammalian nephrons


  • Short-looped or cortical nephron
  • Long-looped or juxtamedullary nephron

  
• Short-looped or cortical nephron


  Nephron is shallow in terms of its position in cortex, loops of Henle are short
• Long-looped or juxtamedullary nephron


  Loop is long, responsible for concentrating the urine, the longer the loop the higher the urine concentration
• When carbohydrates & lipids are fully metabolized they yield CO2 & H2O which are easily excreted
• Proteins & nucleic acids also yield CO2 & H2O, but additionally they give rise to nitrogen containing excretory products: ammonia, urea & uric acid
• Deamination


  The amino group (-NH2) is removed from amino acids, forming ammonia (NH3)
• Animals grouped by excretory product


  • Ammonotelic (ammonia)
  • Ureotelic (urea)
  • Uricotelic (uric acid)
• There is no clear correlation between phylogenetic relationship of vertebrates & their major excretory products
• The compound an animal excretes depends chiefly on the habitat/environment it occupies
• Ammonia (NH3)


  Excess ammonia if retained in body is very toxic; it can be lost through any surface in contact with water & need not be excreted by the kidney
• Urea


  Easily soluble in water & has low toxicity; urea diffuses into blood plasma which can tolerate reasonably high levels
• Urea is excreted by some teleosts, & in elasmobranchs, amphibians & mammals it is the main nitrogen excretory product
• In elasmobranchs (sharks & rays), crab-eating frog (Rana crancrivora) & Latimeria, urea is retained & serves a major role in osmoregulation
• Uric acid


  Excreted by land snails, insects, most reptiles & birds, as an adaptation to water conservation in a terrestrial habitat
• The semisolid white portion of bird droppings is urine & consists mostly of uric acid - very little water is used for its excretion
• Excretion of uric acid in vertebrates occurs in reptiles & birds as opposed to mammals & amphibians (which form urea), primarily correlated with their mode of reproduction
• Birds & reptiles complete their embryonic development in a cleidoic egg - (only gases are exchanged with environment & all excretory products remain within the eggshell)
• In the cleidoic egg, limited water supply and ammonia is too toxic to be tolerated in large quantities, so uric acid is produced and eliminated as crystals in the allantois, which serves as an embryonic urinary bladder
• Interstitial fluid


  Fluid that lies between cells and other tissue components
• Blood


  Transports substances by way of the circulatory system