ch12. excretory system

• We can survive with water but without food for ~2-3 months. BUT, without food and water, we'd only survive for ~1 week!!
• Water helps your body

  • Keep a normal temperature
  • Deliver O2 around body and remove CO2
  • Lubricate and cushion joints
  • Protect your spinal cord and other sensitive tissues
  • Get rid of waste through urination, perspiration, and bowel movements
• Osmoregulation


  The process of keeping the right balance of water and solutes (dissolved substances, e.g., carbohydrates and ions) in body fluids
• Osmoregulation includes


  • Compensating for water loss
  • Avoiding excess water gain
  • Maintaining the proper osmotic concentration (osmolarity) of the body fluids
• Osmolarity


  The number of particles of solute (e.g., ions, carbohydrates, amino acids or something else), per liter of solution
• Osmosis


  The process by which the molecules of a solvent (e.g., water) pass from a solution of low concentration to a solution of high concentration through a semi-permeable membrane
• Osmotic pressure


  The external pressure that is required to stop osmosis. Can be measured with a device called an osmometer. Osmotic pressure of sea water is ~27 atm
• Osmosis is a key process that is essential for maintaining the balance of water and solutes, ensuring optimal cellular function
• Osmoconformers


  Marine organisms whose cells have the same osmotic pressure as the sea
• Examples of osmoconformers


  • Marine invertebrates such as echinoderms (e.g., starfish), mussels, marine crabs, lobsters, jellyfish, ascidians (e.g., sea squirts), and scallops
• The osmotic pressure of ocean water and internal environment of osmoconformers is ~27 atm
• Even though the osmotic pressures are the same, the types of ions in the two environments differ a lot
• Osmoregulators


  Organisms that must actively take on, conserve, or excrete water or salts to maintain their internal water-mineral content
• Fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments
• Osmoregulation mechanisms of fish in fresh water

  • They tend to take up water because the environment is relatively hypotonic (low salt)
  • They don't drink much water
  • They pass a lot of very dilute urine
  • They achieve electrolyte balance by the active transport of salts through the gills
• Osmoregulation mechanisms of fish in a marine environment (hypertonic; salty)


  • They lose water through the skin
  • They drink sea water
  • They excrete the excess salts through their gills and their urine
• Nitrogenous waste products can be highly toxic
• Amino group


  The toxic group (NH2) released when amino acids are catabolized
• Ways the amino group can be dealt with

  • Salvaged/recycled for the re-synthesis of amino acids
  • Excreted from the body
• Dilution (adding water) is one form of protection from a toxic substance
• There is a close link between osmoregulatory functions and processes involved in the elimination of nitrogenous waste
• In animals with limited water supply, there is conflict between conserving water and preventing the accumulation of toxic nitrogenous wastes
• Forms of amino groups excreted

  • Ammonia (one nitrogen molecule)
  • Urea (2 nitrogen molecules)
  • Uric acid (4 nitrogen molecules)
• Ammonia


  Very toxic, needs a lot of water to remove it from the system: 300 to 500 ml of water required to dissolve and remove 1 g of ammonia
• Ammonia is suitable for animals in aquatic environments, but not suitable for terrestrial (land) animals
• Urea


  Less toxic than ammonia, needs 50 ml of water required to dissolve and remove 1 g of urea
• Urea is the choice of mammals
• Uric acid


  Far less toxic than ammonia, only 10 ml water required to dissolve and remove 1 g of uric acid
• Uric acid is produced by birds, reptiles, and most terrestrial arthropods (insects, spiders, scorpions, centipedes)
• Birds excrete a "pasty precipitate" called GUANO - contains white crystals of uric acid
• Guano is mined and used as a fertilizer for plants due to high levels of nitrogen, phosphate and potassium
• General functions of the excretory system

  • Filtration: Water and solutes in the blood are collected by the kidney's tubules
  • Reabsorption: the blood takes back the good stuff
  • Secretion: waste is transferred into the excretory tubule
  • Excretion: everything in the tubule (urine) ejected from the body
• Kidney


  A paired compact organ in adult humans (and other mammals), ~10 cm long and located beneath the diaphragm
• Pathway of urine


  • Urine is made in the kidneys
  • Urine is transferred from the kidneys to the urinary bladder via tubes called ureters
  • Urine passes from the urinary bladder to the exterior via a tube called the urethra
• Ureters


  Tubes made of smooth muscle, usually ~20 –30 cm long and ~3–4 mm in diameter
• Urinary bladder


  Contains a big involuntary (smooth) muscle called the detrusor muscle, which covers the whole bladder
• External urinary sphincter


  A voluntary "gate" made of skeletal muscle
• In females the urethra is separate from the genital tract, while in males, the vas deferens (carry sperm) empty into the urethra, and both urine and semen pass through the urethra to reach the exterior
• Renal cortex


  The granular outer section of the kidney
• Renal medulla


  The smooth, striated inner section of the kidney