1. Intro to Osmotic Balance
    1. Why does it matter
      1. Maintains balances between intra and extracellular compartments
      2. Regulates between extracellular compartments and environment
    2. Types of osmotic exchanges
      1. Obligatory osmotic exchanges - in response to physical factors; no control (ex diffusion gradients)
      2. Regulated osmotic exchanges - physiologically controlled; aids in maintaining internal homeostasis
  2. Obligatory exchange of ions and water (factors)
    1. Gradients between animal and environment
    2. Surface to volume ratio (larger for small animals)
    3. Permeability of the integument (body covering)
      1. Water will move more quickly between cells than it will through cells (via water channels - aquaporins)
      2. Gills are highly permeable in frogs and fish
      3. Birds, reptiles, mammals, insects have relatively impermeable respiratory systems
    4. Feeding, Metabolic Factors, and Excretion
      1. Water and ions come in with food; water is required for the elimination of salts
      2. Metabolic water- end product of cellular metabolism
      3. Waste removal - major loss of water
        1. Some animals concentrate their wastes to reduce water loss
    5. Temperature, Exercise, and Respiration
      1. Water loss (evaporation) leads to heat loss (cooling)
      2. Respiration helps with cooling and causes water loss
        1. Nose helps prevent loss of fluid by a counter-current mechanism. As air leaves, moisture condenses in the nasal passages, then is used to humidify the next breath of air coming in (14-6)
        2. Loss is dependent on surroundings- more loss in cold, dry climates; less loss in warm, humid climates
  3. Osmoregulators and osmoconformers (14-7)
    1. Osmoregulators - control internal osmotic environment
    2. Limited Osmoregulators - control internal osmotic environment over a limited range
    3. Osmoconformers - follow the environment - do not control their internal environment
  4. Osmoregulation in aqueous and terrestrial environments
    1. Water-breathing animals
      1. Freshwater animals
        1. Body fluid is hyperosmotic to the environment
          1. Water comes in causing swelling
          2. Loss of salts
        2. Urine produced is very dilute
        3. Active transport is used to take salts up from the environment
        4. Generally water is not consumed from the environment
      2. Marine animals
        1. Isosmotic for invertebrates and primative chordates
        2. Hagfish, sharks, skates, and rays are isosmotic, but have decreased numbers of inorganic electrolytes (excreted via kidneys and rectal gland) and high amounts of organic osmolytes (urea and trimethylamine oxide)
        3. Marine teleosts are hypotonic
          1. Drink seawater to gain water
          2. Absorb Na, K, Cl via active and co-transport
          3. Most divalent cations exit in fecal matter
          4. Chloride cells in the gills help maintain Na, K, and Cl levels by secreting them out of the body
      3. Air-breathing animals
        1. Marine reptiles (sea snakes, iguanas, crocodiles) and birds - drink saltwater and secrete salts via salt gland (kidneys are unable to concentrate urine)
        2. Marine mammals - avoid drinking seawater and produce a very hyperosmotic urine (compared to blood)
          1. Gain water from food intake and metabolic water
          2. Consumption of seawater will cause a water debt to remove excess salts causing dehydration (because the urine will still be hypotonic to the seawater)
          3. Water loss via respiration is reduced by blowholes and labyrinth nasal passages
        3. Desert living animals
          1. Avoid daytime heat (if small) - cool burrows reduce heat and water loss (nasal countercurrent mechanism)
          2. Form highly concentrated urine and "dry" fecal pellets
          3. Use metabolic water as their main water source.
          4. Camel - uses large mass to store cool night temperatures, so it will take a long time for its mass to heat the next day. Also concentrates feces and urine - can store urea in there bodies until sufficient water can be found.
        4. Terrestrial arthropods
          1. Some extract moisture from the air (against the gradient between air and hemolymph)
            1. Create a high salt solution in mouth or rectum to absorb water
          2. Fecal matter is very low in moisture
  5. Osmoregulatory organs
    1. Specialized transport epithelium is found in the gills, skin, kidneys, and gut
      1. Apical surface (mucosal, luminal) - faces the external
      2. Basal surface (serosal, basolateral) - faces the internal (where cells from the rest of the body tissue are)
  6. Mammalian Kidney (14-13, 14-14)
    1. Anatomy (2 kidneys, receive 20-25% of bloodflow)
      1. Renal artery - blood enters kidney here - can constrict and dilate to help control blood pressure
      2. Afferent arteriole - divides to form the glomerulus
      3. Glomerulus - where blood is filtered (fluid, ions, etc leave) - it's located in Bowman's capsule
      4. Bowman's capsule - collects ultrafiltrate from the blood -first portion of the proximal nephron
      5. Proximal tubule (nephron) - first decending portion of the nephron
      6. Loop of Henle - hairpin loop in the middle of the nephron; contains both acending and decending limbs
      7. Distal tubule - ascending latter portion; between loop of Henle and collecting duct
      8. Juxtaglomerular apparatus (JGA) - section of the distal tubule which winds past the glomerulus so that the concentration of the urine can be determined and hormones released to adjust concentration if necessary
      9. Vasa recta - arteriole/venule which comes from the glomerulus and reabsorbs fluid, ions, etc from the collecting duct and nephrons
      10. Collecting duct - collects urine from individual nephrons; opens into renal pelvis
      11. Ureter - drains urine from renal pelvis and carries it to the bladder
    2. Urine Production
      1. Filtration
        1. Dependent on pressure difference between blood and Bowman's capsule (fig 14-18)
          1. Blood pressure (55) minus Bowman's capsule pressure (15) minus colloidial pressure (30) = net 10 mm Hg pressure forcing fluid out
          2. Blood pressure is regulated by dilation/constriction of renal artery and production of renin by JGA
            1. Renin ® angiotensin I® angiotensin II ® aldosterone ® increased reabsorption of Na and water, and increased secretion of K
        2. Negative charges on the basement membrane repel negative charges on proteins, keeping them in the blood
        3. Filtration slits provide a physical size barrier to particles (prevents proteins from being filtered)
      2. Reabsorption (99% of water and salts are reabsorbed)
        1. Use renal clearance to determine if a substance if reabsorbed or secreted in the kidney(by comparing clearance to glomerular filtration rate) Spotlight 14-1
          1. (Vu) (Ui) / (GFR) (Pi) = 1
          2. If GFR > C, the substance is not freely filterable or it is reabsorbed
          3. If GFR < C, the substance is secreted
        2. Tubular perfusion can be used to determine what each portion of the tubule does (14-23) (hint: remember that what comes out of the tubule is what will become the urine)
          1. If the level of X substance is higher coming out than it was going in, it means that portion of the nephron secretes that substance
          2. If the level of X substance is lower coming out than it was going in, it means that portion of the nephron reabsorbs it.
          3. If the levels of X are the same going in and coming out, there is not net secretion or reabsorption
        3. Roles (14-24)
          1. Proximal tubule - brush border yields a high surface area; 75% of the ultrafiltrate is absorbed here
          2. Decending limb- permeable to water (reabsorption), no active salt transport, decreased permeability to NaCl and urea
          3. Thin portion of ascending loop of Henle - no active transport, increased permeability to NaCl and decreased permeability to urea and very dec. for water
          4. Medullary thick ascending limb - active transport of NaCl (reabsorption), decreased permeability to water
          5. Distal tubule (14-25) - K, H, and NH3 are secreted into the lumen and Na, Cl, and HCO3 are reabsorbed from the lumen into the interstitial fluid (water follows)
          6. Collecting duct - permeable to water (reabsorbed); distal end is permeable to urea (reabsorbed)
        4. Hormones affecting reabsorption
          1. Renin ® aldosterone - increases reabsorption of NaCl and water by one or more of three possible mechanisms
            1. More Na/K ATPase pumps
            2. More ATP for Na/K ATPase pumps
            3. Apical membrane (lumen) more permeable to NaCl
          2. Vasopressin / Antidiuretic hormone - increases water channels in distal tubule and collecting duct to increase water reabsorption
          3. Atrial Naturitic Peptide (ANP) - increases Na excretion and urine production
      3. Tubular Secretion
        1. K, H, NH3, organic acids and bases are secreted
        2. Liver modifies "new" substances to have recognizable sections so the kidney can secrete them
    3. Regulation of pH
      1. Carbonic Anhydrase catalyzes CO2 + OH- + H+ « HCO3-+ H+
      2. H+ is secreted and HCO3- is reabsorbed in the distal tubule
      3. H is secreted along the whole tubule
      4. NH3/NH4+ and HPO42-/H2PO4- also serve as buffers which are secreted in the urine
    4. Urine concentration / Water reabsorption Mechanisms
      1. Differential permeabilities (14-24)
      2. Hormonal Control
      3. Countercurrent mechanisms (14-33) and concentration gradients (14-32)
  7. Nonmammalian Vertebrate Kidneys
    1. Hagfishes - glomeruli leads directly to collecting ducts; very little or no osmoregulation
    2. Amphibians - no loop of Henle; can't produce a hypertonic urine
  8. Extrarenal Osmoregulatory Organs in Vertebrates
    1. Salt glands (no filtration of blood occurs here)
      1. Elasmobranchs (sharks) - active transport of salts in rectal gland
      2. Birds and reptiles - use countercurrent mechanisms and active transport (both under hormonal control: corticosterone increases NaCl and KCl secretion)
    2. Fish gills
      1. Chloride cells (marine fish) -actively transport salts (Na/K ATPase and Na / 2 Cl / K co-transporter)
      2. Pavement cells - in freshwater fish remove Na from the water (net uptake of Na)
  9. Invertebrate Osmoregulatory Organs
    1. Filtration-Reabsorption systems
      1. Mollusks and crustaceans
        1. Evidence for filtration
          1. Inulin appears in urine
          2. Little or no glucose in urine unless glucose transport is blocked
        2. Filtration occurs across the heart into the pericardial cavity, then filtrate is conducted to a "kidney"
        3. High energy cost, but allows the animal to remove unknown/unwanted chemicals without a separate transport system
    2. Secretory-Reabsorption systems
      1. Malpighian tubules (in insects)
        1. No pressure difference between tubules and hemolymph, so urine must be secreted; some things are reabsorbed
        2. Pre-urine is isotonic or hypertonic to hemolymph
        3. Contains high K concentrations
        4. Formation of urine is increased by high K conc.
        5. Na in surrounding fluids has basically no effect on formation
  10. Excretion of Nitrogenous Wastes
    1. Amino group from amino acids must be re-used or removed (it is toxic)
    2. Excretion is in the form of ammonia, urea, or uric acid (based on water availability)
    3. Ammonia - excreting (ammonotelic) animals
      1. Most teleosts and aquatic invertebrates
      2. Allows for passive diffusion of NH3
      3. Ammonia is the most toxic form and requires the most water to remove it from the body; however it is not energy costly
    4. Urea-excreting (ureotelic) Animals
      1. Less toxic form of nitrogenous wastes, uses less water, removes 2 N atoms/molecule
      2. Synthesized in the liver via the ornithine-urea cycle by most vertebrates
      3. Teleosts and inverts. Use the uricolytic pathway
    5. Uric Acid-excreting (Uricotelic) Animals
      1. Birds, reptiles, and most terrestrial arthropods
      2. 4 N atoms/ molecule
      3. Uses least water/ used by animals with limited water availability
      4. Can't break down uric acid because they lack uricase