The excretory system

  1. Excretory systems regulate one or more of following:
    1. volume of extracellular fluids
    2. composition of extracellular fluids
    3. osmotic potential of extracellular fluids
  2. Overview

    1. All known systems = tubules
    2. fluid flows or is pumped in,
    3. active secretion & selective reabsorption alter composition. 
    4. NOTE:  water is not pumped in at a molecular level.  Within cells, water follows when salts are moved (osmotic potential)
  3. Review:  Osmotic potential:
      1. iso osmotic - isotonic = same solute concentration
      2. hypoosmotic = hypotonic = lower relative concentration of solutes
      3. hyperosmotic = hypertonic = higher relative concentration of solutes
    1. 0smoconformers = same osmotic concentration;
        Usually not ion conformers
    2. 0smoregulators = body fluids different concentration from environment.
      1. Control of blood volume & solutes involves the control of salts.
        1. Control of salts is usually not a problem for terrestrial organisms.
        2. Marine organisms secrete salt
  4. Excretory systems rid the body of nitrogenous wastes
    1. Nitrogenous wastes are poisonous
      1. see text table 39.1, pg 754, for comparison
      2. "choice" of nitrogenous waste for excretion balances water availability and energetic costs
  5. The vertebrate excretory system
    1. Nephron (fig. 39.14): vertebrates.
      1. vascular component
        1. afferent arteriole (blood coming into area),
        2. glomerulus (a knot of capillaries with porous vessels),
        3. efferent arteriole peritubular capillaries surrounding tubule of nephron.
      2. Tubular:
        1. closed end enveloping glomerulus: Bowmans capsule.
        2. Filtrate passes down tubule
      3. The renal tubules control composition of urine, the peritubular capillaries bring to tubules wastes & carry away reabsorbed nutrients.
    2. Evolution:
      1. strong evidence that first vertebrates = fresh water,
      2. nephron evolved for water balance:
        1. excretion of water from hypertonic animal into hypotonic environment.
  6. Mammalian kidney:
    1. Anatomy:
      1. cortex, medulla, collection tubules -> ureters (left & right) -> bladder -> urethra.
    2. Function:
      1. Glomerular filtration driven by blood pressure
      2. Fluid is “pushed” into Bowman’s capsule and later concentrated in tubules
      3. Type of transport epithelium varies over length of tubule:
        1. active and passive movement,
      4. Tubular reabsorption:
        1. most solutes (sugars, amino acids) are actively reabsorbed in proximal convoluted tubule
        2. the osmotic potential of the fluid remains similar to blood.
        3. Concentration of urine occurs in collecting tubule
        4. primary solute involved are urea and salt (NaCl)    
    3. The loop of Henle
      1. establishes both a counter-current flow & a medullar osmotic potential gradient. 
      2. Together, these act to draw water out of lumen of the collecting tubule.
    4. the salute concentration of the interstitial fluids highest at the center of the kidney.
    5. The collecting duct then descends through the medullar concentration gradient:
    6. water is drawn out of the lumen of the collecting duct,
    7. urine is isotonic inner medulla hypoertonic
  7. The entire process is driven by permeability of tubules to water and sodium pumps controlling local interstitial concentrations of solute
  8. Regulation
    1. urine concentration and volume can vary
      1. Regulation of glomerular filtration rate
        1. blood must enter glomeruli at constant high rate, driven by blood pressure
        2. To maintain blood pressure
          1. local or autoregulation
          2. hormonal regulation: angiotensin and aldosterone
          3. hormonal regulation part 2: antidiuretic hormone (ADH)
          4. ADH and aldosterone have different, complementary effects
Study questions:
1.  Compare and contrast the anatomy and  function of the mammalian nephron and the bird salt gland.

2.  Compare ammonia, urea, and uric acid in terms of their efficiency in water conservation and expense.

3.  Draw a picture of a mammalian nephron, label the parts, and discuss the movement of water and solutes at each point. When is the movement passive and when is it active?

4.  Boney fish evolved in fresh water and migrated to the salt-water oceans. What might be different in the excretory system of salt water vs. fresh water fishes if they had evolved independently? (hint: consider problems of osmosis and the nitrogen waste product produced by each group).

5.  In what way is function of the proximal tubule limited by the surface area : volume ratio?  What anatomical features have evolved to counter these limitations?  

6.  In the discussion of the small intestine, the Na/Glucose cotransporter (a cell membrane channel) was described.  Where in the kidney would you expect to find it (which part of the nephron)?

7.  In what manner are aldosterone and ADH complimentary or antagonistic in their effects?

8.  Monitoring kidney function involves sensing of what variable, and where?


See also content review - all problems;  conceptual review # 2, #5;  applying ideas:  #1,  #5


Answers