Urinary /Excretory  System
   Kidneys, Ureters, Urinary Bladder, Urethra

Multiple Functions

   Osmoregulation - maintain blood osmolarity
   Regulation of ECF volume
   Maintenance of ion balance   
   Regulation of pH
   Waste  and foreign chemical removal
   Hormone production
   Gluconeogenesis

Functional units  Fig 2, 3, 5

   Vascular elments: arterioles, glomerulus, peritubular capillaries
   Nephron tubule:  Bowman's capsule, PCT, Henle's loop, DCT, collecting 
                    tubule
   
Renal processes   F 6

   1] Glomerular filtration - F 8
          Fenestrated capillaries, negative memb prots - prevent loss of 
             plasma prots
          Bulk flow of H2O and dissolved, low-mw solutes
          Net glomerular filtration pressure =  PGC  -  P BS -  pGC
          Glomerular filtration rate - measure of efficiency  180L/day
          Regulated by 
              local self-regulated changes in arterial pressure
              reflex control by sympathetic neurons and angiotensin (both 
                causing vasoconstr)  and prostaglandin (vasodilation)

   2] Tubular reabsorption
          Osmotic pressure favors movement of fluid into cap          
          Molecules will move due to
               concentration gradients - passive diffusion
               mediated transport - both facilitated diffusion and active 
                  transport (ATPase pumps)
               transport  maximum - rate at saturation (all carriers are 
                  occupied with substrate)
          Transcytosis

   3] Tubular secretion
          Impt in homeostatic reg of ions, esp H + and K+
          Movement of many organics, such as creatinine (also penicillin)

   4] Metabolic modification
         Catabolism of peptides
         Release of glucose into blood during fasting 
         Syn and secretion of NH4


Fluid and Electrolyte Balance - Integrative Physiology
 
Four parameters/variables to be controlled:  Volume, osmolarity, [ion], pH

Systems involved:  Excretory, cardiovascular, respiratory, nervous
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Water and sodium balance:  intake/gain = loss [T 16-3,4]  

    Water and sodium readily filtered, 99% reabsorbed; no secretion
         
    Can modify GFR [change in arterial bl press or sympathetic stim] F 16

    Different mechanisms for reabsorption of Na+in different regions of 
      the nephron tubule
         
      Transport of Na+ contributes to movement of other substances
         cotransport of Na+ and glucose out of lumen in PCT - 
           reabsorption of both
         antitransport of Na+ and H+ in PCT - reabs of Na+ and 
           secretion of H+
   
    Water movement - Related to osmolarity gradients; solutes - Na+,
          urea, et al      F13
             
Regulation by hormones

    ADH regulates permeability of tubular membranes [col.duct] to H2O (fig)
        osmoreceptors in hypothalamus - if osm >280 mOsm/L (stimulus),
            initiates secretion of ADH (by hypoth via post. pit); 
            response to hormone - H2O reabs reduces osmol    F 21
        baroreceptors sense decr in bl pressure - signal hypothal 
            to release  ADH;
            response - H2O reabs, conserves body fluid vol.   F 20

    Aldosterone (from adrenal cortex)  regulate the balance of Na+ and K+
       [also stim Na+ absoption in lg intest., sweat and salivary glands]
       Aldost. promotes reabs of Na+ and secretion of K+; this affects 
         plasma in reverse manner -  Na+ retention and K+ loss
       Affects all Na+ and K+ channels and pumps  
      
       Although much (90%) of the filtered Na+ is reab by the time the filtrate
         reaches the DCT, there is still a large amt of Na+ present in 
         lumenal fluid 
         Primary region for reabs of this last amt of Na+ is in cortical CD
                  
       No K+ in urine unless aldost. promotes secretion of ion; 
         also in cortical CD
          Why [K+] impt: 
           high blood [K+]called hyperkalemia - fatal cardiac arrhythmias
           low bl  [K+]        - hypokalemia - ditto and muscle weakness

       Complex mechanism for prod/release of aldosterone  F 17, 18,28

Acid - base Regulation

           Filtered bicarbonate converted to H2O and CO2 diffuses into PCT
           H+ secreted via Na+/H+ antiport protein