URINARY SYSTEM

Functions

·        regulate the composition of the body fluids

·        rids the body of the wastes of metabolism and removal of foreign chemicals, drugs, and food additives

·       a minor endocrine organs - erythropoietin

 

 

Mammalian kidneys

 

·        Gross structure of the urinary system

·        Microscopic structure of the kidney

 

          1.  Nephrons - tubules and associated blood vessels

 

          2.  Renal blood vessels -two capillary beds in series--               glomerulus & peritubular

 

          3.  Nephron tubules -

          a.  glomerular capsule (Bowman's) -

         

          b.  proximal convoluted tubules -

                   1.  wall - single layer of cuboidal cells,

                   2.  microvilli - increase surface area

                   3.  salt, water ... transported from the lumen,

                             through the tubular cells, and into

                             peritubular capillaries

                   4.  Function -reabsorption

 

          c.  loop of Henle - thin and thick portions

                   1.  located in medulla and cortex

                   2.  descending and ascending loops

                   3.  juxtamedullary nephrons (inner 1/3 of

                             cortex) have longer loop

                   4.  responsible for concentrating urine

 

          d.  distal convoluted tubules -

                   1.  shorter than proximal t.

                   2.  few microvilli

                   3.  empties into  collecting duct. 

 

          e.  collecting duct-

                   1.  fluid drain from the cortex to the medulla

          2.  fluid - urine passes into a minor calyx, funnled                              through the renal pelvis and into the ureter

                   3.   wall- permeable to water, not salt

                             -regulated by ADH to increase permeability                  

 

Mechanisms( filtration, reabsorption and

secretion) and regulations-

         

·        Glomerular filtration - fluid and solutes in the blood

          plasma of the glomerulus pass into the glomerular                            capsule.

 

·        Tubular reabsorption-99% filtrate-transported             actively or passively out of the tubular lumen into the

         interstitial fluid, then into the peritubular capillaries.

 

          1.  Water leaves by osmosis and enters capillaries.

          2.  ADH regulates permeability of the collecting duct

 

·        Tubular secretion - noxious substances - H ions, K, poisons,

drugs, penicillin, metabolic toxins are actively transported from the peritubular capillaries into the interstitial fluid, then into the

tubular lumen.

 

·       Regultions-

                   1. Rate regulated by sympathetic nervous system:

 

                             (Flow chart)

         

                   2.  autoregulation - the ability of the kidneys

                        to maintain a constant GFR despite                                                     changes in mean arterial pressure -

 

                             L in blood pressure  - afferent arteriole dilates

 

How do the kidneys regulate the concentration of urine?

 

·       Countercurrent multiplier system-

 

1.  osmotic gradient necessary to transport water

2.  renal medulla - 4X osm. pressure than cortex

3.  geometry of loop of Henle- ascending & descending   

       ascending  limb- active 

 

·        countercurrent multiplication -

·        Vasa recta- blood capillaries- freely permeable to dissolved solutes

·        countercurrent exchange

 

Renal control of electrolyte balance.

·        Aldosterone stimulates Na reabsorption, K                    secretion in the distal convoluted tubules and cortical      region of collecting duct

                   e.g. No aldosterone- 2% of salt filtered and                                       excreted-

                   30 gm/day excreted in urine; all K+                                                     reabsorbed;  no K+ is excreted

                            

              With aldosterone, all Na+ delivered is                                             reabsorbed;  K+ excreted in urine

·        Aldosterone  secretion - stimulated directly -I blood               K, indirectly L blood Na

·        Aldosterone -  stimulates the secretion of H+, K+ into           the filtrate in exchange for sodium.

 

Renal control of acid-base balance

·       Lungs regulate PCO2 and carbonic acid               concentration; the kidneys regulate the             bicarbonate conc.

         

·       Filtered HCO3 + H+------->  H2CO3------> H2O +   CO2

 

                   1.  carbonic anhydrase - in the membranes of                                    microvilli in the tubules catalyzes the reaction

 

2.     CO2 reabsorbed - by tubule cells or red blood

cells

 

1.    kidneys excrete H, buffered by NH4 and                                   HPO4 -this prevent pH drop

 

                             HPO4 + H ----->  H2PO4

                             NH3 + H -----> NH4+

 

Human- renal plasma flow= 0.7 L/min

Hematocrit = 45%

blood flow to kidney= 1.25 L/min

Cardiac output = 5 to 6 L/min