Describe the process of urine formation in the mammalian kidneys.

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   The  afferent  arteriole  which  is  a  branch  of  the  renal  artery  supplies  blood  to  the  glomerulus;  the afferent  arteriole  has  a  wider  lumen/diameter  than  the  efferent  arteriole;  which  takes  away blood  from  the  glomerulus;  the  differences  in  the  diameter  of  the  afferent  and  the  afferent vessels  causes  high  pressure;  leading  to  ultrafiltration  of  blood;  the  walls  of  the  blood capillaries  are  one-cell  thick;  hence  glucose,  amino  acids,  vitamins,  hormones,  salts,  creatine, urea  and  water  filter  into  the  Bowman’s  capsule;  to  form  glomerular  filtrate;  white  blood  cells, red  blood  cells,  plasma  proteins  such  as  globulin  and  platelets  are  too  large  to  pass  through  the capillary  wall;  hence  remain  in  the  blood  capillaries;  useful  substances  in  the  human  body  are selectively  reabsorbed;  back  into  the  blood  stream  at  the  proximal  convoluted  tubule;  the tubule  is  highly  coiled;  to  increase  the  surface  area  for  reabsorption  of  the  substances;  the useful  substances  include  amino  acids,  glucose,  vitamins,  hormones,  sodium  chloride  and water;  many  mitochondria  found  at  the  proximal  convoluted  tubule;  provide  energy  for reabsorption  of  these  substances  against  a  concentration  gradient;  the  glomerular  filtrate  flows into  the  descending  and  the  ascending  limb  of  the  loop  of  Henle;  blood  in  the  capillaries  and the  glomerular  filtrate  in  the  loop  of  Henle  move  in  opposite  directions/counter-current  flow; this  provides  a  steep  concentration  gradient  that  leads  to  maximum  absorption  of  water  through osmosis;  sodium  chloride  is  actively  absorbed  from  the  ascending  limb  into  the  blood capillaries;  under  the  influence  of  aldosterone  hormone;  the  glomerular  filtrate  flows  into  the collecting  tubule  from  where,  more  water  is  reabsorbed  into  the  blood  stream;  antidiuretic hormone  influences  the  amount  of  water  to  be  reabsorbed  depending  on  the  osmotic  pressure of  the  blood;  the  glomerular  filtrate  from  several  collecting  tubules  now  referred  to  as  urine;  is emptied  into  the  collecting  duct;  the  urine  passes  through  pyramid,  pelvis  and  ureter  into  the bladder;  where  it  is  stored  for  some  time.  The  sphincter  on  the  urethra  relaxes  to  allow  urine  to be  released  from  the  body;  Max.  20  mks 

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