Kidney, Osmoregulation and Urine Formation

KIDNEY

 1.      Each kidneys contains approximately one to two million tiny functional units (nephrons).

2.      Each nephron consists of:
      -          Bowman’s capsule
      -          proximal convoluted tubule
      -          loop of Henle
      -          distal convoluted tubule
      -          collecting duct.

3.      Each nephron tubule - is extensively surrounded by a capillary network.
4.      Blood:
      -          enters kidney through renal artery
      -          leaves through renal vein.
      -          afferent arteriole carries blood à to glomerulus
      -          efferent arteriole carries blood ßout of  glomerulus.

5.      High blood pressure (in glomerulus) forces blood plasma à into Bowson’s capsule due to:

  à renal artery is short + close to aorta.
  à diameter of the efferent arteriole is smaller than afferent arteriole.

URINE FORMATION

1.      Three processes involved in the urine formation are:

      -          ultrafilteration,
      -          reabsorption
      -          secretion.

2.      Ultrafilteration:

      -          a process
      -          blood plasma is forced out from the glomerulus
      -          into the Bowman’s capsule
      -          due to high hydrostatic pressure.

3.      Glomerular filtrate contains:

      -          ions,
      -          water
      -          nutrients
      -          nitrogenous waste.

4.      Reabsorption of  glucose +  amino acids

      -          occurs at the proximal convoluted tubule
      -          an active transport.

5.      Reabsorption of water:

      -          occurs at descending limb of loop of Henle.

6.      Secretion of hydrogen ions (H⁺), creatinine, and toxic waste

      -          occurs at distal convoluted tubule.

7.      Urea is excreted, while water, some urea and sodium ions (Na⁺)
      -          reabsorbed at collecting duct.
      -          composition of urine =  ions, urea, water, toxins + creatinine.
     

OSMOREGULATION

1.      Osmoreceptors (in hypothalamus) - constantly monitor the amount of water in blood.
2.      ADH:
  è released by the posterior lobe of pituitary
  è regulates the reabsorption of the water
  è by kidney.

3.      When osmotic pressure (OP↑) in blood increase:

  è ADH is secreted
  è increases permeability of collecting duct + distal tubule
  è to reabsorb more water.
  è Consequently, less more concentrated urine is produces.

4.      When osmotic pressure (OP↓) decreases:

  è adrenal cortex releases aldosterone (2^nd  hormone)
  è stimulates distal convoluted tubule +  collecting duct
  è to reabsorb more sodium ions (Na⁺)
  è thru active transport,
  è increases blood volume ↑.
  è regulates blood concentration + volume.

5.      rennin-angiotensin mechanismà trigger the release of aldosterone.

6.      Negative feedback mechanism:
  è a system
  è which brings an imbalance back to normal
  è by responding to a condition it controls.

7.      Blood pH is maintained - between 7.35 to 7.45.

      -     relative exchange of bicarbonate (HCO₃^-) + hydrogen ions (H⁺)
      -     between blood +  tubules
      -     help adjust acid-base balance of blood.

8.      When blood pH ↓ decreases:
      -          H⁺ are secreted into distal tubule
      -          while HCO₃^- are reabsorbed,
      -          resulting in blood pH returning to normal.

9.      When blood pH ↑ increases:

      -          HCO₃^- will be removed
      -          from blood àinto distal tubule.
      -          few H⁺ à move into blood capillary.

10.  Urine pH vary between 4.5 -  8.2 (normal pH = 6.0).