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  • The kidney is the body's filtration system

  • and produces urine in its thousands of nephrons - the kidney's basic unit.

  • Each nephron begins with the Bowman's Capsule wrapped around a capillary ball - the glomerulus -

  • which becomes a tubule that eventually forms the Loop of Henle,

  • before collecting into tubules and ducts that empty into the bladder.

  • Fluid from the blood enters the nephron by filtration

  • and passes along it, where substances are added from the blood -

  • by secretion - or removed from it by re-absorption.

  • Potassium and Hydrogen ions, organic acids and bases, metabolites and foreign substances

  • are secreted across a selective membrane.

  • The Potassium and Hydrogen are exchanged with Sodium along the tubule

  • 99% of the filtrate is reabsorbed through a mixture of passive diffusion -

  • down an osmotic or concentration gradient - and active transport, by pumping against a concentration gradient.

  • The kidneys can conserve water.

  • The only way that the body can move water is by moving Sodium Chloride first.

  • You have to create a concentration gradient so that water can move by osmosis

  • in the direction of the highest concentration.

  • The Loop of Henle creates a hyperosmotic region in the medulla of the kidney,

  • acting as a sponge, drawing water out from the collecting ducts,

  • if the duct becomes permeable under the influence of Vasopressin, also known as Antidiuretic Hormone, or ADH.

  • The ascending limb of the loop of Henle - which is impermeable to water -

  • pumps out Sodium Chloride, via Sodium Potassium Chloride co-transporters,

  • from the urine into the blood in the Vasa Recta.

  • Blood in the Vasa Recta carries this Sodium Chloride into the medulla,

  • making the entire region hyperosmotic.

  • The blood - which is moving in the opposite direction to the urine in the Loop of Henle -

  • then ascends parallel to the descending limb of the Loop of Henle.

  • This descending limb is water-permeable

  • allowing water to move out of it into the Vasa Recta.

  • This entry of water into the ascending Vasa Recta

  • reduces the osmotic potential of the blood to normal as it returns to the body.

  • The interplay between the Loop of Henle and the Vasa Recta is called the Counter Current Multiplier.

  • Essentially the medulla acts as a sponge,

  • retaining and releasing water to address

  • the water requirements of the body.

  • ADH acts on the collecting duct to make it permeable to water,

  • allowing water to move into the hyperosmotic medulla

  • where it is drawn up by the Vasa Recta and returned to the body.

The kidney is the body's filtration system

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