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.