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Where is the major site for water reabsorption in the nephron?



What exactly happens when ADH is released into the circulation?

ADH stimulates water reabsorption by stimulating insertion of "water channels" or
Aquaporin into the membranes of kidney tubules. These channels allow solute-free
water through tubular cells and back into blood.


What will happen in the kidney if plasma osmolarity falls too low?

No ADH therefore collecting ducts are impermeable to water. Filtrate passes down the
collecting duct through the concentrated medulla without any water being reabsorbed
and a large volume of dilute urine will be produced. Effect is to increase the
osmolarity of plasma back towards normal by loss of water.


What role does urea play in the concentrating ability of the kidney?

ADH allows urea recycling from the collecting duct (moves through the Aquaporin
channels along with water). Urea accumulates in the interstitium and aids in
increasing the interstitial concentration thus increasing the kidneys ability to produce
hyperosmotic urine.


Why it is that water leaving the descending limb of the loop of Henle and the
collecting duct does not dilute the concentration of the interstitial fluid thus
destroying the concentration gradient?

Blood in the Vasa recta flowing in the opposite direction has just passed through the
hyperosmotic region deep in the kidney parenchyma. It has become very
concentrated; this blood is therefore able to reabsorb water as it leaves the
descending limb, drawn out by the increasing concentration of the interstitium, and as
the blood in the Vasa recta is flowing towards the cortex the reabsorbed water is
drawn away from the concentrated middle of the kidney thus preserving the
concentration gradient.


Is unbound calcium freely filtered in the glomerulus?

All calcium or phosphate that is not bound to large proteins is freely filtered by the
glomerulus. The ions that are bound to protein are not filtered because the proteins
cannot pass through the filtration barrier.


Label the diagram to show where, and with what percentage calcium and
phosphate ion reabsorption occurs

PCT (70%),
ascending limb
of LOH (20%),
DCT (~9%),
collecting duct (~1%).

This means
that normally 1% excreted in urine.
Phosphate (pO4
)-PCT (80%), DCT
(10%), excreted (10%)


How are phosphate ions reabsorbed with sodium ions in the proximal tubule?

Phosphate reabsorption in the proximal tubule is by co-transport with the oppositely
charged sodium ions. This movement occurs down a sodium ion gradient created
by the pumping of sodium ions out of the cell by the 3Na-2K-ATPase transporter at
the basolateral surface (the surface away from the tubular lumen). Sodium ions
enter the cell from the tubule and carries with it phosphate.


Is calcium secreted in the kidney tubules?

There is no secretion of calcium or phosphate and both are reabsorbed along the
nephron. There is a limit to the amount of phosphate that can be absorbed and if
this limit is exceeded, then the excess phosphate remains in the tubular fluid and is
excreted in the urine.


Why is it important that calcium and phosphate regulation be linked?

If calcium levels increase under conditions of high phosphate there would be a
tendency for calcium phosphate stones to be created


Describe the effect of Elevated PTH on the handling of calcium ions in the kidney

PTH’s action is to increase renal calcium reabsorption. In the kidney, PTH promotes
calcium ion reabsorption in the ascending loop of Henle, distal tubule, and
collecting tubule


Approximately 80% of phosphate is absorbed in the PCT. What mechanism best
describes the regulation of Phosphate reabsorption in the PCT:
A) active co-transport with calcium ions
B) active transport with chloride ions
C) passive diffusion down its electrochemical gradient
D) Is inhibited by calcitonin
E) Is inhibited by parathyroid hormone