Calcium Reabsorption and ADH Effect of Kidneys

4. Explain the role of kidneys in
4.1 Calcium metabolism 

4.2 Maintenance of the extracellular fluid osmolarity 

4

4.1 Calcium exists as ionized, plasma protein bound, and non-ionized complexed forms in blood.
The free forms (around 50%) are filtered and reabsorbed in the kidney but not secreted.
Normally about 99% of the filtered Ca is reabsorbed in the following locations.
1. 65% - PCT
2. 2. 25 – 30% - Loop of Henle
3. 4 – 9 % - distal and collecting tibule
 

The Ca reabsorption in the kidney is mainly influenced by parathyroid hormone secreted from
the parathyroid gland. This hormone which is released when the blood Ca levels fall causes
increased Ca++ reabsorption in the think ascending loops of Henle and the distal tibules, thus
increasing Ca++ loss in urine.
Another important function of the kidney is convertion of inactive vitamin D into the active
form calcitriol. Calcitriol plays an important role in increasing blood Ca levels by increasing Ca
reabsorption in the gut and reducting Ca loss in urine.

4.2 

ECF osmolarity depends on the concentration of solutes and electrolytes present. Any change
occurring in these levels in detected by osmoreceptors which stimulates the posterior pituitary
to alter it’s ADH secretion accordingly. When the ECF water levels decrease or the electrolyte
level increases there in an increase in osmolarity. The osmoreceptors are stimulated and in
turn they cause the posterior pituitary to increase secretion of ADH. ADH makes the late DCT,
CD and CT permeable to water by inserting aquaporin 2 channels into the luminal membrane,
allowing for more water to be retained. ADH also makes the inner medullary CD permeable to
urea. Urea is reabsorbed into the renal interstitium and this creates 50% of the osmotic
gradient needed for water to be reabsorped in the presence of ADH. The retained water
increases the ECF water volume which dilutes the ECF and reduces osmolarity. The opposite
happens when osmolarity reduces.