Control of extracellular osmolarityWater & Na regulation :

Dr. M.A.M.Shaikhani.

Water regulated through the following mechanisms:1.Water excretion (if there is excess water in the body) or water

conservation( if less water in the body) by: A. Formation of diluted urine(secreting excess water in urine if there

is excess water in the body) by a simple process through ADH feedback inhibition mechanism.

B. Formation of concentrated urine(through more water conservation, reabsorbing& secreting less water in the urine)by a more complex process involving 2 main processes:

1.Maintinance of hyperosmotic renal medulla by:A. Countercurrent multiplier in the LH.B. Countercurrent exchange in vasa recta.C. Recirculation of urea in the renal medulla between the collecting

ducts & LH.2.High serum ADH concentrations.

2.Control of extracellular Na+ by: A.Osmotic ADH feedback system. B.Thist.

C.Aldosterone. D.Salt appetite. E. Cardiovascular reflexes.

Formation of diluted urine:Is by a simple process through inhibition of ADH, when the ECF is

hypo osmolar I.E high water& low Na+, there will be inhibition of ADH secretion from Hypothalamus &ADH acts on DT segments decreasing water reabsorption(antidiuretic)i.e increasing renal water excretion but still other solutes & waste products continues to be excreted, so diluted urine will be formed & urine osmolarity will decrease from 3oo in BC to 65 finally ,while the interstitum osmolarity will increase from 300 in the cortex to 700 in the medulla.

Formation of diluted urine:

Passive absorption(diffusion) of water, CL & urea:1.These pass passively when the interstitum osmolarity increase as

Na+ is absorbed to the interstitum specially water &to less degree urea(50% of that of water).

2.CL moves passively to Na+ to maintain electrical neutrality,except in the thick ascending LH in which it is secondarily active absorbed & not passively absorbed.

3.Most of these passive diffusion occur through the tight junctions &small amounts through the brash border of the tubular cells which are more loose in the PT &tighter in the DT.

Formation of concentrated urine:There is a limit for maximum concentration ability of the kidneys so that 500

mls./day of urine must be excreted by the kidneys to excrete the body solutes. this obligatory loss together + insensible loss of water through the sweating&

respiration leads to dehydration & death if water is not available for long time.

Some desert mouse have extreme ability to concentrate urine so they can survive in the hot climate of deserts without drinking water for long time .

Some aquatic animals as beaver have less urine concentrating ability than human & can not survive without water even for short periods. For the same reason humans can not depend on sea water for long time as severe dehydration will ensue as excess water needed to be lost with the urine to excrete the high Na+ present in sea water.

Formation of concentrated urine:

Urine concentration is accomplished by :        1.The presence of hyperosmolar medullary interstitum         2.High serum ADH concentrations:

A.Hyperosmolar medullary interstitum: is maintained by:1.Countercurrent multiplier in LH: The repetitive active reabsorpion of Nacl without water by thick

ascending LH & continued flow of new Na+ into the LH ,helps to add more & more Nacl to the interstitium which is carried deep to the medulla to build a high medullary osmolarity ,also more Na+ is absorbed from DT &CD to the interstitium under the effect of aldosterone helping in augmenting the interstitum hyperosmolarity. By this process the interstitial osmolarity increases from 300 in the cortex to 1200 deep in the medulla.

2.Countercurrent exchange in the vasa recta: The vasa recta is sluggish with only 2% of RBF , so preventing the

wash out of medullary hyperosmalarity produced by the counter current multiplier system of LH and also this U shaped venous system maintains medullary hyperosmolarity by continuous inflow of Na+ to its descending limb & continuous outflow of water out of this limb & vice versa occur in its assending limb, by this process the medullary hypermolarity is produced & maintained.

3.Continous recirculation of urea:between CD, interstitium & LH , makes the medullary interstitium

more hyperosmolar to aid in forming of a concentrated urine. The urea passes passively as water is absorbed under the effect of ADH as below.

B.High serum ADH contribution to urine concentration: occurs in response to body water deficit, high serum Na+

&High osmolarity , these will stimulate ADH secretion by the hypothalamus. This helps to pull water into the medullary interstitium by reabsorption through the collecting ducts