Regulatory functions of the kidney1-Regulation of water balance.2-Regulation of electrolyte balance .3-Regulation of acid –base balance.

Regulation of water balance Anti-diuretic hormone (A.D.H) influence permeability of distal convoluted tubule, collecting tubules

-Stimulus for Secretion of A.D.H1-Decrease blood volume as in hemorrhage leads to ↑ADH secretion ( through stimulation of volume receptor in big veins)2-Increase osmotic pressure of blood (osmolarity) ,stimulation of osmoreceptor of hypothalamus leads to increase ADH3- IF their excess water in the body no stimulation of ADH .SO excess water pass in urine .

Water balance depend upon balance between water gain & water loss

Regulation of electrolyte balance Sodium is present mainly outside cells ,so it is essential for

maintenance of osmotic of the pressure of plasma and extracellular fluids, Potassium is presents mainly inside the cells and it is responsible for most of intracellular osmotic pressure.

In proximal convoluted tubuleIn proximal convoluted tubule; 85% OF na+ reabsorption leads to reabsorption of HCO3-,PO4

and cl. 100% of K+ is actively absorped from renal tubule. 85% of H2O is obligatory reabsorped .

In distal convoluted tubulesIn distal convoluted tubules; ; The remaining 15% of Na+ Is reabsorbed in exchange with K+ ,H

( k+ is controlled by aldosterone) K+ & H+ Compete for exchange with na+. The reabsorption of H2o in DCT Is controlled by ADHIs controlled by ADH .

Regulation of acid base balanceRegulation of acid base balance It is regulated through secreting acidic urineacidic urine down to PH 4 by direct excretion of acids like

phosphoric ,beta hydroxy butyric ,creatinine ,other organic acids ,also the kidney maintenance acids base balance by preserving the alkali reserve of the blood by preserving the alkali reserve of the blood

(NAHCO3)(NAHCO3) The bicarbonate (HCO3) is the main alkali reserve of the blood and available for

neutralization of acids Reduction of alkali reserve is called acidosis .if accompany with change in ph called academiaacademia

.Increase in alkali reserve is called alkalosis ,if accompany with change in ph called alkalemiaalkalemia.Renal stabilization of bicarbonate occurs byRenal stabilization of bicarbonate occurs by;;

1-Complete reabsorption of bicarbonate in glomerular filtrate . 2- Excretion of any excess of bicarbonate in the body. . 3-renewing of the depleted reserve by the process of hydrogen ion and ammonia excretion. This mechanism is determined by the nature of the buffer in the tubular urine at a site of

exchange (H+ excretion & na+ reabsorption ).

A- Buffer is A- Buffer is mainly mainly bicarbonatebicarbonate

If the buffer is mainly bicarbonate (HCO3), the exchange of hydrogen for sodium form carbonic acids in the tubular fluids.

H+NaHCO3→ Na+ +H2CO3.

This carbonic acid is change to CO2& H2O CO2 diffuse into tubular cells In the tubular cell CO2 is hydrated to reform

carbonic acids. CO2 + H2O→HHCO3 This cellular carbonic acids is the source of

both the hydrogen ions secreted into the tubular fluids ,and of the bicarbonate returned to the blood in association with na+ .

B- buffer is mainly B- buffer is mainly phosphatephosphate;

If the major buffer in the urine is phoshate po4, the exchange of hydrogen ions for sodium ions converts the filtered dibasic phoshate (Na2Hpo4)to an acid monbasic form(naH2po4)The hydrogen ion are excreted as titratable monobasic acid salts. H+Na2Hpo4→naH2po4+na The carbonic acids which is the

source of the cellular hydrogen ions exchange for Na + ions,is derived in part from CO2 produced in cellular metabolism and in part from CO2 brought to the cell in peritubular blood .

CO2+ H2O→H+ +HCO3.

Buffer is mainly Buffer is mainly amonia:amonia:

If little buffer is present in the tubular urine ,the exchange of H2 for sodium decrease because of the accumulation of free unbuffered hydrogen block the pump.

Under these condition ,low bicarbonate ,low phosphate in tubular urine ,the free base NH3 is formed in large amount.

glutamine→glutamic acid+ NH3 The NH3 formed diffuses into the acidic

urine ,buffer hydrogen ion ,and prevent their accumulation in high concentration

NH3+ H→NH4+ Removal of NH3 used in buffering the

excess hydrogen ion occurs in the form of NH4

To summarize the process of exchange of hydrogen excretion with na absorption the

following occurs;Most of sodium ions are absorbed and combine with

bicarbonate .simultaneously the hydrogen ions combine with bicarbonate to form carbonic acid, in the tubular lumen. this acids dissociate to form CO2& H2O

When HCO3- is not sufficient the hydrogen combine with na2hpo4 to convert it into nah2po4.

If phosphate is not sufficientIf phosphate is not sufficient the hydrogen ions combine with the synthesized ammonia to form NH4

If acidity is still excessive the NH4 combine with chloride to be excreted as NH4CL.

In all these process the bicarbonate is formed in tubular cells to combine with

absorbed Na+ or K+ to keep the bicarbonate at normal blood concentration.