renal acid base.pptx

renal physiology

1. ACIDIFICATION OF URINE AND RENAL DUCT FEATURE Dr. SHAMI POKHREL MD. PEDIATRICS

2. The kidneys play a key role in the maintenance of acid–base balance and to do this they must excrete acid in the amount equivalent to the production of nonvolatile acids in the body. The kidneys must also filter and reabsorb plasma bicarbonate, and thus prevent the loss of bicarbonate in the urine.

3. Kidney regulate extracellular fluid hydrogen ion concentration through three basic mechanism Secretion of hydrogen ions Reabsorption of filtered bicarbonate ions and Production of new bicarbonate ions

4. Kidney each day filters 4320meq of bicarbonate. Because bicarbonate ion must react with a secreted H+ to form H2CO3 before it can be reabsorbed, 4320 meq of H+ must be secreted each day just to reabsorb the filtered bicarbonate An additional 80meq of H+ must be secreted to rid the body of non volatile acids produced each day, for a total of 4400 meq of H+ secreted into the tubular fluid each day

5. Concentration of hydrogen ion of body fluid normally is kept at a very low level. If we compare the concentration of H+ with that of sodium ion concentration, the latter’s concentration is about 3.5million times greater Na+ : 135meq/L H+ : 0.00004meq/L (40nEq/L)

6. Because H+ concentration normally is low, and because these small numbers are cumbersome, it is customary to express H+ concentration on a logarithm scale, using pH units. pH is related to the actual H+ concentration by the following formula (H+ concentration is expressed in equivalents per liter) pH= -log [0.00000004] pH= 7.4

7. Extracellular fluid pH of arterial blood = 7.4 pH of venous blood = 7.35 pH of interstitial fluid = 7.35 Intracellular fluid = 6.0-7.4 Urine = 4.5-8.0 Gastric HCL = 0.8

8. Bicarbonate acts as a major buffer system in blood but it has limitations as it can only bind to H+ but not remove it from the body. To remove H+ from body, kidney comes into play. Phosphate and ammonia are other urinary buffers In normal health, little excess of H+ is produced in the body and this is excreted from urine

9. 80-90% of bicarbonate is reabsorption occurs in proximal tubule. 10% is reabsorbed in thick ascending loop of Henle and remainder approx 5% in distal tubule and collecting duct

10. Hydrogen ion secretion and HCO3- reabsorption occur in virtually all parts of the tubules except the descending and ascending thin limbs of loop of Henle

11. In proximal tubule and thick ascending loop of Henle , hydrogen ion is secreted by sodium-hydrogen counter transport There is sodium-potassium pump in basolateral membrane cells which helps in taking sodium out of cells to interstitium . So, sodium will be attracted from the lumen into the cells via conc gradient and H+ counter transported into tubules

12. About 95% of the bicarbonate is reabsorbed in this manner, requiring about 4000mEq of H+ to be secreted each day by the tubules. This mechanism, however, does not establish a very high H+ concentration in the tubular fluid; the tubular fluid becomes very acidic only in the collecting tubules and collecting ducts

13. After secretion into the tubule, H+ will react with filtered HC03- to form carbonic acid. This is a very weak base and it difusses through apical membrane and enter into tubular cell and combine with water and again forms carbonic acid. It finally disassociate into bicarbonate and hydrogen ions. Bicarbonate formed in this way gets back into circulation. For every H+ secreted into tubule equals amount of HCO3- will be absorbed into circulation

14. H+ secretion occurs by yet another process in intercalated cells in colleting tubules and collecting ducts Cells here contain proton- pump mechanism and this pumps H+ into tubular fluid. About 15-20 % of H+ is secreted into tubule in this way Other 8 0% of secreted H+ into tubule will react with phosphate buffer and form monosodium dihydrogenphosphate and will be excreted in urine. Meanwhile a new bicarbonate will be formed and released into circulation

15. Here, the tubular epithelium secretes H+ by primary active transport It occurs at the luminal membrane of the tubular cell, where H+ is transported directly by a specific protein, a hydrogen-transporting ATPase Although the secretion of H+ in the distal tubule and collecting tubules accounts for only about 5 percent of the total H+ secreted, this mechanism is important in forming maximally acidic urine

16. PHOSPHATE BUFFER SYSTEM Once all the bicarbonate has been reabsorbed and is no longer available to bind with hydrogen ions, any excess H+ can combine with HP04 -- and other tubular buffers. After H+ combines with HP04- to form H2PO4-, it can be excreted as a sodium salt (NaH2P04), carrying with it the excess hydrogen

17. Therefore, whenever a H+ secreted into tubular lumen combine with a buffer other than bicarbonate, the net effect is addition of a new bicarbonate ion to the blood rather than merely a replacement of filtered bicarbonate

18. Phosphate buffer system carries excess H+ into the urine and generates new HCO3- Therefore, although phosphate is not an important extracellular fluid buffer, it is much more eefective as a buffer in the tubular fluid

19. AMMONIA BUFFER SYSTEM Ammonium ion is synthesized from glutamine. Here, two NH4+ is secreted and two HCO3- is reabsorbed. For each NH4 excreted, a new HCO3- is generated and added to the blood In chronic acidosis, the dominant mechanism by which acid is eliminated is excretion of NH4+

20. Once inside the cell, each molecule of glutamine is metabolized in a series of reactions to ultimately form two NH4+ and two HCO3- The NH4 is secreted into the tubular lumen by a counter-transport mechanism in exchange for sodium, which is reabsorbed. The HCO3- is transported across the basolateral membrane along the reabsorbed Na+, into the interstitial fluid and is taken up by the peritubular capillaries