The Renal Role in Acid Base Balance

1. The Renal Role in Acid Base Balance Dr. Dave Johnson Associate Professor Dept. Physiology UNECOM

2. Review of Basics • Acid/Base refers to anything having to do with the concentrations of free H+ ions in aqueous solutions • pH = - log [H+] • Therefore, the ‘normal’ pH of 7.40 means there are 10-7.40 moles of free H+ ions in a liter of plasma. • This is equivalent to about 40 nMol / L

3. Acid Base Pairs • An acid is a compound that can donate a proton to a solution. • An base is a compound that can take up a proton from a solution. • When an acid loses it’s proton, it becomes the conjugate base of that acid.

4. Biological Buffers • The 3 major buffering systems of biological fluids are: • Bicarbonate buffering system • Protein buffering system • Phosphate buffering system

5. Isohydric Principle • The isohydric principle simply denotes the fact that, even though there are 3 principle types of buffering systems in biological fluids, in an acid/base crisis, they all work together. This is because the H+ ion is common to all of them.

6. Bicarbonate Buffer • The major components of the bicarbonate buffering system are carbon dioxide (C02), which serves as the conjugate acid, and bicarbonate ion (HCO3-), which serves as the conjugate base. • This acid/base pair is unusual, since C02 has no proton associated with it - therefore it is usually described as a ‘potential’ acid, since increases in C02 can potentially increase free H+ ion concentrations, and thus lower pH. • C02 + H2O  H2CO3 H+ + HCO3- • The concentration of H2CO3 is about 340 times LESS than dissolved C02 and 6800 times LESS than HCO3- at normal pH, so it is usually ignored and the equation is written as: • C02 + H2O  H+ + HCO3- • This reaction is greatly accelerated by the presence of carbonic anhydrase!

7. Dissolved CO2 • CO2 is a gas, and only CO2 dissolved in the ECF is available to participate in acid base reactions. • It is known that the ‘normal’ partial pressure exerted by CO2 in plasma is 40 mm Hg (ie, pCO2 = 40). • It is also known that the solubility constant of C02 (ie, how much C02 gas dissolves for each mm Hg of partial pressure exerted by the gas in solution) is 0.03. Therefore: 0.03 mMol CO2 / L / mm Hg • Thus, at normal pCO2 of 40 mmHg, there is 1.2 mMol/L plasma of dissolved CO2 in the ECF, that can participate in acid base reactions (40 x .03 = 1.2)

8. Henderson-Hasselbalch Equation • It is important to recognize that it is the RATIO of the log of the conjugate base to the log of the conjugate acid of ANY buffering system in solution that determines the pH of that solution: pH = pK + log [A-] / [HA] Plugging in the values for the plasma concentration of ANY buffering pair in the ECF would give you the pH of the ECF (isohydric principle). For the bicarbonate buffering system, it is written as follows: pH = 6.1 + log [HCO3-] / 0.03 x PC02 pH = 6.1 + log [24] / 0.03 x 40 pH = 6.1 + log (24 / 1.2) pH = 6.1 + log 20 pH = 6.1 + 1.3 pH = 7.4

9. Role of the Kidneys • There are 3 major roles the kidneys play in maintaining acid base balance: • They must recapture the daily filtered load of HCO3- ions by reabsorbing them. • They must excrete into the urine any excess free H+ ions which are added to the body fluids daily • The kidneys must also replace any HCO3- used up titrating these excess acids produced daily.

10. “Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism, but against hydrogen ions". H.L. Mencken

11. Recapturing Filtered HCO3- • HCO3- is readily filtered into Bowman’s space, but normally very little escapes into the urine. • Around 85% of the HCO3- filtered load of is reabsorbed in the proximal tubules, 10-15% in Henle’s loop, and only 3-5% at more distal sites. • Note the mechanism utilized: secreted protons combine with the filtered HCO3-.

12. Recapturing Filtered HCO3- • It is important to recognize that the loss of any free HCO3- into the urine is equivalent to the addition of free H+ ions to the ECF: C02 + H2O  H+ + HCO3- • The loss of HCO3- from the ECF lowers the ratio of base (HCO3-) to acid (CO2) in the ECF, and will therefore result in an increase the free H+ ion concentration (and thus a decrease the pH!)

13. Generating New HCO3- CO2 + H20  H+ + HCO3- • During a metabolic acidemia, free H+ ions are added to the ECF for some reason, which “uses up” HCO3- in the buffering process. • The equation above shifts to the LEFT, generating CO2. • This HCO3- that buffered the excess H+ ions is lost for good, and MUST BE REPLACED to bring plasma HCO3- levels back up to approximately 24 mMol/ L.

14. HCO3- Generation in the Proximal Tubules using Titratable Acids • PROXIMAL TUBULE: Similar to what you saw previously for HCO3- REABSORBTION here, except now a H+ is excreted into the urine, generating a new HCO3- . • In this scenerio, filtered sodium monohydrogen phosphate (Na2HPO4) serves as a proton acceptor (base), and is converted to the acid, Na2H2PO4.

15. HCO3- Generation in Distal Tubules and Collecting Ducts using Titratable Acids • DISTAL TUBULE AND COLLECTING DUCTS: Similar to what you saw here previously for HCO3- REABSORBTION here, except now a H+ is excreted into the urine, generating a new HCO3- • As you just saw in the proximal tubule, a filtered Na2HPO4 serves as the proton acceptor, and is converted to Na2H2PO4.

16. What is Titratable Acidity? • The amount of strong base (such as NaOH) that it takes to titrate a patient’s urine that is acidic back to normal pH (~7.42) is approximately equal to the amount of titratable acids that were in the urine (ie, if 45 mMol of NaOH were required to titrate urine pH up to 7.42, the assumption can be made that 45 mMol of H+ ion were buffered by titratable acids, and 45 mMol of ‘new’ HCO3- were generated). • Dihydrogen phosphate is the major titratable acid measured in urine. • A healthy individual can easily generate some 50 to 100 mEq’s of H+ ions daily, However, titratable acidity normally can account for the excretion of only about 10 to 40 mEq of H+ ion per day.

17. Limitations of Titratable Acids • As the filtrate passes from Bowman’s space to the collecting tubules, the pH can drop all the way to about 4.50. This is an important concept, because urinary pH cannot drop below approximately 4.50. • Unfortunately almost all titratable acids will be fully protonated when the urine pH reaches about 5.20.

18. Importance of Urinary Acid Buffering….. • Assumption: individual has to excrete 100 mEq (mMol) of H+ ion a day to stay in acid / base balance (this is about average). • As noted, the minimum pH that can be achieved by the urine is about 4.50. Although urine with a pH of 4.50 has a H+ concentration about 1000 times greater than healthy plasma (7.42 vs 4.50…..about 3 log units), the H+ ion concentration of this urine with a pH of 4.5 is still only about 40 uMol/L (normal plasma is 40 nMol/L). • Thus, to get 100 mMol’s of unbuffered H+ ion into the urine each day you would have to produce about 2500 liters of this urine !! (2500 L x 40 uMol H+ ion/L = 100,000 uMol H+ ion= 100 mMol of H+ ion)

19. Ammonia Buffering • Many years ago, it was observed that in those patients experiencing metabolic acidemia, there was not only a rise in urinary titratable acid’s, but also in urinary ammonium ion (NH4+). • We now know that ammonium ion is a very important renal buffer, because the amount available is not directly dependant on diet or filtration, like titratable acids such as monohydrogen phosphate.

20. Ammonia Buffering • Ammonium ion can actually be produced in the cells lining the nephron, predominately in the proximal tubule, mostly (but not exclusively) from the deamination of of the amino acid glutamine. • The synthesis of ammonium ion in the proximal tubule occurs as follows: Glutamine----> 2NH4+ + -ketoglutarate

21. How Does This Help? • The subsequent metabolism of  -ketoglutarate in the proximal tubular cell results in the CONSUMPTION OF TWO H+ ions. Removal of two H+ ions is equivalent to the GENERATION OF TWO NEW HCO3- ions in these cells. These two new HCO3- ions are transported across the basolateral membrane of the cell via a Na+/ HCO3- symporter, and returned to the general circulation. • The ammonium ion (NH4+) is transported into the luminal fluid, mostly by substituting for H+ on the Na+/H+ antiporter, and passed out into the urine. Once in the tubule, it cannot diffuse back in due to it’s charge, and is thus lost in the urine. • The urinary excretion of NH4+ plays NO DIRECT ROLE in removing protons: NH4+is merely a side product - or marker - of the formation of -ketoglutarate in renal proximal tubular cells.

22. It Works • Therefore, proximal tubular secretion and subsequent urinary excretion of each NH4+ ion is linked to the generation of a new HCO3- ion in proximal tubular cells, which will then be returned to the circulation to replace HCO3- lost buffering excess plasma H+ ions.

23. Graphic Proof • Notice that AKG metabolism to C02 and H20 in proximal tubule cells consumes two H+ ions. • Now, an intracellular HCO3- in equilibrium with a H+ becomes a ‘free’ HCO3- • NH4+MUST be excreted in the urine after it is secreted from the cell. If it were reabsorbed, it would eventually be converted to urea in the liver, a process which generates two H+ ions (which would then consume two HCO3- ions).