The Renal Role The Renal Role in Acid Base in Acid Base

BalanceBalanceDr. Dave JohnsonDr. Dave Johnson

Associate ProfessorAssociate Professor

Dept. PhysiologyDept. Physiology

UNECOMUNECOM

Review of BasicsReview 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

Acid Base PairsAcid 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.

Biological BuffersBiological Buffers

The 3 major buffering systems of The 3 major buffering systems of biological fluids are:biological fluids are:

1.1. Bicarbonate buffering systemBicarbonate buffering system

2.2. Protein buffering systemProtein buffering system

3.3. Phosphate buffering systemPhosphate buffering system

Isohydric PrincipleIsohydric 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.

Bicarbonate BufferBicarbonate 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!

Dissolved CODissolved CO22

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)

Henderson-Hasselbalch Henderson-Hasselbalch EquationEquation

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 40pH = 6.1 + log (24 / 1.2)pH = 6.1 + log 20pH = 6.1 + 1.3pH = 7.4

Role of the KidneysRole of the Kidneys There are 3 major roles the kidneys play in There are 3 major roles the kidneys play in

maintaining acid base balance:maintaining acid base balance:

1. They must recapture the daily filtered load of HCO3- ions by

reabsorbing them.

2. They must excrete into the urine any excess free H+ ions which are added to the body fluids daily

3. The kidneys must also replace any HCO3- used up titrating

these excess acids produced daily.

““Life is a struggle, not against sin, not Life is a struggle, not against sin, not against the Money Power, not against against the Money Power, not against

malicious animal magnetism, but against malicious animal magnetism, but against hydrogen ions". H.L. Menckenhydrogen ions". H.L. Mencken

Recapturing Filtered Recapturing Filtered HCOHCO33

--

HCOHCO33-- is readily filtered into is readily filtered into

Bowman’s space, but Bowman’s space, but normally very little escapes normally very little escapes into the urine.into the urine.

Around 85% of the HCOAround 85% of the HCO33--

filtered load of is reabsorbed filtered load of is reabsorbed in the proximal tubules, 10-in the proximal tubules, 10-15% in Henle’s loop, and only 15% in Henle’s loop, and only 3-5% at more distal sites.3-5% at more distal sites.

Note the mechanism utilized: Note the mechanism utilized: secreted protons combine secreted protons combine with the filtered HCOwith the filtered HCO33

--..

Recapturing Filtered Recapturing Filtered HCOHCO33

--

It is important to recognize It is important to recognize that the loss of any free that the loss of any free HCOHCO33

-- into the urine is into the urine is equivalent to the addition of equivalent to the addition of free Hfree H+ + ions to the ECF:ions to the ECF:

C02 + H2O H+ + HCO3-

The loss of HCOThe loss of HCO33-- from the from the

ECF lowers the ratio of base ECF lowers the ratio of base (HCO(HCO33

--) to acid (CO) to acid (CO22) in the ) in the ECF, and will therefore result ECF, and will therefore result in an increase the free Hin an increase the free H++ ion ion concentration (and thus a concentration (and thus a decrease the pH!)decrease the pH!)

Generating New HCOGenerating New HCO33--

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.

HCOHCO33-- Generation in the Generation in the

Proximal Tubules using Proximal Tubules using Titratable AcidsTitratable 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.

HCOHCO33-- Generation in Distal Generation in Distal

Tubules and Collecting Ducts Tubules and Collecting Ducts using Titratable Acidsusing 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.

What is Titratable What is Titratable Acidity?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.

Limitations of Titratable Limitations of Titratable AcidsAcids

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.

Importance of Urinary Acid Importance of Urinary Acid Buffering…..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 Hthe H++ ion ion concentration of this urine with a pH of 4.5 is still only concentration of this urine with a pH of 4.5 is still only about 40 uMol/L (normal plasma is 40 nMol/L).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 /L = 100,000 uMol H+ ion = 100 mMol of H+ ion )

Ammonia BufferingAmmonia 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.

Ammonia BufferingAmmonia 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

How Does This Help?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.

It WorksIt 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.

Graphic ProofGraphic 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).