Renal Day 6

What is an Acid/Base

• Acids donate H+

• Bases accept H+

• Free H+ can damage proteins and important cellular structures

• All acids and bases have conjugates of themselves

• Acidity is measured by pH

What is pH?

• pH = −log [H+]

• pH scale : measures acidity/basicity

• 7 = −log [H+]

  • [H+] = 100 nmol/L

• 2.5 = −log [H+]

  • [H+] = 3.2 mmol/L

• 9.5 = −log [H+]

  • [H+] = 0.39 nmol/L

Acid strength and dissociation

• The stronger an acid, the weaker the conjugate base

• In solution, acids dissociate

  • This means the H+ is free to interact with other things

• Like any chemical reaction, it goes both ways

• Each acid dissociates at different rates

• The greater the dissociation, the stronger the acid

  • The strongest acids dissociate completely

• Dissociation constant tells the ratio of conjugates in solution

• Like pH, is described logarithmically

Henderson–Hasselbalch equation

• Equation that allows us to estimate the pH of acids/bases in solution

• To calculate, you need to know the pKaof your substance, and the concentrations of each conjugate

• In solution, when pK = pH, concentrations of acid and conjugate base will be equal

• When ratio goes up, there is less sequestered H+

• When the ratio goes down, less H+ is free

• Since pK is a property of the acid, it remains fixed

• So when pH of the solution shifts, so does the acid-base balance

Buffers

• Some acids/bases can act as buffers

• This means that they prevent changes to pH when in solution

• This works when both conjugates are highly present in solution

• Buffering works best when the pH of the solution is close to the pKa of the acid/base pair

• For a substance that has a pK near 7, a tritration curve looks like this

• At certain pH values, the solution will not change pH much despite significant acid/base additions

• This ability to resist change in pH is called buffering power

Regulating Blood pH

• Blood and interstitial fluid is maintained within a tight physiological range pH ~7.35 -7.45

  • [H+] ~35-45 nmol/L

  • Arterial blood pH drops below 7.35 : Acidosis

    • Headache

    • Vomiting

    • Muscle weakness

  • Arterial blood pH rises above 7.45 : Alkalosis

    • Confusion

    • Vomiting

    • Muscle spasms

Blood pH is challenged regularly

• Normal blood [H+] is 40nmoles/L

  • Only 5nmoles/L tolerance in either direction

  • The body contains 6L of blood

  • 240nmoles of H+ in the blood

  • Can tolerate 30nmoles

• On average the body has a net gain of 70 mmoles H+/day

  • The body’s metabolism produces 40 mmoles H+/day

  • Average diet nets 20 mmoles

  • Egestion loses 10 mmoles/day of OH-

  • Daily acid challenge is 300,000x the amount of H+ the body normally has

    • 1.8million times tolerable range

    • The body needs to prevent acidosis

How is the body protected from acid overload?

• The goal is to prevent H+ ions from roaming free

  • In other words, prevent the acid load from changing the pH of the blood

• We showed how buffering prevents solutions from changing pH

• Buffering works by neutralizing free H+ ions, preventing them from causing damage

• This is important, because free H+ will react with something

  • it’s better to have that something be unimportant than important

• Buffering is mainly done by Bicarbonate

Blood buffering

• There are two main buffers in the blood

  • Phosphates

  • Bicarbonate

• HCO3− is the most important for buffering the blood

  • [HCO3-] ≈ 22-29mmol/L

  • pKa = 6.1

  • Even though this is slightly lower than 7.4, it is still effective at high concentrations

• CO2 (carbon dioxide) is also highly present in the blood

  • [CO2] ≈ 1-2mmol/L

  • These are conjugates of, creating the necessary circumstances for buffering

  • It is important to maintain both CO2 and HCO3- to maintain equilibrium

  • If either changes, equilibrium is thrown off

The kidneys do what lungs can’t

• Ventilation plays a huge role in pH homeostasis

• Breathing removes CO2 via exhalation

  • Reduces reaction products in the equation

• Hyperventilation can act quickly to help rid the body of excess acid load

• Exhaling is not good enough on its own

  • Does not remove neutralized H+

• The kidney works to excrete acids, and to reabsorb bicarbonate to maintain blood buffering power

• While the respiratory system responds faster, the kidneys are more powerful, and can replace lost buffering agents

What do the kidneys do?

• The kidneys fight blood acidification in 3 ways:

  • Reabsorption of HCO3- to prevent excretion

  • Generation of HCO3 to replace what is lost from neutralizing acid load

  • Excretion of H+ ions (buffering alone is not enough; the body must remove excess)

Bicarbonate reabsorption happens in the PT

• Nearly all bicarbonate is reabsorbed by the end of the Proximal Tubule

• Proximal tubule transport is necessary transport, and is unregulated by outside actors

• Bicarbonate enters in the filtrate

• The membrane is impermeable to HCO3 so it must be transported or changed

Bicarb Reabsorb in PT stages

1. Bicarbonate enters the nephron in the filtrate

2. Extracellular Carbonic Anhydrase (CA) takes HCO3 + H+ and makes CO2+H2O

3. These can pass into the cell (recall the PT is permeable to water)

4. Once in the cell, intracellular CA reverses the process

5. Bicarbonate can then be reabsorbed by NBC (Sodium Bicarbonate Cotransporter)

6. The free H+ ion gets secreted by NHE (Sodium Hydrogen Exchanger)

7. The free H+ can be reused

no H+ is eliminated here

Bicarbonate is generated in the PT

• If we recall, 70 mmol of H+ are added to the body each day

• 70 mmol of HCO3 are used to neutralize this

• So, 70 mmol of new HCO3 must replace this

• Some of this comes from ammoniagenesis

• New bicarbonate is formed

Bicarbonate Generation in the PT Stages

1. Glutamine is brought into PT epithelial cells

2. Glutamine gets metabolized into ammonium ions (NH4+) and bicarbonate

3. This bicarbonate gets reabsorbed the same way filtered bicarbonate does,by NBC

4. NH4 is secreted by the same protein H+ ions are, NHE

5. NH4 can then be excreted in the filtrate

Phosphate is a good buffer

• With a pK of 7.2, phosphate is a good buffer in the body’s system

• But there is a lot less phosphate in the body than bicarbonate

• 0.8-1.5 mmol/L compared the bicarbonate at 22-29 mmol/L

• In the urine, we need something available in the filtrate

• Bicarbonate gets almost completely reabsorbed; phosphate doesn’t

• This makes phosphate a good candidate for buffering filtrate

H+ in the tubule must be buffered

• Just like in the blood, free H+ ions can be dangerous

• If all 70mmol of H+ ions were excreted in our ~1.5L/day of urine, the pH would be around 1.3 (almost as acidic as stomach acid!)

• To prevent this, buffers from the blood are filtered into the tubule lumen as well

• Specifically, phosphates act as extracellular buffers

• They combine with the H+ in the tubule lumen, and get excreted in the urine

• Because the proton is excreted, we have a net gain of HCO3 in the body

It all works together

• In the big picture, it all works together

• Filtered bicarbonate gets reabsorbed

• Filtered phosphate buffers secreted H+

• New bicarbonate is generated to replace what was lost

• Ammonium ions get excreted as a balance for the production of bicarbonate

• This is all a big balancing act

  • At one extreme, infinite bicarbonate would have every proton be recycled, but that is too much bicarbonate

  • At the other, with no bicarbonate, every proton would need to be excreted with phosphate or ammonium to generate a new bicarbonate molecule

The kidney needs to respond to altered metabolic states

• What we’ve discussed so far is how homeostasis is maintained

• This does not cover what happens when homeostasis is disrupted

• Blood pH can end up too high

  • Excessive vomiting causes loss of H+ ions

  • Excessive ingestion of bicarbonate antacids

• Blood pH can be too low

  • Excessive dietary intake of acids

  • Overproduction of metabolic acids

• The kidney needs to respond to these

Collecting Duct cells respond to alkalosis

• β-intercalated cells respond to alkalosis

• During alkalosis, the blood pH is too high

• There is too much HCO3 in the blood

• Some must be excreted

• Carbonic Anhydrase (CA) forms bicarbonate and H+ from CO2

• Bicarbonate is excreted through a protein called Pendrin

  • Exchanges bicarbonate for chloride ion

  • This excretion removes bases from the body

• H+ gets reabsorbed through ATPase

  • This returns acid to the blood, also helping to correct pH

CD cells respond to chronic acidosis

• α-intercalated cells respond to chronic acidosis

• Normal acid loads are dealt with by the PT

• During chronic acidosis, the body tells the CD that the PT can’t keep up with acid load

  • Filtrate may be lower than 6.0 by the time it reaches the CD

  • Phosphate has been used up, so something else must be done

• Need to excrete more acid

• CA forms H+ and HCO3 from H2O and CO2

• H+ gets secreted into the tubule fluid by 2 different types of ATPases

• Bicarbonate gets transported back to the interstitial space by Anion Exchanger 1 (AE1)

• The reabsorbed bicarbonate can help neutralize the acid load in the blood