top of page

Acid Base Balance and Buffers

image.png

pH

  • —pH is a measure of [H+]

  • —[H+] is an indicator of acidity

  • —pH = neg log10 of [H+]

    • —The negative log makes it an inverse relationship

    • —When [H+] increases, pH decreases

  • —The lower the pH is, the more acidic, because the more [H+] present

  • —Log units work in increments of 10

    • —1 pH unit = 10x change in [H+]

    • —A log is the number (or power or exponent) to which 10 is raised

image.png

Dissociation of Acids

  • —Bronsted-Lowrey acids and bases

    • —Acids are proton [H+] donors

    • —Bases are proton [H+] acceptors

  • —In water, acids tend to dissociate

  • ​A- is the conjugate base - has the potential to be a proton acceptor

​​

​

  • —Strongs acids the dissociation is nearly 100%

  • —Biological fluids contain weak acids

  • —Partial dissociation: they dissociate much less than  100%

  •   In weak acids, some stays as HA​

​

Examples

Screenshot 2024-07-31 at 3.31.33 PM.png
Screenshot 2024-07-31 at 3.34.37 PM.png

study question: 

  • Why do strong acids have a higher concentration of hydrogen ions (H⁺) in solution compared to weak acids when both are at the same concentration?

image.png
image.png
image.png

  • —We tend to think of H+ as the acid, but under the Bronsted-Lowrey definition, it’s not

    • —The compound that gave up the H+ is the acid

    • —If add more H+, then the reaction would go back

​

​

  • —At the pH of blood, most organic acids are completely dissociated

    • —Often use the suffix “ate”

    • “ate” indicates the conjugate base form

Screenshot 2024-07-31 at 3.39.34 PM.png

  • —Each acid has its own dissociation constant (K or Ka)

    • —Unique to each acid

    • —Is based on the ratio of dissociated to undissociated acid

  • —Ka is expressed as a molar concentration

  • —The dissociation varies with each acid and depending on the pH of a solution

    • —Ka indicates the tendency of an acid to lose its proton

    • —Can determine how the pH is affected by Ka of an acid

    • —K is a known value (determined by analytical chemists in the lab)

    • —We use the Henderson-Hasselbach Equation to look at the relationship between pH and pK

Screenshot 2024-07-31 at 3.42.23 PM.png
Screenshot 2024-07-31 at 3.43.36 PM.png

Physiological Significance

  • —How do we maintain acid-base balance?

    • —Metabolic processes disturb acid-base balance

      • —Diet—minor role

      • —Metabolism—generally ­increases [H+] and CO2

      • —Diseases can ­ increases or decrease [H+]

        • —Metabolic

        • —Respiratory

      • —Drugs can ­ increase or decrease [H+]

    • —Because metabolism produces carbon dioxide, which decreases pH

    • —Metabolic products also include organic acids, which dissociate to release more H+, which decreases pH

  • —Part of homeostatic mechanisms of body

    • Normal compensation to correct acid-base imbalance includes

    • Buffers

      • ​—Variety of substances and systems that keeps body fluids in narrow pH range

      • Like a sponge - can hold lots of H+, but when it gets full, it must get squeezed out

    • Respiratory (lungs)

    • Renal (kidney)

Buffers

  • —Buffer effectiveness depends on

    • —Dissociation constant or pK

    • —Concentration

  • Normal pH range is 7.35 - 7.45

  • —Buffer systems—minimize pH changes temporarily

    • —Proteins in cells (~50%)

    • —Blood proteins [plasma proteins (~1%) and RBC hemoglobin (~6%)]

    • —Bicarbonate system (~42%)

    • —Phosphate system (separate, via urine)

  • —Ultimately, excess CO2 is exhaled through the lungs and excess H+ through kidneys

    • —Buffers “buy time” while excesses are being eliminated

image.png

study question: 

  • What is a buffer solution, and how does it work to maintain a stable pH when small amounts of acid or base are added?

Proteins as Buffers

  • —Why do proteins make good buffers?

    • —Proteins are made of amino acids

    • —Functional groups on amino acids

      • —Each have a  NH2 (amino) and a COOH (carboxyl)—

  • —There is potential for giving up or donating H+

Screenshot 2024-07-31 at 3.58.12 PM.png
image.png
image.png
image.png
image.png

Bicarbonate Buffer System

Screenshot 2024-07-31 at 5.36.59 PM.png

  • —Contribute to two systems

    • —Respiratory compensation: Breathing CO2 out

    • —Renal compensation Excreting H+ through kidney

  • —The direction of the bicarbonate system depends on the part of the body (cell type) where compensation is occurring

  • Henderson-Hasselbalch Equation

    • —To maintain normal arterial pH = 7.4

      • —[HCO3-] : [CO2] must be 20:1

    • —Respiratory system regulates [CO2]

    • —Kidneys regulate [HCO3-]

Screenshot 2024-07-31 at 5.52.23 PM.png
image.png
Screenshot 2024-07-31 at 5.55.09 PM.png

Renal Regulation

  • —Alternative to respiratory regulation

  • —Renal compensation

    • —Takes time to react

    • —Long-term

    • —Involves

      • —Bicarbonate

      • —Disodium /monosodium phosphate

      • —Ammonia

  • —Renal compensation for elevated ­[H+]

  • —H+ can’t be exhaled—it must go out in kidney

    • —Most CO2 from metabolism goes out by lungs

  • —After kidney filters blood and it goes through tubules, the distal tubule is where final adjustments are made

  • —CO2 dissolves into blood

  • —As passes through distal tubular cell

    • —Carbonic anhydrase is there (inside the cell)

    • —Makes bicarbonate (same reaction as before)

    • —Spontaneously dissociates

    • —The bicarbonate (HCO3 – ) is drawn back into blood (reabsorbed)

    • —Tubular fluid has HPO42 – and it picks up H+

    • —Tubular cell kicks out H+ in exchange for a Na+

      • —Keeps electroneutrality

      • If get rid of a positive charge, need to replace it

image.png
image.png

study question: 

  • How do the kidneys contribute to maintaining the body's pH balance, and what role do buffer systems play in this process?

image.png

Phosphate Buffer System

  • —Renal Compensation

  • —Phosphate group— P binds with oxygens

  • —H3PO4  - Phosphoric Acid

  • —Can donate 3 H+

    • —One pK is in physiological range

    • Has 3 pKs—if add base, draw off H+

  • Consists of phosphoric acid (H3PO4) in equilibrium with dihydrogen phosphate ion (H2PO4-) and H+

  • acid-base pair

    • H2PO4–  is the acid  or donor

    • HPO42–  is the conjugate base

image.png
Screenshot 2024-07-31 at 6.03.42 PM.png
Screenshot 2024-07-31 at 6.06.34 PM.png

Acid-Base Compensation

Screenshot 2024-08-01 at 11.36.00 AM.png

  • —When ­increased acidity, the reaction is pushed to the right

    • —The lungs compensate by excreting CO2

  • —When this system reaches it’s limit, the kidneys get rid of more H+ using the phosphate buffer system

Screenshot 2024-08-01 at 11.37.34 AM.png

Assessment of Acid-Base Balance

  • —Body attempts to self-correct changes in pH – making assessment difficult

  • —Common lab measures

    • —ABGs

    • —Serum chemistries

  • —pH alone not adequate – why?

image.png

Acid Base Disorders

Respiratory Acidosis

  • —Excess acid in blood secondary to carbon dioxide retention

  • —Hypercapnia

  • —Due to respiratory dysfunction – renal regulatory systems compensate

  • —Labs

    • —Decreased pH, elevated pCO3

    • —Slightly elevated bicarbonate

    • —Increase in serum Ca, K, Cl

  • —Hypoxemia (responsible for most symptoms)

  • —Restlessness, apprehension, lethargy, muscle twitching, tremors, convulsions, coma

  • —Treatment

    • —Correct underlying condition

    • —Increase oxygenation

    • —Mechanical ventilation

Respiratory Alkalosis

  • —Relative excess amount of base d/t reduction of CO2

  • —Hyperventilation

  • —Shift of acid from ICF to ECF bicarbonate moved into cells in exchange for chloride– renal compensation

  • —pH > 7.45,

  • —plasma HCO3- low in chronic, PaCO2 low in acute

  • —Cardiac, CNS, respiratory symptoms

  • —Treat underlying cause

  • —Correction of hypoxia

Metabolic Acidosis

  • —All types not caused by excessive CO2

  • —Diarrhea most common cause

  • —d/t excessive loss of bicarbonate – bicarbonate-carbonic acid buffer system is stimulated

  • —Kussmaul breathing (extreme acidosis)

  • —Cardiac and neurological

  • —Treat underlying cause

  • —Raise pH to safe level – not too quickly

Metabolic Alkalosis

  • —Excess amount of base

  • —Fluid imbalance – with volume decrease

  • —Without fluid imbalance – without volume decrease

  • —Underlying event determines pathophysiology

  • —No specific signs and symptoms

  • —pH >7.45 elevated HCO3

  • —Look at electrolytes and volume

  • —Volume imbalance corrected with isotonic saline with KCl

  • —Treat underlying condition

  • —Carbonic anhydrase in severe cases

image.png
image.png

©2023 by Syracuse University Dr.Margaret Voss

bottom of page