This document provides an overview of acid-base balance and the key buffer systems that maintain it, including: 1. The bicarbonate buffer system works in extracellular fluid to balance pH by converting carbonic acid to bicarbonate and hydrogen ions. 2. The phosphate buffer system works in intracellular fluid and red blood cells, and is more powerful than bicarbonate. 3. Protein buffers also help maintain pH levels in extracellular fluid and red blood cells through amino acid groups and hemoglobin. 4. The lungs and kidneys play important roles in excreting acids and balancing pH levels. The lungs remove carbon dioxide from blood, while the kidneys secrete hydrogen ions through b

1. PHYSIO-BIOCHEMISTRY OF
ACID BASE BALANCE
Dr. Aniket Anil Shilwant
BAMS, MD (Ayurveda Physiology)

2. TABLE OF CONTENTS
• ACIDS
• BASES
• ACID BASE BUFFER
• BICARBONATE BUFFER
• PHOSPHATE BUFFER
• PROTEIN BUFFER
• RESPIRATORY MECHANISM
• RENAL MECHANISM

3. ACIDS – BASES
• An acid is the proton donor (the substance that liberates hydrogen ion).
• A base is the proton acceptor (the substance that accepts hydrogen ion).
• The normal H concentration in the extracellular fluid (ECF) is 38 to 42 nM/L.
• An increase in H+ ion concentration decreases the pH (acidosis) and a reduction
in H + concentration increases the pH (alkalosis).
• An increase in pH by onefold requires a tenfold decrease in H+ concentration.

4. BICARBONATE BUFFER SYSTEM
• It works at ECF level – Plasma
• It is Less powerful / effective as it is unable to cope with the pH difference to
that of Blood.
• Blood pH – 7.4
• Bicarb buffer pK – 6.1
• Chemical Equations –
HCl + NaHCO3 ----- H2CO3 + NaCl
Hydrochloric acid Sodium Bicarbonate Carbonic Acid (Weak) Sodium Chloride
NaOH + H2CO3 ----- NaHCO3 + H2O
Sodium Hydroxide Sodium Bicarbonate Carbonic Acid (Weak) Water

5. PHOSPHATE BUFFER SYSTEM
• It works at ICF and also at RBC level.
• It is More powerful / effective as it aptly matches the blood pH with least difference.
• Blood pH – 7.4
• Phosphate buffer pK – 6.8
• Chemical Equations –
HCl + Na2HPO4 ----- NaH2PO4 + NaCl
Hydrochloric acid Di-Sodium Hydrogen Phosphate Sodium Di-Hydrogen Phosphate Sodium Chloride
NaOH + NaH2PO4 ----- Na2HPO4 + H2O
Sodium Hydroxide Sodium Di-Hydrogen Phosphate Di-Sodium Hydrogen Phosphate Water
Note: At RBC level, due to excess concentration of Potassium, there is Potassium Di-
Hydrogen Phosphate and Di-Potassium Hydrogen Phosphate buffer.

6. PROTEIN BUFFER SYSTEM
• It works at both - ECF (Plasma) and RBC level
At ECF (Plasma) level
• At Plasma level weak acids that participate are –
 Carboxyl and Amino group of Glutamic acid
 Amino group of Lysine
 Imidazole group of Histidine
• Protein buffer systems in plasma are more powerful because of their high
concentration in plasma and because of their pK being very close to 7.4.

7. PROTEIN BUFFER SYSTEM
At RBC level
• Hemoglobin six times more powerful buffer due to its high concentration than
Plasma Proteins.
• Deoxygenated Hb is more powerful than Oxygenated Hb due to higher pK.
• Deoxygenated Hb easily binds the free H+ ions which are released when CO2 enters
capillaries. This attachment prevents fall in pH of blood.

8. RESPIRATORY SYSTEM
• Lungs maintain Acid-Base balance by removing CO2 produced during metabolic activities
in the body.
• CO2 combines with H2O to form Carbonic acid (H2CO3).
• Since carbonic acid is unstable, it splits into H+ and HCO3
CO2 + H2O → H2CO3 → H+ + HCO3-
• This entire reaction is reversed in lungs when CO2 diffuses from blood into the alveoli of
lungs.
H+ + HCO3– → H2CO3 → CO2 + H2O
• Where CO2 and H2O is blown off by ventilation.

9. RESPIRATORY SYSTEM
Sequence of Respiratory events to cope up with Metabolic Acidosis
1. Increase in metabolic activity
2. More CO2 is produced in tissues
3. Concentration of H+ ions also increases
4. State of Acidosis
5. Activation of Chemoreceptors
6. Hyperventilation – Pulmonary ventilation increases
7. Excess of CO2 is evaporated through breathing (Along with Insensible water loss)

10. RENAL MECHANISM
Kidneys play an important role in preventing Metabolic Acidosis by excretion of
H+ ions through 3 ways –
• Bicarbonate mechanism
• Phosphate mechanism
• Ammonia mechanism

11. H+ RELEASE VIA KIDNEYS – BICARBONATE MECHANISM
 For every H+ ion secreted into lumen of tubule, one bicarbonate ion is reabsorbed from the
tubule.
 In this way, kidneys conserve the HCO3.
 This reabsorption of filtered HCO3 is helpful in maintaining pH of the body fluids.

12. H+ RELEASE VIA KIDNEYS – BICARBONATE MECHANISM
INTERSTITIAL
FLUID
TUBULUAR CELL TUBULAR LUMEN
NaHCO3
Na+
H+ HCO3-
H2CO3
CO2 + H2O
H2O + CO2
H2CO3
HCO3- + H+
Na
Na+ + HCO3-
Na
HCO3-
NaHCO3

13. H+ RELEASE VIA KIDNEYS – PHOSPHATE MECHANISM
 The excess of H+ ions are secreted in renal tubules in the form of Sodium dihydrogen
phosphate.
 This excess H+ ions added to Urine in form of Sodium Dihydrogen Phosphate makes Urine
more acidic.
 This happens mainly in distal tubule and collecting duct because of the presence of large
quantity of sodium hydrogen phosphate in these segments.

14. H+ RELEASE VIA KIDNEYS – PHOSPHATE MECHANISM
INTERSTITIAL
FLUID
TUBULUAR CELL TUBULAR LUMEN
Na2HPO4
Na+
H+ NaHPO4
NaH2PO4
H2O + CO2
H2CO3
HCO3- + H+
Na
Na+ + NaHPO4
Na
HCO3-
NaHCO3

15. H+ RELEASE VIA KIDNEYS – AMMONIA MECHANISM
 On of the most important mechanism by which kidneys excrete H+ ions and make the urine
acidic.
 In the tubular epithelial cells, ammonia is formed when the amino acid glutamine is
converted into glutamic acid in the presence of the enzyme glutaminase.
 Ammonia (NH3) formed in tubular cells is secreted into tubular lumen in exchange for
sodium ion.
 Here, it combines with H+ ions to form Ammonium ions (NH4+).

16. H+ RELEASE VIA KIDNEYS – AMMONIA MECHANISM
 Thus, here the excess of H+ ions are secreted into renal tubules in the form of Ammonium
ions (NH4) resulting in acidification of urine.
 For each NH4 ion excreted, one HCO3- is added to interstitial fluid.
 This process takes place mostly in the proximal convoluted tubule because glutamine is
converted into ammonia in the cells of this segment.
 Thus, by excreting H+ and conserving HCO3, kidneys produce acidic urine and help to
maintain the acid-base balance of body fluids.

17. H+ RELEASE VIA KIDNEYS – AMMONIA MECHANISM
INTERSTITIAL
FLUID
TUBULUAR CELL TUBULAR LUMEN
Ammonia (NH3)
H2O + CO2
H2CO3
HCO3- + H+
Na Na+
Na
HCO3-
NaHCO3
Glutamine
Glutamic Acid
Ammonia (NH3)
H+
Ammonium ions NH4

18. Thank You!!!