Biochemistry: Acids, Bases, and pH Scale

• Biochemistry explores chemical processes in living organisms and is lab-based, combining biology and chemistry

Acids

• Acids are compounds containing ionizable hydrogen (H+) or protons
• Acids serve as proton donors
• Strong acids dissociate completely and irreversibly in solution (e.g., HCl -> H+ + Cl-)
• Weak acids dissociate incompletely and reversibly in solution (e.g., CH3COOH ⇆ CH3COO- + H+)

Bases

• Bases accept ionizable hydrogen (H+) or protons
• Bases act as proton acceptors
• Bases can provide OH- ions
• Strong bases dissociate completely and irreversibly in solution (e.g., NaOH -> Na+ + OH-)
• Weak bases dissociate incompletely and reversibly in solution (e.g., NH4OH ⇆ NH4+ + OH-)

pH Scale

• pH is the negative logarithm of hydrogen ion concentration: pH = -log[H+]
• [H+] represents the concentration of hydrogen ions
• The pH scale ranges from 0 to 14
• At high [H+], pH < 7, indicating an acidic solution
• At low [H+], pH > 7, indicating a basic or alkaline solution
• pH = 7 indicates a neutral solution

Buffers

• Buffers resist pH changes upon addition of small amounts of acids or bases
• They neutralize small amounts of added acid or base, maintaining pH stability
• Excessive amounts of acids or bases can cause significant pH changes, even with buffers, due to buffer capacity
• Acidic buffers are composed of a weak acid and a salt of that acid with a strong base
• Basic buffers are composed of a weak base and a salt of that base with a strong acid

Physiological Buffers

• Physiological buffers maintain constant pH in blood and tissues
• The physiological pH is around 7.4, optimal for plasma enzyme activity
• Body enzymes are sensitive to pH changes
• Physiological buffer systems include bicarbonate, phosphate, protein, and hemoglobin buffers
• Hemoglobin provides about 60% of the protein buffering capacity
• Hemoglobin buffers carbon dioxide added to the blood by tissues

Bicarbonate Buffer System

• The bicarbonate buffer system is present in higher concentrations than other buffers
• Carbonic acid is easily formed from CO2 by carbonic anhydrase in tissues
• The bicarbonate to carbonic acid ratio is 20:1 at pH 7.4
• Added acid is converted of HCO3 - to H2CO3, then to CO2 + H2O by carbonic anhydrase, disposing of CO2 by the lungs
• The ratio of bicarbonate to carbonic acid decreases below 20:1 when acid is added to the blood

Organs Controlling Blood pH

• Lungs
• Chemoreceptors in the medulla sense pH changes, adjusting breathing rate and depth to compensate
• Kidneys regulate blood pH through reabsorption of HCO3- and excretion of H+ in the urine

Acidosis

• Acidosis is a condition where the bicarbonate to carbonic acid ratio is decreased below 20:1
• Types include respiratory and metabolic acidosis

Alkalosis

• Alkalosis is a condition where the bicarbonate to carbonic acid ratio exceeds 20:1
• Types include respiratory and metabolic alkalosis

Metabolic Acidosis

• Metabolic acidosis may be caused by increased acid production, renal failure, or excessive base loss

Increased Acid Production

• Severe muscular exercise leads to lactic acid accumulation
• High protein diets result in increased acid production
• Ketosis (excessive ketone body formation) occurs in uncontrolled diabetes mellitus and starvation

Renal Acidosis

• Renal acidosis is due to the failure of the kidneys to excrete acids

Excessive Base Loss

• Excessive base loss is caused by severe vomiting and diarrhea

Compensation of Metabolic Acidosis

• Compensations include an Increased rate of respiration to increase disposal of CO2

Respiratory Acidosis

• Respiratory acidosis involves increased blood levels of H2CO3 due to lungs failing to excrete CO2.
• Extensive lung disease (severe bronchial asthma, extensive emphysema, massive pneumonia, asphyxia)
• Depression of the respiratory center (for example: morphine poisoning)

Compensation of Respiratory Acidosis

• Increase the reabsorption of HCO3 –
• Increase the excretion of H+ in the urine

Respiratory Alkalosis

• Respiratory Alkalosis decreases the blood levels of H2CO3 due to hyperventilation
• Hyperventilation leads to increased excretion of CO2 by lungs (for example : in fevers and High altitudes )

Compensation of Respiratory Alkalosis

• Increase the excretion of HCO3 –
• Decrease the excretion of H+ in the urine

Metabolic Alkalosis

• Metabolic Alkalosis includes increase loss of acids and excessive absorption of bases

Increase Loss of Acids

• Loss of gastric juice (containing HCl) by vomiting in cases of pyloric stenosis or by prolonged suction in major operations.

Excessive Absorption of Bases

• Intake of large amounts of vegetables and fruits in diet which are rich in salts of organic acids (for example: sodium and potassium citrates) and bicarbonates.
• Administration of alkalis (for example: sodium and potassium citrates and bicarbonates) for treatment of peptic ulcer and hyperacidity.

Compensation for Metabolic Alkalosis

• Decrease the rate of respiration to increase CO2 in blood.
• Increase the excretion of HCO3 –
• Decrease the excretion of H+ in the urine