Acid-Base Balance Disorders: pH Homeostasis in the Body

pH Homeostasis

• pH of blood is tightly controlled between 7.35-7.45
• Acidosis (acidemia) occurs when pH is below 7.35
• Alkalosis (alkalemia) occurs when pH is above 7.45
• Small changes in pH can lead to disturbances in enzyme function, hormone and receptor function, and muscle, heart, and nervous system function

Defence Mechanisms

• Buffer systems: weak acid and base pairs that bind or release H+ ions to maintain pH
• Respiratory system: regulates CO2 levels, which affects pH
• Kidneys: regulate acid-base balance by reabsorbing and regenerating bicarbonate, excreting hydrogen ions, and eliminating large amounts of acids

Buffer Systems

• Bicarbonate buffer system: NaHCO3/H2CO3, main buffer in extracellular fluid
• Hemoglobin buffer system: HbO2/Hb
• Phosphate buffer system: HPO4/H2PO4
• Protein buffer system: carboxyl and amino groups

Respiratory System

• Concentration of carbonic acid directly proportional to pCO2
• Body pH can be adjusted by changing rate and depth of breathing

Kidneys Regulation

• Reabsorb and regenerate bicarbonate
• Excrete bases and eliminate large amounts of acids
• Failures in kidney function lead to pH balance failures

Acid-Base Balance Parameters

• pH: 7.35-7.45
• pCO2: 36-44 mmHg (40 mmHg)
• HCO3: 22-26 mmol/L (24 mmol/L)
• BE (base excess): 0±2
• Other parameters: Na+, K+, Ca2+, Mg2+, Cl-, lactate, total proteins, albumin, urea, creatinine, glycemia, anion gap

Acid-Base Balance Disorders

• Metabolic disorders: acidosis (decrease in pH) and alkalosis (increase in pH)
• Respiratory disorders: acidosis (increase in pCO2) and alkalosis (decrease in pCO2)
• Combined disorders: metabolic acidosis and respiratory alkalosis, metabolic alkalosis and respiratory acidosis

Compensation and Correction

• Compensation: mechanisms that help restore pH balance
• Correction: complete restoration of pH balance
• Respiratory compensation: hyperventilation or hypoventilation to change CO2 levels
• Metabolic compensation: kidneys regulate acid-base balance by reabsorbing and regenerating bicarbonate, excreting hydrogen ions, and eliminating large amounts of acids

Metabolic Acidosis

• Bicarbonate deficit: HCO3- < 22 mmol/L
• Causes: accumulation of acids, kidney failure, loss of bicarbonate, intoxication with acids
• Compensation: increased ventilation, renal excretion of hydrogen ions, potassium exchange with excess hydrogen ions

Metabolic Alkalosis

• Bicarbonate excess: HCO3- > 26 mmol/L
• Causes: excessive vomiting, excessive use of alkaline drugs, certain diuretics, endocrine disorders, severe dehydration
• Compensation: decreased ventilation, difficult due to limited hypoventilation

Respiratory Acidosis

• Carbonic acid excess: pCO2 > 45 mmHg
• Causes: chronic conditions (depression of respiratory center, paralysis of respiratory or chest muscles, emphysema) and acute conditions (ARDS, pulmonary edema, pneumothorax, asphyxia)
• Compensation: metabolic compensation by kidneys

Respiratory Alkalosis

• Carbonic acid deficit: pCO2 < 35 mmHg
• Causes: hyperventilation, oxygen deficiency at high altitude, pulmonary disease, fever, acute anxiety, early salicylate intoxication, cirrhosis, Gram-negative sepsis
• Compensation: kidneys decrease secretion of hydrogen ion and excrete bicarbonate ion