17. Disturbances in acid-base balance – general characteristics, regulation, parameters, compensations. Respiratory and metabolic acidosis.

DISTURBANCES IN ACID-BASE BALANCE - GENERAL CHARACTERISTICS, PARAMETERS AND COMPENSATION

• Acids are H+ donors
• Bases are H+ acceptors or give up OH- in solution
• Acids and bases can be:

1. a) Strong → dissociate completely in solution (HCl)
2. b) Weak → dissociate partially (lactic acid)

pH

• Normal pH value = 7.35 - 7.45
• If [H+] is high → acidic, pH < 7
• If [H+] is low → basic, pH > 7
• Generally, in the body there are more acids than bases because:
• More acids are taken in with food
• More acids are produced by breakdown of lipids and proteins
• CO2 produced by cellular metabolism
• Small changes in pH can produce major disturbances:
• Enzyme function
• Electrolytes
• Hormones

Regulation of pH

1) Buffer systems

• Function almost instantaneously
• Weak acids and weak bases make good buffers
• They take up H+ or release H+ as conditions change
• They exchange a strong acid/base for a weak one resulting in a much smaller pH change
• Examples = bicarbonate system, phosphate, hemoglobin, plasma proteins

2) Respiratory mechanisms

• Takes several minutes to hours
• Exhalation of CO2
• CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3 -
• Powerful mechanism, but only works with volatile acids (carbonic acid) → doesn’t affect fixed acids like lactic acid
• The body’s pH can be adjusted by changing the rate and depth of breathing

3) Kidney excretion

• May take several hours to days
• Can eliminate large amounts of acid, and can also excrete bases
• Can conserve and produce bicarbonate ions
• Kidneys also possess tubular buffer systems (ammonia and phosphate)
• It is the most effective regulator of pH
• If the kidneys fail, pH balance fails also

Lines of Defence against pH shift

Summary

• First line of defense against pH shift: Chemical buffer system
• Bicarbonate buffer system
• Phosphate buffer system
• Protein buffer system
• Second line of defense against pH shift: Physiological buffers
• Respiratory mechanism (CO₂ excretion)
• Renal mechanism (H⁺ excretion)

Normal Laboratory Values

Arterial Blood Gas

Plasma Electrolytes