Ch26_ pt 2

Disorders of Acid-Base Balance

Metabolic Compensation

• Metabolic and renal compensation for respiratory diseases that can lead to acidosis involves the conservation of bicarbonate ions.

• In respiratory acidosis:

  • The kidneys increase bicarbonate conservation.

  • H+ ions are secreted through an exchange mechanism.

  • This process raises the concentration of bicarbonate in the blood, re-establishing the correct balance with carbonic acid.

• In respiratory alkalosis:

  • The kidneys decrease bicarbonate production.

  • H+ ions are reabsorbed from the tubular fluid.

  • This mechanism can be limited by the K+-H+ exchange (antiporter) used by renal cells.

Diagnosing Acidosis and Alkalosis

• Lab tests for pH, CO2 partial pressure (PCO2), and HCO3 are crucial in identifying acidosis and alkalosis.

• The tests indicate whether the imbalance is metabolic or respiratory and assess the effectiveness of compensatory mechanisms:

  • Blood pH Value: Classifies as acidosis, normal, or alkalosis.

  • PCO2 and Total HCO3 Values: Help determine the nature of the imbalance and compensatory status.

• Conditions and Laboratory Results (see Table 26.3 for reference values):

  • Metabolic acid-base imbalances often stem from kidney disease, with respiratory system compensation.

Types of Acidosis and Alkalosis

• Acidosis and Alkalosis Characteristics

  • Metabolic Acidosis: pH ↓, PCO2 N → N, then ↓

  • Respiratory Acidosis: pH ↓, PCO2 ↑ → N, then ↑

  • Metabolic Alkalosis: pH ↑, PCO2 N → N, then ↑

  • Respiratory Alkalosis: pH ↑, PCO2 N → N, then ↓

• Reference Values (Arterial):

  • pH: 7.35-7.45

  • pCO2: Male: 35-48 mm Hg, Female: 32-45 mm Hg

  • Total venous bicarbonate: 22-29 mM

  • N = Normal; ↑ = Increased; ↓ = Decreased.

Problematic Conditions

• Metabolic Acidosis:

  • Characterized by lower-than-normal bicarbonate in the blood.

  • Normal PCO2 initially; decreases with compensation, aiming to restore bicarbonate and carbonic acid ratio.

• Respiratory Acidosis:

  • Presence of excess CO2 in the blood.

  • Normal bicarbonate levels initially; if compensated, bicarbonate increases to restore balance.

• Alkalosis:

  • Higher-than-normal pH levels.

  • Metabolic Alkalosis: Presence of excess bicarbonate.

    • Initially normal PCO2; increases with compensation to restore balance.

  • Respiratory Alkalosis:

    • CO2 deficiency in the bloodstream, normal bicarbonate levels initially; renal compensation leads to decreased bicarbonate concentration to restore pH.