Notes on Renal Control in Acid-Base Balance

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Buffering Systems:

Control concentration of H+ ions and other elements that affect H+ concentration.

Metabolic Disorder: Plasma pH: Plasma HCO3- Arterial PCO2 Compensatory Response:
Metabolic Acidosis low low low Hyperventilation; increased excretion of H+ and titratable acids
Metabolic Alkalosis high high high Hypoventilation; increased absorption of H+ and titratable acids
Mixed Acidosis low low high
Mixed Alkalosis high high low
Respiratory Acidosis low high high Increased renal HCO3- reabsorption; increased H+ excretion
Respiratory Alkalosis high low low Decreased renal HCO3- reabsorption; reduced H+ excretion

Main Acid-Base Equation:

CO2 equals H+ plus HCO3-

Respiratory Problems (left of above equation):

Respiratory Acidosis:

• Hypoventilation.

• Increased CO2, H+.

• Decreased pH.

• Increased bicarbonate ion.

• Acute or uncompensated: increased HCO3- (less than 30 mEq/L).

• Chronic or completely compensated: increased HCO3- (greater than 30 mEq/L).

Respiratory Alkalosis:

• Hyperventilation.

• Decreased CO2, H+.

• Increased pH.

• Uncompensated: decreased CO2.

• Decreased bicarbonate ion.

Combined:

• CO2 depressed, bicarbonate elevated.

Metabolic problems (right of above equation):

Metabolic Acidosis:

• Gain in fixed acid.

• Increased H+.

• Extremely low bicarbonate ion.

• Compensation: reaction forced to left, consuming H+.

• Compensation is hyperventilation: decreased CO2.

• Compensation is bicarbonate production by kidney.

• Partially compensated: slightly decreased bicarbonate; decreased CO2.

• Uncompensated: extremely low bicarbonate.

Metabolic Alkalosis:

• Decreased H+.

• Compensation: reaction forced to right, producing H+.

• Extremely high bicarbonate ion.

• Compensation is hypoventilation: increased CO2.

• Compensation is bicarbonate excretion by kidney.

• Partially compensated: slightly increased bicarbonate.

• Uncompensated: extremely high bicarbonate.

• Next step in managing patient with suspected metabolic alkalosis would be to measure urinary chloride levels; metabolic alkalosis due to vomiting results in decreased chloride levels, and metabolic alkalosis due to diuretics and Conn syndrome results in increased urinary chloride levels.

Combined:

• CO2 and bicarbonate concentrations move in opposite directions.

To Determine the Type of Disorder:

• pH, then CO2.

• Acidic lung = compensatory basic body (and vice versa) (?).

Normal Anion Values:

• pH = 7.4 +/- 0.1.

• PCO2 = 40 mm Hg +/- 10.

• HCO3- = 24 mmol/L.

Anion Gap:

• Due to unmeasured anions.

• Anions are estimated from plasma Cl- and HCO3-.

• Cations are estimated from plasma Na+.

Henderson-Hasselbalch Equation:

• pH = pK + log [HCO3- / (0.03 x pCO2)]

• pK ~ 6.1

Additional Readings:

Basic Nephrology

1. Renal Control in Acid-Base Balance
2. Renal Processes
3. Renal Clearance
4. Regional Transport
5. What are the indications for hemodialysis?

Related Topics

1. Kidney Disorders
2. Histology of the Urinary System
3. Acid-Base Disturbance: Acidotic or Alkalotic? [Size: 427 kB; Format: PDF]
4. Abdominal Examination for Internal Medicine

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