Metabolic acidosis

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Metabolic acidosis

Renal system


Metabolic acidosis


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Metabolic acidosis

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USMLE® Step 1 style questions USMLE

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A 62-year-old man comes to his primary care physician for a routine visit. The patient has no acute complaints, but he feels out of breath after walking 3-4 blocks. The patient has a 50-pack-year smoking history and consumes one pack of cigarettes per day. His temperature is 37.0°C (98.6°F), pulse is 77/min, blood pressure is 159/83 mmHg, and respirations are 12/min. Physical exam reveals a barrel shaped chest, bilateral wheezing, and a prolonged expiratory phase. Routine labs are ordered. Which of the following sets of findings would be expected given this patient’s clinical presentation?

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Acetazolamide p. 253, 573, 632

metabolic acidosis p. 616

Addison disease p. 355

metabolic acidosis in p. 616

Diabetic ketoacidosis (DKA) p. 357

metabolic acidosis p. 616


metabolic acidosis from p. 616

Ethylene glycol

metabolic acidosis p. 616


metabolic acidosis p. 616


metabolic acidosis p. 616

Lactic acidosis

metabolic acidosis p. 616

Metabolic acidosis p. 616

adrenal insufficiency p. 355

neonatal respiratory distress syndrome p. 685

renal failure p. 627

symptoms of p. 616

Renal tubular acidosis

metabolic acidosis p. 616


metabolic acidosis p. 616

Spironolactone p. 633, 669, 679, 682

metabolic acidosis p. 616


metabolic acidosis p. 616


With metabolic acidosis, “acidosis” refers to a process that lowers blood pH below 7.35, and “metabolic” refers to the fact that it’s a problem caused by a decrease in the bicarbonate HCO3− concentration in the blood.

Normally, blood pH depends on the balance or ratio between the concentration of bases, mainly bicarbonate HCO3−, which increases the pH, and acids, mainly carbon dioxide CO2, which decrease the pH.

The blood pH needs to be constantly between 7.35 and 7.45, and in addition the blood needs to remain electrically neutral, which means that the total cations, or positively charged particles, equals the total anions, or negatively charged particles.

Now, not all of the ions are easy or convenient to measure, so typically the dominant cation, sodium Na+, which is typically around 137 mEq/L and the two dominant anions, chloride Cl−, which is about 104 mEq/L, and bicarbonate HCO3−, which is around 24 mEq/L, are measured.

The rest are unmeasured. So just counting up these three ions, there’s usually a difference, or “gap” between the sodium Na+ concentration and the sum of bicarbonate HCO3− and chloride Cl− concentrations in the plasma, which is 137 minus 128 (104 plus 24) or 9 mEq/L.

This is known as the anion gap, and normally it ranges between 3 and 11 mEq/L. The anion gap largely represents unmeasured anions like organic acids and negatively charged plasma proteins, like albumin.

So, basically, metabolic acidosis arises either from the buildup of acid in our blood, which could be because it’s produced or ingested in increased amounts, or because the body can’t get rid of it, or from excessive bicarbonate HCO3− loss from the kidneys or gastrointestinal tract.

The main problem with all of this is that they lead to a primary decrease in the concentration of bicarbonate HCO3− in the blood.

They can be broken down to two categories, based on whether the anion gap is high or normal. So, the first category of metabolic acidosis is a high anion gap metabolic acidosis.

In this case, the bicarbonate HCO3− ion concentration decreases by binding of bicarbonate HCO3− ions and protons H+, which results in the formation of H2CO3 carbonic acid, which subsequently breaks down into carbon dioxide CO2 and water H2O.


Metabolic acidosis is a condition in which the body produces too much acid (e.g. acetic acid or ketones), or excessively loses bicarbonate ions (e.g. diarrhea or renal failure). In metabolic acidosis, the respiratory center is stimulated in order to compensate for the acidosis and the individual hyperventilates, leading to dyspnea. Other associated symptoms are related to the underlying cause, for example, in diabetic ketoacidosis there's nausea and vomiting.


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  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Metabolic acidosis: pathophysiology, diagnosis and management" Nature Reviews Nephrology (2010)
  6. "Metabolic acidosis" Acta Med Indones (2007)
  7. "Management of the Metabolic Acidosis of Chronic Kidney Disease" Advances in Chronic Kidney Disease (2017)
  8. "Respiratory Acid–Base Disorders in the Critical Care Unit" Veterinary Clinics of North America: Small Animal Practice (2017)
  9. "Respiratory acidosis" Respir Care (2001)

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