Acid-base disturbances: Pathology review

00:00 / 00:00



Acid-base disturbances: Pathology review



Acid-base disturbances: Pathology review

USMLE® Step 1 questions

0 / 9 complete


USMLE® Step 1 style questions USMLE

of complete

A 57-year-old man comes to the emergency department for evaluation of acute-onset shortness of breath and chest pain that began three hours ago. The patient reports poorly-localized chest pain worse with inspiration. He is a business executive and recently returned from an overseas trip. Past medical history includes hypertension and hyperlipidemia. His temperature is 37.3°C (99.1°F), pulse is 91/min, blood pressure is 142/71 mmHg, and respirations are 21/min. Breath sounds are clear to auscultation bilaterally. Examination of the lower extremities reveals swelling and erythema in the right calf. Dorsalis pedis and posterior tibial pulses are 3+ bilaterally. A chest CT is ordered and reveals the following:

Image reproduced from Radiopedia

An arterial blood gas is pending. Which of the following findings will most likely be present on laboratory analysis?  


Two people came into the Emergency Department one day. The first one is 33 year old Muriel who came in with abdominal pain, a severe headache and hyperventilation. One of Muriel’s friends said that she caught her drinking antifreeze. The other one is 35 year old Eustace who came in with confusion and hypoventilation. Eustace also has duodenal ulcers, for which he has been taking antacids. Among other tests, an ABG was done for both individuals. The results showed that Muriel had low pH, along with low levels of bicarbonate and low levels of pCO2, while Eustace had high pH, along with high levels of bicarbonate and high levels of pCO2.

Okay, based on lab results, both individuals seem to have acid-base disturbances. Now, let’s go back to the basics for a bit. So, in plasma you can find carbon dioxide or CO2 and water or H2O. They are constantly mixing together in order to make bicarbonate ion or HCO3− and hydrogen ion or H+. Similarly, HCO3− and H+ can form CO2 and H2O.

Now, HCO3 − is mostly regulated by the kidneys and metabolism, while CO2 is regulated by the lungs. The blood pH which corresponds to the hydrogen ion concentration needs to stay in a very narrow range, between 7.37 and 7.42. Basically, the more hydrogen ions, the more acidic the blood is and the lower the pH. Less hydrogen ions means the blood is more alkaline, and the higher the pH. So, let’s say that HCO3− levels decrease for some reason. In this case, the equation shifts to the right and more HCO3− and H+ will be produced and as a result the blood becomes more acidic, so pH levels decrease. On the other hand, if HCO3− levels rise, less H+ will be produced and the pH rises. Now, if CO2 increases, then the equation shifts to the right and the pH drops. If CO2 decreases, then the equation shifts to the left and the pH rises. Stay with us here. In practice, the Henderson-Hasselbalch equation is used to calculate the pH based on HCO3 and pCO2 values, where pCO2 represents the partial pressure of carbon dioxide. Now, In order not to overcomplicate things here, just remember, If HCO3 goes up or if pCO2 goes down, then pH increases and if HCO3 goes down or if pCO2 goes up, then pH decreases.


Acid-base disturbances are a type of electrolyte imbalance that occurs when the body's pH balance is disturbed. The blood pH is maintained in a narrow delicate range of 7.35 to 7.45, which is optimal for many biological processes taking place in our body. Below that range, the blood is too acidic, and above it, it's too alkalic, which is not ideal.

The acid-base disturbances are divided into two major groups due to their causes and the clinical picture of the patient. First, there are metabolic disturbances that can either be metabolic acidosis or alkalosis, which are reflected by disturbances in the serum HCO3 ��. The second group consists of respiratory disturbances, which can be either respiratory acidosis or alkalosis, depending on the blood's Pco2. There are a variety of causes for acid-base disturbances, including dehydration, hypoventilation, kidney failure, and diabetic ketoacidosis.


  1. "Robbins Basic Pathology" Elsevier (2017)
  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Practical Renal Pathology, A Diagnostic Approach E-Book" Elsevier Health Sciences (2012)
  4. "Physiology E-Book" Elsevier Health Sciences (2017)
  5. "The Renal System" Churchill Livingstone (2010)
  6. "Metabolic acidosis: pathophysiology, diagnosis and management" Nature Reviews Nephrology (2010)
  7. "Treatment of acute metabolic acidosis: a pathophysiologic approach" Nature Reviews Nephrology (2012)

Copyright © 2023 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Cookies are used by this site.

USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.