Acid-Base Disturbances

Table of Contents

How Acid-Base Balance is Maintained

Our bodies are huge and complex chemical factories with many reactions going on within our cells. Every day, our metabolism produces huge amounts of acids that our bodies must manage to avoid deadly acidemia. These acids are expired/exhaled (by the lungs), excreted (by the kidneys), metabolized to non-charged neutral molecules, and/or buffered to avoid fatal acidemia. Three major classes of acids produced are¹:

Carbon dioxide which combines with water to form carbonic acid. i.e. CO₂ + H₂O <—> H₂CO₃ <—> HCO₃^– + H⁺. More CO₂ is produced when we exercise.
Organic acids e.g. lactic acid and citric acid. These acids are metabolized to neutral products (such as glucose) and to CO2 and water. Under normal circumstances, the quantities of these acids that are produced are equal to the quantities used so that at a steady state, their concentrations are low and stable.
Nonvolatile acids (mostly sulfuric acid derived from the metabolism of sulfur-containing amino acids in the diet).

Acid-base balance is maintained as the lungs exhale carbon dioxide (as part of their normal routine), our metabolism uses the organic acids produced, and our kidneys excrete nonvolatile acids.

Acid-base balance is usually assessed in terms of the bicarbonate-carbon dioxide buffer system:

Dissolved CO₂ + H₂O <—> H₂CO₃ <—> HCO₃^– + H⁺

“Changes in pH are caused by an imbalance in the CO₂ (respiratory) or HCO₃– (metabolic). These work as buffers to keep the pH within a set range and when there is an abnormality in either of these the pH will be outside of the normal range.” (Geeky Medics)

pH = – log ([H⁺] .i.e. pH is equal to the negative log of the hydrogen ion concentrations. That means if the hydrogen ions go up, the pH goes down.

The Henderson-Hasselbalch gives us the pH.
pH = 6.10 + log ([HCO₃^–] ÷ [0.03 x pCO₂])
That is:

In simple terms, pH= constant x ([HCO₃^–] ÷ [paCO₂])

Definitions

Acidemia and Alkalemia vs. Acidosis and Alkalosis
The difference b/n an –emia and an –osis. An –emia is a state of being e.g. hyperkalemia, hyponatremia. It is a state that has been reached. An –osis is a process that is occurring. You can have many such processes occurring at the same time. You can be alkalemic and still have an acidosis going on. You can have an acidosis and an alkalosis going on in the same person simultaneously. E.g. if a person is vomiting, that is an alkalosis process. If they are having diarrhea at the same time, that is an acidosis process. But at a given time, a person can only be defined as having acidemia or alkalemia but not both.
Acidemia = an arterial pH <7.35, and alkalemia = an arterial pH > 7.45
Acidosis = a process that increases [H⁺] and tends to lower the pH. This can be caused by either a decrease in the serum bicarbonate (HCO3) concentration or a rise in PCO2 or both.
Alkalosis = a process that decreases [H⁺] and tends to increase the pH. This can be caused by either a rise in the serum HCO3 concentration or a fall in PCO2 or both.

Primary Disorders

Primary Acid-Base Disorders.

Compensation by the Lungs and Kidneys

From the Henderson-Hasselbalch equation, pH is determined by the ratio of bicarb and pCO₂_^. None of them alone determines the pH.

Respiratory compensation by the lungs is done either by hyperventilating or hypoventilating to alter the pCO₂ and counteract a primary metabolic process. Note that the lungs can only do one at a time. The lungs can’t both hypoventilate and hyperventilate simultaneously.
Renal compensation by the kidneys is done by excreting or retaining H⁺or HCO₃^– to counteract a primary respiratory process. e.g. In respiratory acidosis, pCO2 rises causing the pH to go down (acidosis). The kidneys compensate by increasing H⁺secretion (which increases the serum [HCO₃^–]) which counteracts the pH change. In respiratory alkalosis, the pCO2 decreases producing alkalosis. The kidneys respond by increasing H⁺retention (which decreases serum HCO3).

It’s important to note two things:

Respiratory compensation occurs within minutes while full renal compensation takes 3-5 days to complete.
The compensation never fully corrects the pH. If the pH is normal, consider a mixed disorder. For example, if there is a primary metabolic acidosis that lowers the pH, the respiratory compensation will raise the pH towards normal, mitigating the decrease in pH but never fully correcting it and bring the pH to normal. The same is true if it were a primary respiratory acid-base disorder with metabolic compensation.
In primary metabolic disorders, the pH matches in the same direction as the pCO2 (and bicarb). That is the pH changes in the same direction as the pCO2 and bicarb. E.g. in metabolic acidosis, the bicarb goes down, the pH goes down as well, and then lungs compensate by reducing the pCO2 as well. In a primary metabolic disorder, everything matches in the same direction.
In a primary respiratory disorder, the pH goes in a reverse direction to pCO2.
Compensation for a primary respiratory acid-based disorder occurs in two phases. This is because the kidneys take time (3-5 days) to do their job. In the initial acute phase, there is a small rapid change in serum HCO3, to counteract the respiratory acid-based disorder. This initial change in bicarb comes from buffering mechanisms throughout the whole body. The kidneys are not yet involved. If the respiratory disorder lasts for minutes to hours, the kidney response kicks in and the kidneys produce larger changes in serum HCO3. These acute and chronic HCO3 changes counteract the changes in pH. Because of this, the expected acute response (from whole-body buffering without significant renal compensation) and the chronic response (full renal compensation) in respiratory acid-base disorders are very different.
The expected degree of compensation for each acid-base disorder has been determined by empirical studies in humans. For primary metabolic acid-based disorders, the degree of compensation is defined by the decrease or increase in arterial PCO2 from its normal range. For primary respiratory acid-based disorders, the degree of compensation is defined by the decrease or increase in serum HCO3 from its normal range.
Compensation Equations for Acid-Base Disorders.

My Approach to Diagnosing Acid-Base Disturbances

My Approach to Diagnosing Acid-Base Disturbances.

What happens when the body doesn’t maintain the pH within appropriate levels?

Proteins get denatured, enzymes don’t work as they should.

Acid-Base Articles

Key Points

“When the HCO3- is measured in venous blood, it is usually measured directly as “total CO2” with an ion-selective electrode. The directly measured venous “total CO2” is generally approximately 2 mEq/L greater than the simultaneously calculated arterial HCO3-.” UTD

Excellent videos on the topic

References / Resources

A) Tsapenko, Mykola V. “Modified delta gap equation for quick evaluation of mixed metabolic Acid-base disorders.” Oman medical journal vol. 28,1 (2013): 73-4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3562975/

B) www.uptodate.com/contents/simple-and-mixed-acid-base-disorders

C) www.uptodate.com/contents/approach-to-the-adult-with-metabolic-acidosis

D) www.aliem.com/2014/11/henderson-hasselbalch-belong-ed/

E) https://geekymedics.com/abg-interpretation