Physiology, Acid Base Balance

In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan.


To maintain homeostasis, the human body employs many physiological adaptations. One of these is maintaining an acid-base balance. In the absence of pathological states, the pH of the human body ranges between 7.35 to 7.45, with the average at 7.40. Why this number? Why not a neutral number of 7.0 instead of a slightly alkaline 7.40? A pH at this level is ideal for many biological processes, one of the most important being the oxygenation of blood. Also, many of the intermediates of biochemical reactions in the body become ionized at a neutral pH, which causes the utilization of these intermediates to be more difficult.

A pH below 7.35 is an acidemia, and a pH above 7.45 is an alkalemia. Due to the importance of sustaining a pH level in the needed narrow range, the human body contains compensatory mechanisms. This discussion intends to impart a basic understanding of acid-base balance in the body while providing a systematic way to approach patients who present with conditions causing alterations in pH.

The human body experiences four main types of acid-based disorders: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. If one of these conditions occurs, the human body should induce a counterbalance in the form of an opposite condition. For example, if a person is experiencing a metabolic acidemia, their body will attempt to induce a respiratory alkalosis to compensate. It is rare for the compensation to make the pH completely normal at 7.4. When using the term acidemia or alkalemia, one is denoting that overall the pH is acidic or alkalotic, respectively. While not necessary, it can be useful to employ this terminology to distinguish between individual processes and the overall pH status of the patient since multiple imbalances can happen at the same time.

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