What is acid-base balance and how is it maintained in the body?

From the Guidelines

Acid-base balance is crucial for maintaining optimal cellular function, and it refers to the body's regulation of hydrogen ion concentration (pH) in bodily fluids, particularly blood, which must be maintained within a narrow range of 7.35-7.45. The body maintains acid-base homeostasis through three primary mechanisms: buffer systems, the respiratory system, and the kidneys. First, buffer systems provide immediate chemical neutralization of acids or bases, with the bicarbonate buffer system being most important in blood, as noted in the BTS guideline for oxygen use in adults in healthcare and emergency settings 1. Second, the respiratory system regulates carbon dioxide levels through breathing rate and depth; increased ventilation eliminates CO2 (a potential acid) while decreased ventilation retains it, with carbon dioxide levels in the blood controlled by chemical sensors in the carotid body and brainstem 1. Third, the kidneys provide longer-term regulation by excreting or retaining hydrogen ions and bicarbonate, generating new bicarbonate, excreting excess acids in urine, and reabsorbing filtered bicarbonate. These systems work together continuously, with the buffers responding within seconds, the respiratory system within minutes, and the kidneys over hours to days. Disruptions to acid-base balance can result in conditions like acidosis (pH below 7.35) or alkalosis (pH above 7.45), which may be metabolic or respiratory in origin depending on the primary cause, and permissive hypercapnia may be tolerated in certain conditions, such as ARDS, with a target pH above 7.2 1. Key points to consider in acid-base balance include:

• The normal pH range of the blood in humans is between 7.35 and 7.45 units 1
• Acidosis is defined as a pH<7.35 and alkalosis is defined as a pH>7.45 1
• The respiratory system plays a critical role in regulating carbon dioxide levels, with increased ventilation eliminating CO2 and decreased ventilation retaining it 1
• The kidneys provide longer-term regulation of acid-base balance by excreting or retaining hydrogen ions and bicarbonate 1
• Permissive hypercapnia may be tolerated in certain conditions, such as ARDS, with a target pH above 7.2 1

From the FDA Drug Label

CLINICAL PHARMACOLOGY Intravenous sodium bicarbonate therapy increases plasma bicarbonate, buffers excess hydrogen ion concentration, raises blood pH and reverses the clinical manifestations of acidosis. Plasma concentration is regulated by the kidney through acidification of the urine when there is a deficit or by alkalinization of the urine when there is an excess. Bicarbonate anion is considered “labile” since at a proper concentration of hydrogen ion (H+) it may be converted to carbonic acid (H2CO3) and thence to its volatile form, carbon dioxide (CO2) excreted by the lung Normally a ratio of 1:20 (carbonic acid; bicarbonate) is present in the extracellular fluid. In a healthy adult with normal kidney function, practically all the glomerular filtered bicarbonate ion is reabsorbed; less than 1% is excreted in the urine.

Acid-base balance is maintained in the body through the regulation of plasma bicarbonate concentration by the kidney and the buffering of excess hydrogen ions.

• The kidney regulates plasma concentration by acidifying or alkalinizing the urine.
• The bicarbonate anion plays a crucial role in this process, as it can be converted to carbonic acid and then to carbon dioxide, which is excreted by the lung.
• A normal ratio of carbonic acid to bicarbonate is maintained in the extracellular fluid.
• In healthy adults with normal kidney function, nearly all filtered bicarbonate is reabsorbed, with less than 1% excreted in the urine 2.

From the Research

Definition of Acid-Base Balance

Acid-base balance refers to the mechanism by which the body maintains a stable pH level in the blood and other fluids, despite the continuous production of acidic and basic substances [ 3 ]. This balance is crucial for the proper functioning of various physiological processes.

Mechanisms of Acid-Base Balance

The body has several mechanisms to maintain acid-base balance, including:

• Respiratory compensation: The lungs can remove excess carbon dioxide, a potential acid, through increased ventilation [ 3 ].
• Renal compensation: The kidneys can excrete excess hydrogen ions, regenerate bicarbonate, and adjust the pH of the urine to maintain acid-base balance [ 4 ].
• Buffering: The body has buffer systems, such as the bicarbonate buffering system, that can neutralize excess hydrogen or hydroxide ions [ 5 ].

Classification of Acid-Base Disorders

Acid-base disorders can be classified into two main categories:

• Metabolic disorders: These occur when there is an abnormality in the production or excretion of non-carbonic acids or bicarbonate [ 3 ].
• Respiratory disorders: These occur when there is an abnormality in the ventilation or excretion of carbon dioxide [ 3 ].

Key Factors in Maintaining Acid-Base Balance

Some key factors that help maintain acid-base balance include:

• Bicarbonate reclamation: The kidneys can reabsorb bicarbonate from the urine to maintain plasma levels [ 4 ].
• Bicarbonate regeneration: The kidneys can regenerate bicarbonate from non-carbonic acids to maintain plasma levels [ 4 ].
• Electrolyte balance: The balance of electrolytes, such as sodium, potassium, and chloride, can affect acid-base balance [ 6 ].

Clinical Implications

Understanding acid-base physiology is essential for the diagnosis and treatment of various diseases, particularly in critical care medicine [ 6 ]. Accurate interpretation of acid-base data can help clinicians identify the underlying cause of acid-base disorders and develop effective treatment strategies [ 7 ].