Bicarbonate, often represented as \(\text{HCO}_3\) on a blood test, is a negatively charged ion that functions as a powerful chemical buffer in the bloodstream. It is considered one of the body’s most important electrolytes, working alongside sodium, potassium, and chloride to maintain electrical neutrality and fluid balance. The \(\text{HCO}_3\) level is most commonly measured as part of a standard diagnostic tool called the Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP). This measurement assesses the total amount of carbon dioxide (\(\text{CO}_2\)) content in the blood, which is predominantly bicarbonate, providing a quick estimate of the acid-base balance.
Bicarbonate’s Role in pH Balance
The human body must maintain a very narrow, slightly alkaline blood pH range, typically between 7.35 and 7.45, for metabolic processes to function correctly. The bicarbonate buffer system is the primary mechanism responsible for defending against significant shifts in this delicate balance. Bicarbonate acts as a weak base that can readily absorb excess hydrogen ions (\(\text{H}^+\)), which are the molecular components that determine acidity.
This buffering action is based on a reversible chemical reaction involving carbon dioxide, water, carbonic acid (\(\text{H}_2\text{CO}_3\)), and bicarbonate (\(\text{HCO}_3\)). When the blood becomes too acidic, bicarbonate combines with the excess \(\text{H}^+\) to form carbonic acid, which then dissociates into water and \(\text{CO}_2\). The lungs quickly expel this newly formed \(\text{CO}_2\) through increased breathing, rapidly removing the acid component from the system.
The kidneys provide the long-term control of this system by regulating the amount of bicarbonate in the body. If the body is too acidic, the kidneys conserve bicarbonate and excrete \(\text{H}^+\) into the urine. Conversely, if the body is too alkaline, the kidneys excrete excess bicarbonate, demonstrating a collaborative effort between the lungs and kidneys to preserve the narrow pH range.
The Bicarbonate Blood Test: Measurement and Normal Ranges
The bicarbonate test is typically ordered to evaluate a person’s general metabolic status, screen for electrolyte imbalances, or monitor chronic conditions like kidney disease. The laboratory measurement usually reports the total carbon dioxide (\(\text{TCO}_2\)) content in the blood plasma. Since \(\text{TCO}_2\) is composed of approximately 90% to 95% bicarbonate, the result serves as a close proxy for the true bicarbonate concentration. The test is performed via a venous blood draw.
For most adults, the typical reference range for \(\text{TCO}_2\) or bicarbonate falls between 22 and 30 milliequivalents per liter (\(\text{mEq/L}\)) or millimoles per liter (\(\text{mmol/L}\)). A physician may order this test when a person presents with symptoms like persistent vomiting, unexplained weakness, or difficulty breathing, which can signal an underlying acid-base disturbance. Regular testing is also common for individuals taking medications like diuretics that can affect electrolyte balance or for those with kidney or lung conditions.
Implications of Low Bicarbonate Levels
A low bicarbonate level, generally below \(22\ \text{mEq/L}\), indicates a state known as metabolic acidosis, meaning the blood has become too acidic. This condition arises when the body produces too much acid or loses too much base, and the bicarbonate reserve has been depleted attempting to neutralize the excess acid. Metabolic acidosis can be caused by the loss of alkali from the body, such as through severe, prolonged diarrhea.
Another common cause is the overproduction of acids within the body, most notably seen in Diabetic Ketoacidosis (DKA). In DKA, a lack of insulin causes the body to break down fat for energy, producing acidic ketone bodies that overwhelm the buffer system. Kidney failure is also a frequent cause, as impaired kidneys cannot effectively excrete the normal daily load of metabolic acids or retain enough bicarbonate.
The body attempts to compensate for metabolic acidosis primarily through the respiratory system by increasing the rate and depth of breathing, a pattern known as hyperventilation. When the acidosis is severe, this compensatory breathing can become very deep and labored, which is specifically referred to as Kussmaul breathing. This intense breathing pattern serves to expel more \(\text{CO}_2\), thereby shifting the chemical equilibrium to raise the blood pH.
Implications of High Bicarbonate Levels
An elevated bicarbonate level, typically above \(30\ \text{mEq/L}\), suggests metabolic alkalosis, a condition where the blood has become too alkaline or basic. This imbalance can occur due to either the excessive loss of acid or the gain of too much base in the body. One of the most frequent causes is the loss of gastric acid from the stomach, which occurs with prolonged and severe vomiting or the use of nasogastric suctioning.
The loss of stomach acid leaves the body with a relative excess of bicarbonate, leading to an alkaline state. Certain medications, like specific diuretic “water pills,” can also contribute to metabolic alkalosis by causing the kidneys to excrete more acid and retain bicarbonate. Furthermore, the excessive intake of base, such as from large amounts of antacids containing sodium bicarbonate, can occasionally lead to this condition, especially in individuals with reduced kidney function.
In response to metabolic alkalosis, the body’s compensatory mechanism involves the respiratory system slowing down the breathing rate. This reduction in ventilation is meant to retain \(\text{CO}_2\), which acts as an acid in the blood, helping to lower the overall pH. Symptoms of metabolic alkalosis can range from mild, like tingling in the extremities or muscle cramping, to more severe neurological effects such as confusion or stupor in critical cases.