Bicarbonate (CO₂, HCO₃) Test – Acid–Base Balance & Blood CO₂

What is Bicarbonate (CO₂, HCO₃⁻)?

Bicarbonate (HCO₃⁻) is a chemical form of carbon dioxide (CO₂) that circulates in the blood.
It plays a vital role in maintaining the body’s acid–base balance, ensuring the blood’s pH stays within a narrow range (7.35–7.45).

In simple terms, bicarbonate acts as a buffer, preventing the blood from becoming too acidic or too alkaline.
When doctors order a bicarbonate test, it’s usually part of a basic metabolic panel (BMP), electrolyte test, or arterial blood gas (ABG) analysis.

By checking bicarbonate levels, healthcare providers can understand how well your lungs and kidneys are working together to maintain the right acid–base balance in the body.

Where It Is Produced in the Body

Bicarbonate is not produced by one organ alone — it is part of a complex physiological system involving several organs:

1. Red Blood Cells:
   • Inside red blood cells, an enzyme called carbonic anhydrase converts carbon dioxide (CO₂) and water (H₂O) into carbonic acid (H₂CO₃), which then dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).
2. Kidneys:
   • The kidneys play a crucial role by reabsorbing bicarbonate and regenerating new bicarbonate to replace what is lost.
   • This helps keep the blood’s pH balanced even when acids build up.
3. Lungs:
   • The lungs regulate CO₂ levels through breathing. When you exhale CO₂, the body’s acid level decreases, indirectly influencing bicarbonate balance.

Together, these organs create a dynamic system that keeps the body’s acid-base homeostasis stable.

Main Functions and Importance of Bicarbonate

Bicarbonate serves multiple important roles in the body:

1. Maintains Acid–Base Balance

It helps buffer acids and bases, keeping blood pH between 7.35 and 7.45, which is essential for enzyme activity and normal cellular function.

2. Neutralizes Excess Acid

When acids build up due to metabolism or disease, bicarbonate binds with hydrogen ions to neutralize them, forming carbon dioxide and water.

3. Supports Lung and Kidney Coordination

The lungs remove carbon dioxide (acidic component) through exhalation, and the kidneys maintain bicarbonate concentration by reabsorption or excretion — ensuring balance between both systems.

4. Aids in Carbon Dioxide Transport

Bicarbonate acts as the main form of CO₂ transport from tissues to the lungs — converting back into CO₂ to be exhaled.

Because of these functions, measuring bicarbonate levels helps doctors identify respiratory or metabolic disorders, including acidosis and alkalosis.

Causes of Low Bicarbonate Levels (Metabolic Acidosis)

Low bicarbonate levels mean the blood is more acidic than normal — a condition known as metabolic acidosis.
This can happen due to excessive acid production, acid retention, or bicarbonate loss.

Common Causes:

• Kidney disease or failure: Kidneys lose their ability to retain bicarbonate.
• Severe diarrhea: Loss of bicarbonate-rich fluids from the intestines.
• Diabetic ketoacidosis: Excess acid (ketones) builds up due to uncontrolled diabetes.
• Lactic acidosis: Caused by oxygen deprivation (shock, sepsis, or strenuous exercise).
• Addison’s disease: Low adrenal hormone levels affect salt and acid balance.

Symptoms of Low Bicarbonate Levels

When bicarbonate is low, the body’s pH drops (acidic), leading to:

• Rapid or deep breathing (to remove excess CO₂)
• Fatigue and weakness
• Headache or confusion
• Nausea and vomiting
• Disorientation or drowsiness

If untreated, severe metabolic acidosis can become life-threatening.

Causes of High Bicarbonate Levels (Metabolic Alkalosis)

High bicarbonate levels indicate that the blood is more alkaline than normal, known as metabolic alkalosis.
This usually results from loss of acid or excessive bicarbonate intake.

Common Causes:

• Prolonged vomiting or gastric suctioning (loss of stomach acid)
• Overuse of antacids or bicarbonate supplements
• Chronic lung disease (CO₂ retention): The body compensates by increasing bicarbonate levels.
• Cushing’s syndrome: Overproduction of cortisol causes electrolyte imbalance.
• Hypokalemia (low potassium): Alters kidney bicarbonate handling.

Symptoms of High Bicarbonate Levels

When bicarbonate is high, the blood becomes too alkaline, leading to symptoms such as:

• Slow, shallow breathing (body tries to retain CO₂ to balance pH)
• Muscle twitching or cramps
• Tingling sensation in hands, face, or feet
• Irritability or nervousness
• Confusion or dizziness

If not corrected, metabolic alkalosis can interfere with muscle and heart function.

Reference Ranges

Normal bicarbonate levels may slightly vary depending on the lab and testing method.

Low levels suggest acidosis, while high levels indicate alkalosis.

Sample Type and Collection

Bicarbonate can be measured through two main types of blood tests:

1. Venous Blood Sample

• Collected from a vein in the arm.
• Commonly part of electrolyte panel or metabolic profile.
• No fasting required.

2. Arterial Blood Sample (ABG)

• Taken from an artery (usually wrist) to assess respiratory and metabolic balance.
• Gives detailed information on oxygen (O₂), carbon dioxide (CO₂), and bicarbonate (HCO₃⁻).

Test Preparation

• No fasting or special preparation required for a routine bicarbonate test.
• If undergoing an arterial blood gas (ABG) test, you may be asked to avoid oxygen therapy briefly before the sample.
• Inform your doctor about:
  • Medications such as diuretics, antacids, or steroids, which can affect bicarbonate levels.
  • Any recent illness involving vomiting, diarrhea, or dehydration.

When to Consult a Doctor

You should seek medical advice if you experience symptoms of acid-base imbalance, such as:

• Unexplained fatigue or confusion
• Rapid or shallow breathing
• Nausea, vomiting, or loss of appetite
• Muscle cramps or twitching
• Frequent acid reflux or chronic vomiting

Doctors may recommend a bicarbonate test to evaluate:

• Kidney function
• Respiratory conditions
• Metabolic disorders
• Unexplained electrolyte abnormalities

Early diagnosis can prevent serious complications from acidosis or alkalosis.

Important Word Explanations

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