Venous Blood pH Calculator

Arterial vs Venous Blood Gas

Blood gas analysis measures pH, partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂), and bicarbonate concentration in blood samples. While arterial blood gas (ABG) analysis is the gold standard, venous blood gas (VBG) analysis is increasingly used as a less invasive screening tool.

Venous blood has slightly different reference values compared to arterial blood because it carries metabolic waste products (CO₂) from the tissues back to the lungs. Venous blood has a lower pH, higher PCO₂, and slightly higher bicarbonate than arterial blood.

Parameter	Arterial (ABG)	Venous (VBG)
pH	7.35 – 7.45	7.31 – 7.41
PCO₂ (mmHg)	35 – 45	41 – 51
HCO₃⁻ (mEq/L)	22 – 26	22 – 26
PO₂ (mmHg)	80 – 100	30 – 50

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates pH to the ratio of bicarbonate (the base, or metabolic component) to dissolved CO₂ (the acid, or respiratory component):

Where:

• 6.1 is the pKa of carbonic acid (H₂CO₃) at body temperature
• HCO₃⁻ is bicarbonate concentration in mEq/L (or mmol/L)
• 0.0307 is the solubility coefficient of CO₂ in blood at 37°C (in mmol/L/mmHg)
• PaCO₂ is the partial pressure of CO₂ in mmHg

If PaCO₂ is given in kPa, convert to mmHg by multiplying by 7.5006.

Normal Reference Values

Parameter	Normal Venous Range	Unit
pH	7.31 – 7.41	—
PaCO₂	41 – 51	mmHg
HCO₃⁻	22 – 26	mEq/L
Base Excess	−2 to +2	mEq/L

Acid-Base Diagram

Acid-Base Disorders

Disorder	pH	Primary Change	Common Causes
Respiratory Acidosis	↓	↑ PaCO₂	COPD, pneumonia, hypoventilation, opioid overdose, neuromuscular disease
Metabolic Acidosis	↓	↓ HCO₃⁻	DKA, lactic acidosis, renal failure, diarrhea, toxins (methanol, ethylene glycol)
Respiratory Alkalosis	↑	↓ PaCO₂	Hyperventilation, anxiety, pain, fever, sepsis, high altitude
Metabolic Alkalosis	↑	↑ HCO₃⁻	Vomiting, NG suction, diuretics, volume contraction, Cushing syndrome

Compensation Mechanisms

The body maintains pH homeostasis through compensatory mechanisms. When one system (respiratory or metabolic) is disturbed, the other adjusts to minimize the pH change:

• Metabolic acidosis: Respiratory compensation — hyperventilation decreases PaCO₂ (Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2)
• Metabolic alkalosis: Respiratory compensation — hypoventilation increases PaCO₂ (expected PaCO₂ = 0.7 × HCO₃⁻ + 21 ± 2)
• Respiratory acidosis (acute): Metabolic compensation — HCO₃⁻ increases ~1 mEq/L per 10 mmHg rise in PaCO₂
• Respiratory acidosis (chronic): Renal compensation — HCO₃⁻ increases ~3.5 mEq/L per 10 mmHg rise in PaCO₂
• Respiratory alkalosis (acute): HCO₃⁻ decreases ~2 mEq/L per 10 mmHg drop in PaCO₂
• Respiratory alkalosis (chronic): HCO₃⁻ decreases ~5 mEq/L per 10 mmHg drop in PaCO₂

Full respiratory compensation occurs within minutes to hours, while full renal (metabolic) compensation takes 3-5 days.

Worked Example

A venous blood sample shows HCO₃⁻ = 24 mEq/L and PaCO₂ = 46 mmHg:

This pH of 7.330 falls within the normal venous range (7.31–7.41), indicating normal acid-base status.

Frequently Asked Questions

Can a VBG replace an ABG?

A venous blood gas can be used as a screening tool in many clinical settings. Studies show that venous pH correlates well with arterial pH (difference typically < 0.04 units). However, ABG remains necessary when precise PaO₂ measurement is required (e.g., assessing oxygenation in respiratory failure) or when exact arterial pH values are critical.

Why is venous pH lower than arterial pH?

Venous blood carries CO₂ produced by tissue metabolism back to the lungs. This additional CO₂ dissolves in blood to form carbonic acid, lowering the pH. The typical difference between arterial and venous pH is approximately 0.02-0.04 units.

What does HCO₃⁻ represent?

Bicarbonate (HCO₃⁻) is the metabolic component of acid-base balance. It is regulated by the kidneys and acts as the primary buffer system in blood. A normal HCO₃⁻ is 22-26 mEq/L. Low HCO₃⁻ suggests metabolic acidosis; high HCO₃⁻ suggests metabolic alkalosis (or compensation for respiratory acidosis).

What is the significance of PaCO₂?

PaCO₂ represents the respiratory component of acid-base balance. It is regulated by the lungs through ventilation. High PaCO₂ (hypercapnia) indicates inadequate ventilation and respiratory acidosis; low PaCO₂ (hypocapnia) indicates hyperventilation and respiratory alkalosis.

How do I convert PaCO₂ from kPa to mmHg?

Multiply the kPa value by 7.5006 to get mmHg. For example, 6.0 kPa × 7.5006 = 45.0 mmHg. This calculator handles this conversion automatically when you select kPa as the unit.