What are the physiological differences between arterial and venous blood?

Physiological Differences Between Arterial and Venous Blood

Arterial and venous blood differ fundamentally in oxygen and carbon dioxide content, with arterial blood having significantly higher oxygen tension (PaO2 ~100 mm Hg vs venous ~45 mm Hg) and lower carbon dioxide tension (PaCO2 ~37 mm Hg vs venous ~52 mm Hg), while carboxyhemoglobin levels remain similar between the two under steady-state conditions. 1

Gas Content and Partial Pressures

Oxygen Differences

• Arterial blood maintains a PaO2 of approximately 13 kPa (100 mm Hg) in the pulmonary veins and systemic arterial circulation 1
• Venous blood (mixed systemic) has a markedly lower PaO2 of about 6 kPa (45 mm Hg) in the pulmonary artery 1
• This dramatic rise in oxygen tension occurs as blood passes through the pulmonary capillaries, where PaO2 increases from 45 mm Hg to approximately 120 mm Hg by the end of the pulmonary capillary 1
• The arterial oxygen saturation (SaO2) in healthy individuals should be 94-98%, representing the proportion of hemoglobin bound to oxygen 1

Carbon Dioxide Differences

• The carbon dioxide gradient is much more gradual than oxygen, falling from approximately 7 kPa (52 mm Hg) in the venous system and pulmonary artery to about 5 kPa (37 mm Hg) in the pulmonary vein and arterial system 1
• Carbon dioxide is highly soluble in blood and carried in three forms: bicarbonate (70-85%), dissolved CO2 (5-10%), and bound to hemoglobin (10-20%), with percentages varying between arterial and venous blood 1

Sampling Considerations for Clinical Practice

Carboxyhemoglobin Measurements

• Either arterial or venous blood may be used for carboxyhemoglobin (COHb) measurements, as the levels are similar under steady-state conditions when CO body stores are in equilibrium with lung CO partial pressure 2
• Under non-steady state conditions, venous COHb may be slightly above or below arterial COHb due to CO uptake or egress from tissues 2

Acid-Base Assessment in Circulatory Failure

• In severe circulatory failure and cardiac arrest, substantial arteriovenous differences emerge that are critical for clinical assessment 3
• In severe circulatory failure, mean arterial pH is 7.31 versus central venous pH of 7.21, with arterial PCO2 of 5.8 kPa (44 mm Hg) versus venous 9.0 kPa (68 mm Hg) 3
• During cardiac arrest with maintained ventilation, arteriovenous differences are even more pronounced: arterial pH 7.27 versus venous 7.07, and arterial PCO2 5.8 kPa versus venous 8.6 kPa when bicarbonate is administered 3
• Both arterial and central venous blood samples are needed to assess acid-base status in patients with critical hemodynamic compromise, as venous blood better detects tissue-level hypercapnia and acidemia during severe hypoperfusion 3

Hemorheologic Parameters

Viscosity and Cellular Properties

• Hematocrit, whole blood viscosity, plasma viscosity, and erythrocyte aggregation are statistically significantly higher in venous blood than arterial blood 4
• These differences may be explained primarily by the difference in hematocrit between arterial and venous samples 4
• Erythrocyte deformability does not differ significantly between arterial and venous blood 4
• With modern precise rheological measurement techniques, arterial and venous blood samples are not entirely rheologically comparable and this distinction may be important in clinical studies 4

Molecular and Structural Distinctions

Endothelial Cell Differences

• Arterial and venous endothelial cells are molecularly distinct even before the first embryonic heartbeat, revealing genetic programs coordinating arterial-venous differentiation 5, 6
• The specification of arterial and venous identity is largely genetically determined, not solely dependent on circulatory dynamics 5, 6
• Arteries and veins differ in reactive oxygen species (ROS) metabolism and utilization, with qualitative differences in how each vessel type produces and destroys ROS 7

Clinical Pitfalls to Avoid

• Do not assume pulse oximetry accurately reflects oxygen saturation in carbon monoxide poisoning, as standard two-wavelength pulse oximeters cannot differentiate carboxyhemoglobin from oxyhemoglobin 2
• Avoid relying solely on arterial blood gas analysis in severe circulatory failure, as this misses critical tissue-level acidosis and hypercapnia better detected in central venous samples 3
• Recognize that older blood gas machines without CO-oximeters may report falsely normal oxygen saturation in the presence of significant carboxyhemoglobin, calculating saturation from PaO2 rather than directly measuring oxyhemoglobin 2
• Be aware that venous oxygen saturation (ScvO2) monitoring requires serial measurements and should be complemented with other perfusion markers including skin perfusion assessment and venous-arterial CO2 gradient 8