Oxygen Transport: Hemoglobin, the Oxyhemoglobin Dissociation Curve, and Tissue Delivery

How Oxygen Is Carried in the Blood

Oxygen travels in the blood in two forms. A small fraction dissolves directly in plasma—roughly 0.003 mL of O₂ per 100 mL of blood for every 1 mmHg of partial pressure (PaO₂). This dissolved portion is clinically important but quantitatively minor. The vast majority of oxygen—more than 98% under normal conditions—is carried bound to hemoglobin (Hb) inside red blood cells.

Each hemoglobin molecule contains four heme groups, each capable of binding one oxygen molecule. When oxygen binds, hemoglobin becomes oxyhemoglobin; when it releases oxygen, it becomes deoxyhemoglobin (also called reduced hemoglobin). One gram of fully saturated, normal hemoglobin carries approximately 1.34 mL of oxygen. This constant (sometimes written as 1.36 in some references, but 1.34 is the standard used on NBRC exams and in most US respiratory therapy curricula) is essential for calculating oxygen content.

Arterial Oxygen Content (CaO₂)

The arterial oxygen content equation expresses how much oxygen is actually present in arterial blood:

> CaO₂ = (Hb × 1.34 × SaO₂) + (PaO₂ × 0.003)

• Hb = hemoglobin concentration in g/dL
• 1.34 = mL of O₂ carried per gram of Hb at full saturation
• SaO₂ = fractional arterial oxygen saturation (use decimal form: 0.97, not 97%)
• PaO₂ = arterial partial pressure of oxygen in mmHg
• 0.003 = solubility coefficient of O₂ in plasma (mL/dL/mmHg)

Normal CaO₂ is approximately 20 mL O₂ per 100 mL of blood (often written 20 vol%).

Oxygen Delivery (DO₂)

Oxygen delivery to the tissues depends not only on how much oxygen the blood carries, but also on cardiac output (CO):

> DO₂ = CaO₂ × Cardiac Output × 10

(The factor of 10 converts dL to L, since cardiac output is expressed in L/min.) Normal DO₂ is roughly 900–1,000 mL O₂/min. A drop in hemoglobin, saturation, or cardiac output will reduce delivery and can compromise tissue oxygenation.

The Oxyhemoglobin Dissociation Curve

The relationship between PaO₂ and SaO₂ is not linear—it forms an S-shaped (sigmoid) curve. This shape has important clinical implications.

• Upper flat portion (PaO₂ above ~60 mmHg): Saturation changes very little even with significant drops in PaO₂. This provides a safety margin: a patient whose PaO₂ falls from 100 to 70 mmHg loses only a few percent saturation.
• Steep portion (PaO₂ below ~60 mmHg): Small drops in PaO₂ cause large drops in saturation. Below this threshold, oxygen loading becomes critically impaired.

The P50 is the PaO₂ at which hemoglobin is 50% saturated; the normal value is approximately 26–27 mmHg. A higher P50 indicates reduced hemoglobin affinity for oxygen.

Shifts of the Curve

Several factors change hemoglobin's affinity for oxygen, shifting the curve:

Right shift (decreased affinity — O₂ released more easily to tissues):

• Increased temperature
• Decreased pH (acidosis)
• Increased PaCO₂ (hypercapnia)
• Increased 2,3-DPG (2,3-diphosphoglycerate)

A right shift raises the P50 and is generally beneficial at the tissue level because oxygen unloads more readily.

Left shift (increased affinity — O₂ held more tightly by Hb):

• Decreased temperature
• Increased pH (alkalosis)
• Decreased PaCO₂
• Decreased 2,3-DPG
• Carbon monoxide (CO) binding
• Fetal hemoglobin (HbF)

A left shift lowers the P50. Oxygen loads easily in the lungs but is released less readily to tissues—a potentially dangerous situation.

The Bohr effect describes the influence of pH and CO₂ on the curve: acidosis and hypercapnia shift the curve right, promoting O₂ unloading at metabolically active tissues.

Carbon Monoxide: A Critical Safety Issue

Carbon monoxide binds hemoglobin with an affinity roughly 200–250 times greater than oxygen, forming carboxyhemoglobin (COHb). CO poisoning is life-threatening for two reasons: (1) it occupies O₂-binding sites, dramatically reducing the hemoglobin available to carry oxygen; and (2) it shifts the oxyhemoglobin dissociation curve to the left, impairing O₂ unloading at tissues.

Critical point for patient safety: Standard pulse oximetry (SpO₂) cannot distinguish oxyhemoglobin from carboxyhemoglobin. In CO poisoning, the SpO₂ reading will appear falsely normal or near-normal even while the patient is severely hypoxic. The correct monitoring tool is co-oximetry (a multi-wavelength device, typically measured from an arterial blood sample in the laboratory). Treatment priority is high-flow 100% oxygen via a non-rebreather mask, which accelerates CO elimination by competitive displacement.

2,3-DPG and Physiologic Adaptation

2,3-DPG is a metabolic byproduct produced in red blood cells. Its concentration rises in conditions of chronic hypoxia (e.g., high altitude, chronic anemia) and decreases in stored bank blood. Elevated 2,3-DPG shifts the curve right, improving O₂ delivery to tissues—an important adaptive response. Stored blood low in 2,3-DPG may temporarily impair tissue oxygenation after transfusion.

Clinical Application

When a patient's oxygenation is inadequate, identifying which variable to correct first matters:

• Anemia: CaO₂ is low primarily because Hb is low; transfusion or treatment of the underlying cause addresses the deficiency.
• CO poisoning: SaO₂ (true oxyhemoglobin fraction) is low; high-flow O₂ is the immediate priority.
• Low PaO₂: Supplemental oxygen and ventilatory support are indicated.
• Low cardiac output: Improving cardiac function is essential; increasing CaO₂ alone will not restore adequate DO₂ if the pump is failing.