Oxygen content, delivery and extraction !

Measurement
Exam priority: medium

Tissues care not just about saturation but about how much oxygen the blood carries and how much blood arrives. Three numbers capture it.

Updated 2026-11-01

Also called: CaO₂, DO₂, Oxygen extraction, SvO₂

What it is

Arterial O₂ content (CaO₂) depends almost entirely on hemoglobin and its saturation; dissolved O₂ is tiny. Delivery (DO₂) is content times cardiac output. Tissues normally take about a quarter of what arrives; if delivery falls they raise extraction up to a limit, and below that critical point consumption falls, anaerobic metabolism begins and lactate rises.

Oxygen–hemoglobin dissociation curve

P50 26.8 mmHgArterial blood 97 %Mixed venous blood 75 %O₂ given up to the tissues 23 %

Normal curve. Tap a point (arterial, venous or P50) or move a control.

Current curve
Normal curve (pH 7.40 · 37 °C)
Arterial blood (PO₂ 100)
Mixed venous blood (PO₂ 40)
O₂ given up to the tissues

The drop from 100 to 40 mmHg between artery and vein is resting extraction; with exercise the curve shifts right and more is released.

Equations

Arterial oxygen contentCaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)

Hb: hemoglobin (g/dL)
SaO₂: arterial saturation (fraction)
PaO₂: arterial O₂ pressure (mmHg)

Result in mL O₂ per dL of blood; the dissolved term is almost always negligible.

Oxygen deliveryDO₂ = CO × CaO₂ × 10

CO: cardiac output (L/min)
10: converts dL to L

≈1,000 mL/min at rest.

Fick principle and extractionVO₂ = CO × (CaO₂ − CvO₂) × 10; O₂ER = VO₂ / DO₂

CvO₂: mixed venous O₂ content
O₂ER: extraction ratio

If consumption is fixed and output falls, the arteriovenous difference widens.

Reference values (resting adult)

Parameter	Value	Note
CaO₂	≈18–20 mL/dL
CvO₂	≈15 mL/dL
DO₂	≈900–1,100 mL/min
VO₂	≈200–250 mL/min
O₂ extraction	≈20–30 %	the heart extracts ≈70 %
SvO₂	≈65–75 %	ScvO₂ slightly higher

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

Hypovolemic shock: output falls, extraction rises, SvO₂ drops, then lactate climbs.
Iron deficiency anemia: SpO₂ of 98 % with low content; the heart compensates with tachycardia and high output.
In Bogotá normal SaO₂ is slightly lower than at sea level and normal hemoglobin slightly higher, which preserves content.

Test yourself

Think of the answer, then tap the question to see it.

Why can someone with a hemoglobin of 7 g/dL have a normal SpO₂ yet deliver little oxygen?

Saturation measures what fraction of hemoglobin carries O₂, not how much hemoglobin there is; with half the hemoglobin, content falls by almost half.

What does a low SvO₂ indicate in a patient in shock?

That tissues are extracting more than normal because delivery is not enough: low cardiac output, anemia or hypoxemia.

Why does cyanide poisoning give a high SvO₂ and high lactate?

Cyanide blocks cytochrome c oxidase: mitochondria cannot use the O₂ that arrives, venous blood returns nearly saturated and cells switch to anaerobic glycolysis.

In mild hypoxemia with SaO₂ 92 %, what raises DO₂ more: bringing SaO₂ to 100 % or raising cardiac output by 30 %?

Raising output: going from 92 to 100 % adds less than 10 % to content, while output multiplies the whole delivery.

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 22.5 Transport of Gases (OpenStax (Rice University), 2022); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2026); Bacterial Sepsis (StatPearls (NCBI Bookshelf), 2026); Iron Deficiency and Microcytic Hypochromic Anemia (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 41). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.