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# 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

## Key points

- CaO₂ ≈ 1.34 × Hb × SaO₂ + 0.003 × PaO₂ ≈ 20 mL O₂/dL in a healthy adult.
- DO₂ = cardiac output × CaO₂ × 10 ≈ 1,000 mL/min; consumption (VO₂) ≈ 250 mL/min; extraction ≈ 25 %.
- Anemia lowers CaO₂ with normal PaO₂ and SaO₂: saturation does not detect anemia.
- Mixed venous saturation (SvO₂) ≈ 70–75 %: low means delivery is falling short (hypovolemic or cardiogenic shock, anemia).
- High SvO₂ with high lactate: tissues are not extracting (sepsis with microcirculatory failure, cyanide poisoning).
- Three levers to raise delivery: cardiac output, hemoglobin and saturation (usually in that order of impact).

## 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 mmHg **Arterial 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 content**CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2)

**Hb**
hemoglobin (g/dL)

**SaO 2**
arterial saturation (fraction)

**PaO 2**
arterial O₂ pressure (mmHg)

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

**Oxygen delivery**DO2 = CO × CaO2 × 10

**CO**
cardiac output (L/min)

**10**
converts dL to L

≈1,000 mL/min at rest.

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

**CvO 2**
mixed venous O₂ content

**O 2 ER**
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.

## Memory aid

Truck, load and trips: hemoglobin is the truck, saturation the load, and cardiac output the trips per minute.

## 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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## Related topics

- [Oxygen transport and the dissociation curve](https://durgical.com/en/physiology/oxygen-transport-and-the-dissociation-curve/)
- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Shock physiology](https://durgical.com/en/physiology/shock-physiology/)
- [Anemia physiology and compensation](https://durgical.com/en/physiology/anemia-physiology-and-compensation/)
- [Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy](https://durgical.com/en/physiology/oxygenation-spo-pao-cyanosis-and-oxygen-therapy/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Cyanide poisoning](https://durgical.com/en/conditions/cyanide-poisoning/)

## In Biochemistry

- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Lactate](https://durgical.com/en/biochemistry/molecules/lactate/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases) (OpenStax (Rice University), 2022); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2026); [Bacterial Sepsis](https://www.ncbi.nlm.nih.gov/books/NBK537054/) (StatPearls (NCBI Bookshelf), 2026); [Iron Deficiency and Microcytic Hypochromic Anemia](https://www.ncbi.nlm.nih.gov/books/NBK560876/) (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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