# Sixty-Six Times: Oxygen Held by Haemoglobin

At full saturation, the haemoglobin of a reference adult can bind 20.1 millilitres of oxygen in each decilitre of blood. Clinical formulas give 0.30–0.31 millilitres dissolved at 100 millimetres of mercury. The ratio is 64.8–67.0, or about 66. In the reference case, mixed-venous saturation is 75 percent, while about 76.4 percent of arterial oxygen content returns to the lungs. Without haemoglobin, impossible complete extraction would require 81–83 litres a minute at resting oxygen consumption.

## Core metrics

- **Bound capacity / dissolved at 100 mmHg**: about 66 × (20.1 mL/dL versus 0.30–0.31 mL/dL)
- **Mixed-venous saturation**: 75 % (three-quarters of arterial oxygen returns)
- **Theoretical minimum without haemoglobin**: at least 82 L/min if tissues extracted all dissolved oxygen at a consumption of 250 mL/min

## Why dissolved oxygen is not enough

Oxygen is poorly soluble in warm water. Clinical formulas use 0.0030–0.0031 mL/dL/mmHg, giving 0.30–0.31 mL/dL at 100 mmHg. Hüfner 1.34 mL/g × 15.0 g/dL = 20.1 mL/dL bound. The ratio is 64.8–67.0, or about 66.

## Three-quarters come back

Blood leaving the lung is about 97.5 percent saturated. Returning mixed-venous blood stays about 75 percent saturated. Classical Fick pair: 5.0 L/min × 50 mL/L = 250 mL/min.

## Oxygen stores

Blood ≈ 807 mL O₂. Lung (FRC 2.5 L × 0.15) = 375 mL. Ratio 2.2.

## Method note

CaO₂ ≈ (1.34 × [Hb] × SO₂) + (0.0030–0.0031 × PO₂). The instrument uses the midpoint, 0.00305. Primary sources include [NCBI Oxygen Transport](https://www.ncbi.nlm.nih.gov/books/NBK54103/), [Severinghaus 1979](https://pubmed.ncbi.nlm.nih.gov/35496/) and [Bohr, Hasselbalch and Krogh 1904](https://doi.org/10.1111/j.1748-1716.1904.tb01382.x).

Educational material. The mass-and-pulse control runs a fixed reference scenario and does not estimate personal physiology.