HaemoglobinHemoglobin · Hb

Haemoglobin is the protein that binds oxygen in the lungs and releases it in the tissues. Each molecule holds four iron-containing haem groups, and it is what makes blood red. Its affinity for oxygen shifts with temperature, pH and carbon dioxide, so working muscle automatically gets more oxygen released to it.

It matters for endurance because oxygen delivery is the main limit on aerobic capacity, and haemoglobin concentration sets how much oxygen a given volume of blood can carry. Low levels — anaemia, often from iron deficiency — cause fatigue and a measurable drop in performance, and are considerably more common in menstruating women and in endurance athletes.

In practice this is also why altitude training and, illegitimately, blood doping and EPO work: all raise oxygen-carrying capacity. Persistent unexplained fatigue is worth a blood test rather than a training change.