The molecule that carries oxygen
Each red blood cell is estimated to hold about 270 million haemoglobin molecules. This protein has two kinds of chains (two α and two β in adults), each carrying an iron-containing haem, so it can carry up to four oxygen molecules. After it was crystallised in the mid-19th century, Christian Bohr and colleagues found in 1904 that haemoglobin releases oxygen more readily where carbon dioxide is high and acidic — the Bohr effect — so working muscles get more oxygen. Because binding one oxygen makes the next bind more easily, the binding curve is S-shaped; the 1965 Monod–Wyman–Changeux model explained this cooperativity. In 1936 Linus Pauling studied the oxygen–iron bond through its magnetic properties.
Structure and molecular disease
In 1959–60 Max Perutz solved haemoglobin's 3D structure by X-ray crystallography, sharing the 1962 Nobel Prize in Chemistry. In 1949 Pauling's group showed that haemoglobin from people with sickle cell disease differs electrically, founding the idea of a "molecular disease", and in 1957 Vernon Ingram found the cause was a single amino-acid change in the β chain — an early demonstration that one change in a gene's DNA can cause disease. In 1954 Anthony Allison reported that people with one copy of the sickle trait resist malaria better, explaining by natural selection why the gene is common where malaria is. In 2023 a CRISPR gene-editing therapy was first approved for sickle cell disease.
Levels and health
Blood haemoglobin is the key measure of anaemia. WHO defines anaemia in adult men as below 13 g/dL and in non-pregnant adult women as below 12 g/dL, though thresholds vary with laboratory, age and altitude. The commonest cause is iron deficiency, which can bring fatigue, dizziness and breathlessness; unusually high values can signal dehydration, smoking or some diseases. Carbon monoxide binds haemoglobin over 200 times more strongly than oxygen and blocks oxygen transport, so with headache and dizziness near a possible source, ventilate and get out at once. When blood sugar stays high, glucose attaches to haemoglobin; HbA1c reflects average glucose over roughly the previous 2–3 months. Discuss results and iron supplements with a clinician. This is general information, not a substitute for medical advice.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- What should I eat to raise haemoglobin?
- If iron deficiency is the cause, iron-rich foods such as red meat, liver, legumes and dark leafy greens, with vitamin C to aid absorption, can help. But anaemia has many causes, so get tested first.
- Why is blood red?
- Because of the way haem absorbs light: oxygenated blood is bright red and deoxygenated blood dark red. Veins look blue because of how light passes through skin.
Sources reviewed: 2026-10-26
References
- Bohr, C., Hasselbalch, K., & Krogh, A. (1904). Ueber einen in biologischer Beziehung wichtigen Einfluss, den die Kohlensäurespannung des Blutes auf dessen Sauerstoffbindung übt. Skandinavisches Archiv für Physiologie, 16(2), 402–412.
- Pauling, L., & Coryell, C. D. (1936). The magnetic properties and structure of hemoglobin, oxyhemoglobin and carbonmonoxyhemoglobin. PNAS, 22(4), 210–216.
- Pauling, L., Itano, H. A., Singer, S. J., & Wells, I. C. (1949). Sickle cell anemia, a molecular disease. Science, 110(2865), 543–548.
- Ingram, V. M. (1957). Gene mutations in human haemoglobin: the chemical difference between normal and sickle cell haemoglobin. Nature, 180, 326–328.
- Perutz, M. F., Rossmann, M. G., Cullis, A. F., Muirhead, H., Will, G., & North, A. C. T. (1960). Structure of haemoglobin: a three-dimensional Fourier synthesis at 5.5-Å resolution. Nature, 185, 416–422.
- Monod, J., Wyman, J., & Changeux, J.-P. (1965). On the nature of allosteric transitions: a plausible model. Journal of Molecular Biology, 12(1), 88–118.
- Allison, A. C. (1954). Protection afforded by sickle-cell trait against subtertian malarial infection. British Medical Journal, 1(4857), 290–294.
- World Health Organization. (2011). Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity (WHO/NMH/NHD/MNM/11.1). Geneva: WHO.
- Frangoul, H., et al. (2021). CRISPR-Cas9 gene editing for sickle cell disease and β-thalassemia. New England Journal of Medicine, 384(3), 252–260.