Anaes · Applied cardiovascular & respiratory physiology
Oxygen transport & the dissociation curve
Also known as Oxyhaemoglobin dissociation curve · P50 · Bohr effect · Haldane effect · 2,3-DPG · Oxygen content
Oxygen is carried almost entirely bound to haemoglobin, and the oxyhaemoglobin dissociation curve governs both how it loads in the lung and how it unloads in the tissues. The framework rests on five exam-critical ideas: arterial oxygen content is the sum of haemoglobin-bound oxygen (about 1.34 mL per gram of haemoglobin times the saturation) plus the small dissolved fraction (0.003 times PaO2), so haemoglobin concentration dominates oxygen content; the dissociation curve is sigmoid, with a flat plateau above a PaO2 of about 60 to 70 mmHg (which protects saturation as PaO2 falls) and a steep lower portion (which aids tissue unloading); the P50 (the PaO2 at 50 percent saturation, about 27 mmHg) is the index of haemoglobin-oxygen affinity; the curve shifts RIGHT with raised hydrogen ion, CO2, temperature and 2,3-DPG (easier tissue unloading, the Bohr effect) and LEFT with the opposite, fetal haemoglobin, carbon monoxide and methaemoglobin (tighter binding, harder unloading); and carbon dioxide is carried as bicarbonate (about 70 percent), carbamino compounds (about 20 percent) and dissolved (about 10 percent), with oxygenation aiding CO2 unloading in the lung (the Haldane effect). Built on the P50-outcomes study (Karakurt 2026), the bisphosphoglycerate-mutase structure study (2,3-DPG, Martinez-Rodriguez 2026), the high-altitude oxygen-affinity study (Woyke 2025), the haemodialysis oxygen-affinity study (Sharma 2025), the carbon-monoxide-poisoning blood-gas study (Ke 2026), and the high-affinity Hb Rothschild report (Nuzhnaya 2025).
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Red flags
- Oxygen content is dominated by haemoglobin-bound oxygen (1.34 mL per gram times saturation); a low haemoglobin halves oxygen content long before the saturation falls — anaemia desaturates the PATIENT, not the pulse oximeter.
- The dissociation-curve plateau (above about 60 to 70 mmHg PaO2) means saturation stays near 98 percent even as PaO2 falls from 100 to 60, so the pulse oximeter lags arterial PaO2 — by the time SpO2 falls, PaO2 is already low.
- Carbon monoxide left-shifts the curve AND occupies binding sites, so the pulse oximeter reads falsely high (it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin) while tissue oxygen delivery collapses.
- A right shift (raised H+, CO2, temperature, 2,3-DPG) aids tissue unloading but hinders pulmonary loading; a left shift aids loading but starves the tissues of oxygen despite a high saturation.
- Stored blood loses 2,3-DPG, left-shifting the curve for days after transfusion, so massive transfusion temporarily impairs tissue oxygen unloading despite a normal measured saturation.
Meet the patient
A 32-year-old man is pulled from a house fire. He is confused, tachycardic, and the pulse oximeter reads 97 percent. The ambulance crew is reassured — but his venous gas shows a lactate of 6 and a carboxyhaemoglobin of 28 percent. He is tissue-starved despite a normal-looking SpO2.[5]
This is the dissociation curve betraying the clinician. Carbon monoxide occupies binding sites and left-shifts the curve, so the pulse oximeter reads falsely high while tissue oxygen delivery collapses. Every decision about transfusion thresholds, acid–base, temperature, and oximetry interpretation rests on the physiology that follows.[5]
References6ShowHide
- [1]Karakurt Y, et al. Association of Day-1 Hemoglobin Oxygen Affinity (p50) with Severe Intraventricular Hemorrhage in Preterm Infants Indian Pediatr, 2026.PMID 41949748
- [2]Martínez-Rodríguez S, Torres JM, Sánchez P, et al. New human bisphosphoglycerate mutase structures provide insights into the structural basis of BPGM deficiency and citrate inhibition Int J Biol Macromol, 2026.PMID 41354380
- [3]Woyke S, et al. A new approach to haemoglobin oxygen affinity research at high altitude: Determination of haemoglobin oxygen dissociation curves and 2,3-bisphosphoglycerate in an experimental human crossover hypoxic chamber study Eur J Appl Physiol, 2025.PMID 40314729
- [4]Sharma S, et al. Effects of hemodialysis on hemoglobin oxygen affinity and cardiac function Clin Kidney J, 2025.PMID 40832120
- [5]Ke J, et al. Arterial blood gas analysis in risk stratification of acute carbon monoxide poisoning Indian J Med Res, 2026.PMID 42295710
- [6]Nuzhnaya E, et al. First Symptomatic Pediatric Case of Hb Rothschild (HBB: c.112T>C, p.Trp38Arg): Low-Oxygen-Affinity Hemoglobin Presenting with Persistent Pseudohypoxemia Diagnostics (Basel), 2025.PMID 41464181