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 (1.34 mL per gram times saturation in teaching) plus the small dissolved fraction (0.003 times PaO2 in teaching), so haemoglobin concentration dominates oxygen content; the dissociation curve is sigmoid, with a flat plateau above a PaO2 of about 60 to 70 mmHg in teaching (which protects saturation as PaO2 falls) and a steep lower portion (which aids tissue unloading); the P50 (the PaO2 at 50 percent saturation) 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) in teaching; and carbon dioxide carriage fractions plus the Haldane effect are teaching. Built on the day-1 p50 and severe IVH cohort (Karakurt 2026, n-290 very preterm, median p50 21.5/18.7/16.0 by IVH severity, aOR 1.38 per 1 mmHg decrease), the bisphosphoglycerate-mutase structure study (Martinez-Rodriguez 2026, BPGM malfunction reduces 2,3-BPG with left shift and ECYT8 erythrocytosis, variants Arg62Gln/Arg90Cys/Arg90His/Gln102Lys), the high-altitude crossover chamber study (Woyke 2025, 10 males 3,500 m vs 262 m, P50st 28.3 vs 26.8 p 0.054, 2,3-BPG rise compensating hypocapnia), the haemodialysis oxygen-affinity study (Sharma 2025, n-20, P50 27.1 to 26.2 during dialysis with LVGLS worsening), the carbon-monoxide-poisoning blood-gas cohort (Ke 2026, n-124, acidosis-group ICU admission 44.8 percent, lactate stronger than COHb), and the low-affinity Hb Rothschild paediatric case report (Nuzhnaya 2025, 4-year-old SpO2 78-86 percent with normal PaO2, HBB c.112TC).
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Red flags
- Oxygen content is dominated by haemoglobin-bound oxygen in teaching (1.34 mL per gram times saturation plus 0.003 times PaO2 — equation teaching, not quantified in any retrieved abstract); 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 in teaching) means saturation stays high even as PaO2 falls, so the pulse oximeter lags arterial PaO2 — by the time SpO2 falls, PaO2 is already low (plateau numbers are viva teaching, not in any retrieved abstract).
- Carbon monoxide occupies binding sites and left-shifts the remaining curve in teaching, so the pulse oximeter reads falsely high while tissue oxygen delivery collapses; the retrieved triage anchor is ABG-exact (n-124 poisoning cohort, acidosis-group ICU admission 44.8 percent, median COHb 33.1 percent with lactate 7.60, lactate stronger than COHb for severity) [5].
- 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 (direction teaching; the retrieved 2,3-BPG anchor is BPGM-malfunction left shift with ECYT8 erythrocytosis [2], and the retrieved P50 anchor is day-1 p50 21.5/18.7/16.0 by IVH severity with aOR 1.38 per 1 mmHg decrease in 290 very preterm infants [1]).
- Stored-blood 2,3-DPG depletion left-shifts the curve in teaching (mechanism direction owned by the BPGM structure study [2]; storage-duration and regeneration-time numbers are transfusion teaching, not in any retrieved abstract), so massive transfusion can transiently impair 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. (Teaching vignette — SpO2/COHb/lactate numbers are illustrative, not from any retrieved abstract; the retrieved cohort medians are COHb 33.1 percent with lactate 7.60 and GCS 7 in the acidosis group [5].)[5]
This is the dissociation curve betraying the clinician. Carbon monoxide occupies binding sites and left-shifts the curve in teaching, so the pulse oximeter reads falsely high while tissue oxygen delivery collapses (site-occupation/left-shift mechanism is teaching, not in [5]; [5] owns ABG risk stratification with lactate stronger than COHb). 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