Oxygen-hemoglobin dissociation curve: P50, shifts and content
The oxygen-hemoglobin dissociation curve plots saturation against oxygen pressure. Learn to read it, predict its shifts, and see why a normal saturation can hide a low oxygen content.
Explain the sigmoid oxygen-hemoglobin curve and predict right and left shifts (pH, CO₂, temperature, 2,3-BPG, fetal hemoglobin); recognise that saturation is not content.
The short version
- The curve is sigmoid because of cooperation between the four subunits, so it is flat at high PO₂ and steep in the tissue range of PO₂.
- P50, the PO₂ at which hemoglobin is half saturated, is about 27 mmHg in an adult; a higher P50 is a right shift, a lower P50 a left shift.
- CO₂, acid, heat and 2,3-BPG shift the curve right; the opposites, and fetal hemoglobin, shift it left.
- The curve shows saturation, not content: CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂.
- A normal saturation and PaO₂ can still hide a low oxygen content, as in anemia.
The case: normal saturation, short of oxygen
A 30-year-old woman is pale and breathless on the stairs. Her pulse oximeter reads 98%, and her PaO₂ is normal. Yet her tissues are short of oxygen.
The oxygen-hemoglobin dissociation curve explains most of what is going on, and also shows what it hides. This page covers how to read the curve, how to predict its shifts, and why the curve alone cannot tell you how much oxygen the blood carries. The answer to the opening case comes at the end.
Hemoglobin and the sigmoid curve
Hemoglobin has four subunits. Each has a heme group holding ferrous iron, and each heme carries one oxygen molecule, so up to four oxygen molecules bind per hemoglobin.[2][3]
The curve plots hemoglobin saturation, in percent, against oxygen pressure, in mmHg. It is S-shaped, a sigmoid, because of cooperation: when the first oxygen binds, the molecule changes shape, and the next oxygens bind more easily.[1][2]
The flat part, the steep part and the P50
At a PO₂ of 40 mmHg, which is venous blood, saturation is about 75%. At 60 mmHg it is still about 90%.[1][8]
- Flat upper part: a fall in arterial PO₂ from 100 to 60 mmHg barely changes saturation.[1][8]
- Steep lower part: in the tissues, a small fall in pressure unloads a lot of oxygen.[8]
The marker for the curve's position is the P50, the PO₂ at which hemoglobin is half saturated. It is about 27 mmHg in an adult.[1][7]
Right and left shifts of the curve
| Right shift (P50 up) | Left shift (P50 down) | |
|---|---|---|
| Hemoglobin affinity | Lower: oxygen held more loosely, more released | Higher: oxygen held tighter |
| CO₂ | More | Less |
| Acid–base | More acid (low pH) | Alkalosis |
| Temperature | Higher (and exercise) | Cold |
| 2,3-BPG | More | Less |
| Other | Fetal hemoglobin |
Exercise shifts the curve right through the extra CO₂, acid and heat that working muscle produces.[1][2]
2,3-BPG holds deoxyhemoglobin in the low-affinity state, and chronic anemia raises it.[2][4][5]
Fetal hemoglobin shifts the curve left
Fetal hemoglobin binds 2,3-BPG poorly, so it holds oxygen tighter than adult hemoglobin. That left shift helps the fetus take oxygen across the placenta.[2][6]
Saturation is not oxygen content
The curve shows saturation. It does not show how much oxygen is in the blood. Oxygen content comes from the content equation.[4][5]
mL O₂/dL; Hb in g/dL; SaO₂ as a fraction (98% = 0.98); PaO₂ in mmHg; 1.34 mL O₂ per g Hb is the convention used here, and sources vary from 1.31 to 1.39; 0.003 mL/dL per mmHg is dissolved oxygen
- Bound to hemoglobin: 1.34 × 15 × 0.98 ≈ 19.7
- Dissolved: 0.003 × 100 = 0.3
- Total ≈ 20 mL O₂/dL, nearly all on hemoglobin
- Bound: 1.34 × 8 × 0.98 ≈ 10.5
- Dissolved: 0.3, unchanged
- Total ≈ 10.8 mL O₂/dL
Normal PaO₂, normal saturation, and about half the oxygen content. That is the breathless woman from the opening case.[4]
What to keep from the curve
- ShiftsCO₂, acid, heat and 2,3-BPG shift the curve right; the opposites shift it left.
- Saturation versus contentThe curve shows saturation, not content.
- Hidden deficitA normal saturation and PaO₂ can still hide a low oxygen content.
Practice question
Original · USMLE styleA 22-year-old man cycles hard on an exercise bike. Venous blood from his working leg muscle has a pH of 7.28, a PCO₂ of 62 mmHg and a temperature of 39.5 °C.
Common questions
What is the P50 of hemoglobin?
The P50 is the oxygen pressure at which hemoglobin is half saturated. It is about 27 mmHg in an adult.[7]
What shifts the oxygen-hemoglobin dissociation curve to the right?
More CO₂, more acid, higher temperature, more 2,3-BPG and exercise shift it right. The P50 rises, hemoglobin holds oxygen more loosely, and more oxygen is released.[1][2]
What shifts the oxygen-hemoglobin dissociation curve to the left?
Less CO₂, alkalosis, cold, less 2,3-BPG and fetal hemoglobin shift it left. The P50 falls and hemoglobin holds oxygen tighter.[1][2]
Why does fetal hemoglobin have a higher oxygen affinity?
Fetal hemoglobin binds 2,3-BPG poorly, so it holds oxygen tighter than adult hemoglobin. This helps the fetus take oxygen across the placenta.[2][6]
Can oxygen saturation be normal when oxygen content is low?
Yes. The curve shows saturation, not content, and content depends on hemoglobin as well. In anemia with a hemoglobin of 8 g/dL, saturation and PaO₂ can be normal while content is about half of normal.[4]
Transcript
Read the full narration, by chapter645 words
0:00The case: normal saturation, breathless patient
A 30-year-old woman is pale and breathless on the stairs. Her pulse oximeter reads 98%, and her PaO2 is normal. Yet her tissues are short of oxygen. How can that be? Today: read the oxygen-haemoglobin curve, predict its shifts, and see what it hides. Haemoglobin has four subunits,
0:19Haemoglobin and the sigmoid curve
each with a haem group holding ferrous iron that carries one oxygen molecule: up to four per haemoglobin. The curve plots haemoglobin saturation, in %, against oxygen pressure, in mm Hg. It is an S shape, a sigmoid, because of cooperation: when the first oxygen binds, the molecule changes shape, and the next oxygens bind more easily.
0:46The flat part, the steep part and the P50
At a partial pressure of 40, which is venous blood, saturation is about 75%. At 60, it is still about 90. Up there the curve is flat, so a fall in arterial oxygen pressure from 100 to 60 barely changes saturation. Down in the tissues it is steep, so a small fall in pressure unloads a lot of oxygen. The marker for the curve's position is the P50, the pressure at which haemoglobin is half saturated: about 27 millimetres of mercury in an adult.
1:20Right and left shifts
A higher P50 means haemoglobin holds oxygen more loosely: a shift to the right. A lower P50 means it holds on tighter: a shift to the left. Right shift: more carbon dioxide, more acid, higher temperature, more two three BPG, and exercise. Two three BPG holds deoxyhaemoglobin in the low-affinity state, and chronic anaemia raises it. Left shift is the opposite: less carbon dioxide, alkalosis, cold, less two three BPG.
1:55Fetal haemoglobin and the placenta
Fetal haemoglobin shifts the curve left: it binds two three BPG poorly, so it holds oxygen tighter than adult haemoglobin, helping the fetus take oxygen across the placenta.
2:07Saturation is not content: the oxygen content equation
The curve shows saturation. It does not show how much oxygen is in the blood. Oxygen content, in mL per decilitre, is 1.34 times haemoglobin times saturation, plus 0.003 times the PaO2. Take haemoglobin 15, saturation 98%, PaO2 100. Bound: 1.34 times 15 times 0.98, about 19.7. Dissolved: only 0.3. Total about 20 mL per decilitre, nearly all on haemoglobin. Now severe anaemia: haemoglobin of eight, same saturation, same pressure. Bound about 10.5, plus 0.3: about 10.8. Normal PaO2, normal saturation, and about half the oxygen content. That is our breathless woman.
3:06Your turn: practice question
Your turn. A 22-year-old man cycles hard on an exercise bike. Venous blood from his working leg muscle has a pH of 7.28, a PaCO2 of 62 mm Hg, and a temperature of 39.5 degrees Celsius. Which of the following best describes the effect of these changes on haemoglobin in the muscle capillaries? A, left shift, P50 decreased, less oxygen released at a given partial pressure. B, left shift, P50 decreased, more oxygen released at a given partial pressure. C, right shift, P50 increased, more oxygen released at a given partial pressure. D, right shift, P50 increased, less oxygen released at a given partial pressure. E, no shift; the change is only movement along the same curve. Pause here and decide.
4:08The answer: wrong options first, then the right one
A is out: acid, carbon dioxide and heat lower affinity and raise the P50. B is out: a left shift makes haemoglobin hold oxygen tighter, not release more. D is out: the shift is right, but a higher P50 means oxygen is released more easily, not less. E is out: movement along a curve follows a change in oxygen pressure alone, and these changes move the curve itself. So the answer is C. Low pH, high carbon dioxide and high temperature shift the curve right, raising the P50, so haemoglobin unloads more oxygen.
4:49High-yield recap and next lesson
Three things to keep. One: carbon dioxide, acid, heat and two three BPG shift the curve right; the opposites shift it left. Two: the curve shows saturation, not content. Three: a normal saturation and PaO2 can still hide a low oxygen content. Next, carbon dioxide transport and the Bohr and Haldane effects.
Sources
Numbers in the text, e.g. [1], point here- Nickson C. Oxygen-haemoglobin dissociation curve. Life in the Fast Lane (LITFL). litfl.com/oxygen-haemoglobin-dissociation-curve/
- Deranged Physiology. Factors which influence the affinity of haemoglobin for oxygen. derangedphysiology.com/main/cicm-primary-exam/respiratory-system/Chapter-113/factors-which-influence-affinity-haemoglobin-oxygen
- LITFL. Methaemoglobinaemia (haem and ferrous iron). litfl.com/methaemoglobinaemia/
- LITFL Part One. Hypoxia (oxygen content, Huefner constant, anaemic hypoxia). partone.litfl.com/hypoxia.html
- OpenAnesthesia. Oxygen delivery versus PaO2. openanesthesia.org/o2_delivery_vs-_pao2/
- OpenStax. Anatomy and Physiology 2e, 22.5 Transport of gases (fetal haemoglobin). openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases
- Europe PMC, PMC12318577 (P50 of haemoglobin about 27 mmHg). pmc.ncbi.nlm.nih.gov/articles/PMC12318577/
- Europe PMC, PMC13523615 and PMC12123657 (plateau and steep portions of the curve; venous and arterial PO2). pmc.ncbi.nlm.nih.gov/articles/PMC13523615/ · pmc.ncbi.nlm.nih.gov/articles/PMC12123657/
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): placenta, bicycle ergometer stress test. NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain: schematic haemoglobin (item 469), red blood cell (item 442). Icons, CC0: pulse oximeter (Health Icons, healthicons.org); thermometer by kehan (via bioicons.com). All other diagrams are original and schematic; the curve is a typical adult curve, not patient data.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-RESP-006.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Lung volumes and capacities, explained5:26
- EP 02Obstructive vs restrictive lung disease: FEV1/FVC and loops5:23
- EP 03Lung compliance, Laplace and surfactant: why alveoli stay open5:23
- EP 04V/Q mismatch: shunt, dead space and the oxygen test5:18
- EP 05A-a gradient: the alveolar gas equation and the causes of hypoxaemia5:39
- EP 06Oxygen-hemoglobin dissociation curve: P50, shifts and content5:19
- EP 07CO₂ transport: bicarbonate, the chloride shift, Bohr vs Haldane5:29
- EP 08Central vs peripheral chemoreceptors and oxygen-induced hypercapnia5:27
- EP 09Hypoxic pulmonary vasoconstriction and cor pulmonale5:41
- EP 10CO, cyanide and methaemoglobin: why SpO₂ and PaO₂ fool you5:34