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Human Physiology · Respiratory system

Oxygen and Carbon Dioxide Transport

Blood carries oxygen from the lungs to every cell and brings carbon dioxide back. This page explains how haemoglobin loads and unloads oxygen, how carbon dioxide travels, how to read saturation and oxygen targets, and where pulse oximetry can mislead.

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Part 4 · Respiratory System · Chapter 18 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe how oxygen is carried in blood and calculate oxygen content.
  • Draw and interpret the oxygen–haemoglobin dissociation curve.
  • Explain the factors that shift the curve and why this matters in exercise.
  • Describe the three forms in which carbon dioxide is carried.
  • Apply oxygen saturation targets safely.
  • Recognise the limits of pulse oximetry.

Why it matters

Saturation is checked on almost every patient, and physiotherapists often adjust oxygen during exercise or airway clearance. A number on a screen is only useful if you know what it represents and when it can be wrong.

Understanding oxygen transport also explains why anaemic patients tire quickly, why exercising muscle receives more oxygen, and why too much oxygen can harm some patients.

How oxygen is carried

Oxygen dissolves poorly in plasma: only about 0.3 mL per 100 mL of blood at normal arterial PO2. Almost all oxygen is carried bound to haemoglobin inside red cells.

Each gram of haemoglobin can carry about 1.34 mL of oxygen when fully saturated. With 15 g/dL of haemoglobin, arterial blood carries about 20 mL of oxygen per 100 mL.

Figure 1 · How much oxygen blood carries

Arterial oxygen content in normal blood, anaemia and low saturation Three bars. Normal blood with haemoglobin 15 and saturation 98 per cent carries about 20 millilitres of oxygen per decilitre. Anaemia with haemoglobin 9 carries about 12. Normal haemoglobin at 85 per cent saturation carries about 17. The dissolved fraction is a thin sliver on top of each bar. 20.0 mL/dLNormal12.1 mL/dLAnaemia (Hb 9)17.2 mL/dLLow saturation (85%)bound to HbdissolvedO2 content ≈ 1.34 × Hb (g/dL) × saturation + 0.003 × PO2
Content depends on haemoglobin as much as saturation. An anaemic patient can have a normal saturation and still carry far less oxygen. Calculated from the standard formula.

Oxygen delivery to the body is the oxygen content multiplied by cardiac output: about 20 mL/dL × 50 dL/min, or 1,000 mL per minute at rest. Delivery can fall from low saturation, low haemoglobin or low cardiac output.

The dissociation curve

Figure 2 · The oxygen–haemoglobin dissociation curve

Haemoglobin saturation against oxygen partial pressure, with a right-shifted curve An S-shaped curve. Saturation is about 98 per cent at 100 millimetres of mercury, 91 per cent at 60, 75 per cent at 40 and 50 per cent at about 27. A dashed curve shifted to the right shows lower saturation at each partial pressure, as occurs with acid, carbon dioxide, heat and 2,3-DPG. 0204060801000255075100PO2 (mmHg)Haemoglobin saturation (%)P50 about 27venous: 40 → 75%60 → 91%arterial: 100 → 98%Shift right:more acidmore CO2warmermore 2,3-DPG= more unloading(exercising muscle)
Flat at the top, steep in the middle. The solid curve is computed from Severinghaus’s equation [1]. The flat top protects loading in the lungs; the steep middle releases oxygen to tissues. The right shift is illustrative.

Haemoglobin has four binding sites. Binding one oxygen molecule makes the next easier, which gives the curve its S shape. Severinghaus produced a simple equation that reproduces the standard human curve closely [1].

PO2 (mmHg)Saturation [1]Meaning
100About 98%Normal arterial blood
60About 91%Shoulder of the curve; below this saturation falls fast
40About 75%Mixed venous blood at rest
About 2750% (the P50)Index of haemoglobin’s affinity

The flat upper part means that arterial saturation stays high even if alveolar PO2 falls somewhat. The steep middle part means that a small fall in tissue PO2 releases a lot of oxygen.

Shifts of the curve

ShiftCaused byEffectWhere it helps
RightMore acid, more CO2, higher temperature, more 2,3-DPGEasier unloadingExercising muscle; anaemia; altitude
LeftLess acid, less CO2, cooling, fetal haemoglobin, carbon monoxideTighter binding, poorer unloadingPlacenta (fetal haemoglobin)

The effect of acid and carbon dioxide is called the Bohr effect, and Severinghaus gave equations for both the pH and temperature effects [1]. Working muscle is warm, acidic and rich in CO2, so it receives extra oxygen exactly where needed.

Carbon monoxide

Carbon monoxide binds haemoglobin over 200 times more tightly than oxygen and shifts the curve left. Most finger oximeters read carboxyhaemoglobin as oxyhaemoglobin, so saturation can look normal in poisoning.

How carbon dioxide is carried

Figure 3 · How carbon dioxide is carried

Carbon dioxide conversion to bicarbonate in the red cell and the shares of each transport form In the red cell, carbonic anhydrase converts carbon dioxide and water to carbonic acid, which splits into a hydrogen ion and bicarbonate. Bicarbonate leaves the cell in exchange for chloride, and haemoglobin buffers the hydrogen ion. A bar shows about 70 per cent carried as bicarbonate, 20 per cent bound to haemoglobin and 10 per cent dissolved. Red blood cellCO2 fromtissuesCO2 + H2OcarbonicanhydraseH2CO3H⁺ + HCO3⁻H⁺ buffered byhaemoglobinHCO3⁻ outCl⁻ in(chloride shift)some CO2 binds haemoglobin(carbamino)Bicarbonate about 70%Carbamino about 20%Dissolved about 10%
Mostly as bicarbonate. The same reaction links carbon dioxide to blood acidity, which is why breathing controls pH. Approximate textbook shares.

Carbon dioxide is far more soluble than oxygen. About 10% travels dissolved, about 20% bound to haemoglobin as carbamino compounds, and about 70% as bicarbonate. The enzyme carbonic anhydrase in red cells makes the bicarbonate reaction fast.

Deoxygenated haemoglobin binds carbon dioxide and hydrogen ions more readily than oxygenated haemoglobin. So blood picks up more carbon dioxide in the tissues, where oxygen is released, and gives it up in the lungs.

Oxygen saturation targets

Figure 4 · Oxygen saturation targets

Target oxygen saturation ranges for most acutely ill adults and for those at risk of hypercapnia A scale of oxygen saturation from 80 to 100 per cent. A green band from 94 to 98 per cent marks the target for most acutely ill adults. A red band from 88 to 92 per cent marks the target for patients at risk of hypercapnic respiratory failure. 80%82%84%86%88%90%92%94%96%98%100%Most acutely ill adults: 94–98%Risk of hypercapnia: 88–92%Oxygen is a drug: prescribe to a target range, monitor, and adjust
Targets, not maximum oxygen. The British Thoracic Society guideline recommends these ranges [2]. Too much oxygen can raise carbon dioxide in susceptible patients.

The British Thoracic Society guideline treats oxygen as a drug, prescribed to a target saturation range [2]. For most acutely ill adults the target is 94–98%. For patients at risk of hypercapnic respiratory failure, such as those with severe COPD, it is 88–92% [2].

Giving too much oxygen to these patients can raise arterial carbon dioxide. Aim for the target, not the highest possible number [2].

Practical skill · oxygen during exercise

Check the prescribed target before starting [2]. Monitor saturation during and after exercise. If saturation falls below target, slow down and allow recovery, and follow the prescription for any change in oxygen. Record the lowest value and recovery time.

Pulse oximetry and its limits

A pulse oximeter shines red and infrared light through tissue and estimates saturation from the pulsatile absorption. It is quick and non-invasive, but it is an estimate.

Figure 5 · When the oximeter misleads

Frequency of hidden low oxygen by patient group in paired oximeter and blood gas readings Two bars. Hidden low arterial oxygen, with arterial saturation under 88 per cent while the oximeter read 92 to 96 per cent, occurred in 3.6 per cent of readings in White patients and 11.7 per cent in Black patients. 3.6%White patients11.7%Black patientsArterial saturation below 88% despite oximeter reading 92–96%Paired arterial blood gas and pulse oximetry readings [3]
Know the limits of pulse oximetry. Oximeters overestimated saturation more often in patients with darker skin [3]. Look at the patient, not only the number, and check a blood gas when in doubt.

Sjoding and colleagues compared paired oximeter and arterial readings. Hidden low oxygen, with arterial saturation under 88% while the oximeter read 92–96%, occurred in 11.7% of readings in Black patients and 3.6% in White patients [3].

Source of errorEffect
Darker skin pigmentationSaturation may be overestimated [3]
Poor perfusion, cold fingersWeak signal; unreliable reading
Movement during exerciseArtefact; check waveform
Carbon monoxideFalsely normal reading
Nail varnish, bright ambient lightInterference

Clinical relevance for the physiotherapist

  • Anaemia. Saturation may be normal while oxygen content is low; expect early fatigue and breathlessness.
  • Exercise. Muscle unloads more oxygen as the curve shifts right; desaturation on exercise signals a lung or circulation limit.
  • COPD. Use the prescribed 88–92% target where indicated [2].
  • Monitoring. Check the oximeter waveform and the patient; be alert to overestimation [3].
  • Interpreting values. Below about 90% saturation, small falls in PO2 cause large falls in saturation [1].

Assessment and application

SituationInterpretationAction
SpO2 95% in a patient with Hb 8 g/dL who is breathless on walkingLow oxygen content despite normal saturationPace activity; inform team of exertional symptoms
SpO2 falls from 92% to 84% during stair climbing in COPDExertional desaturation below target [2]Rest, recover; follow oxygen prescription
SpO2 97% but patient drowsy after high-flow oxygen in severe COPDPossible CO2 retentionUrgent medical review; blood gas
Reading jumps around during exerciseMovement artefactCheck waveform; pause to re-read

Common mistakes

  • Equating saturation with oxygen content and ignoring haemoglobin.
  • Expecting saturation to fall in proportion to PO2; the curve is S-shaped [1].
  • Turning oxygen up to 100% saturation in patients at risk of hypercapnia [2].
  • Trusting the oximeter number without looking at the patient [3].
  • Forgetting that carbon monoxide gives falsely normal readings.

Exam preparation

Long answer

Describe the transport of oxygen in blood. Draw the dissociation curve and explain the factors that shift it.

  • Dissolved and haemoglobin-bound oxygen; oxygen content (Figure 1).
  • Dissociation curve with key points [1] (Figure 2).
  • Bohr effect and other shifts.
  • Physiological significance in lungs and tissues.
  • Clinical: saturation targets [2] and oximetry limits [3].

Short notes

  • Bohr effect.
  • Chloride shift.
  • Oxygen therapy targets [2].
  • Pulse oximetry [3].

Viva and practical

  • “What is P50?” — PO2 at 50% saturation, about 27 mmHg [1].
  • “What target would you use in severe COPD?” — 88–92% [2].
  • “Why might an oximeter mislead?” — pigmentation, perfusion, movement, carbon monoxide [3].
  • OSCE: apply an oximeter and interpret the reading.

Current evidence

Exam and traditional teaching

Oxygen is described as harmless, and the oximeter reading is treated as the truth.

What the evidence adds

Oxygen should be prescribed to target ranges, with a lower range for those at risk of hypercapnia [2].

Pulse oximeters overestimate saturation more often in patients with darker skin [3].

What the student should understand

Saturation, haemoglobin and cardiac output together decide oxygen delivery. Use targets, watch the patient, and confirm doubtful readings.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. On the standard dissociation curve, haemoglobin is about 50% saturated at a PO2 of about:
  1. (A) 10 mmHg
  2. (B) 27 mmHg
  3. (C) 60 mmHg
  4. (D) 100 mmHg

Answer: (B) Severinghaus’s equation gives about 50% saturation at roughly 27 mmHg [1].

Q2. At a PO2 of 60 mmHg, saturation is approximately:
  1. (A) 50%
  2. (B) 75%
  3. (C) 91%
  4. (D) 100%

Answer: (C) The equation gives about 91% at 60 mmHg, the shoulder of the curve [1].

Q3. A rightward shift of the dissociation curve means:
  1. (A) Haemoglobin holds oxygen more tightly
  2. (B) Haemoglobin releases oxygen more easily
  3. (C) No change in unloading
  4. (D) Less carbon dioxide in blood

Answer: (B) A right shift raises the PO2 for a given saturation, aiding unloading; pH effects on the curve are quantified by the Bohr coefficient [1].

Q4. The British Thoracic Society target saturation for most acutely ill adults is:
  1. (A) 80–84%
  2. (B) 88–92%
  3. (C) 94–98%
  4. (D) 100%

Answer: (C) The guideline recommends 94–98% for most patients [2].

Q5. For a patient at risk of hypercapnic respiratory failure, the target is usually:
  1. (A) 94–98%
  2. (B) 88–92%
  3. (C) 75–80%
  4. (D) 100%

Answer: (B) A lower range of 88–92% is recommended for those at risk [2].

Q6. In paired readings, hidden low oxygen (arterial saturation under 88% with oximeter 92–96%) occurred in Black patients in about:
  1. (A) 0.1% of readings
  2. (B) 3.6%
  3. (C) 11.7%
  4. (D) 50%

Answer: (C) It occurred in 11.7% of readings in Black patients versus 3.6% in White patients [3].

Q7. The main practical lesson from the oximetry study is:
  1. (A) Oximeters are never useful
  2. (B) Oximeters can overestimate saturation, so interpret readings with clinical signs
  3. (C) Only use oximeters on fingers
  4. (D) Saturation is always exact

Answer: (B) Oximetry overestimated saturation more often in Black patients [3].

Q8. Most carbon dioxide is carried in blood as:
  1. (A) Dissolved gas
  2. (B) Bicarbonate
  3. (C) Carboxyhaemoglobin
  4. (D) Bound to albumin

Answer: (B) About 70% travels as bicarbonate; guideline advice on oxygen use in hypercapnia reflects this CO2 chemistry [2].

Q9. In a patient with severe COPD, giving high-flow oxygen without a target may:
  1. (A) Always be safe
  2. (B) Raise arterial carbon dioxide
  3. (C) Lower carbon dioxide
  4. (D) Cure the COPD

Answer: (B) Excess oxygen can worsen hypercapnia; hence the lower target range [2].

Q10. Increasing the temperature of blood shifts the dissociation curve:
  1. (A) Left
  2. (B) Right
  3. (C) Up
  4. (D) Not at all

Answer: (B) Warming shifts the curve right; its temperature coefficient is given by Severinghaus [1].

Quick review

Key takeaways

  • Oxygen content ≈ 1.34 × Hb × saturation (+ a little dissolved).
  • Dissociation curve: 98% at 100 mmHg, 91% at 60, 75% at 40, 50% at about 27 [1].
  • Right shift (acid, CO2, heat, 2,3-DPG) aids unloading.
  • CO2: about 70% bicarbonate, 20% carbamino, 10% dissolved.
  • Targets: 94–98% for most; 88–92% if at risk of hypercapnia [2].
  • Oximeters can overestimate saturation [3].

Further reading

SourceWhy it earns its place
Severinghaus JW — J Appl Physiol, 1979The equation behind the curve [1]
O’Driscoll BR et al. — Thorax, 2017Oxygen prescribing and targets [2]
Sjoding MW et al. — NEJM, 2020Bias in pulse oximetry [3]
West JB — Respiratory Physiology: The EssentialsGas transport explained

References

  1. Severinghaus JW. Simple, accurate equations for human blood O2 dissociation computations. Journal of Applied Physiology. 1979 Mar;46(3):599–602. doi:10.1152/jappl.1979.46.3.599 PMID 35496 Methods paper
  2. O’Driscoll BR, Howard LS, Earis J, Mak V. BTS guideline for oxygen use in adults in healthcare and emergency settings. Thorax. 2017;72(Suppl 1):ii1–ii90. doi:10.1136/thoraxjnl-2016-209729 PMID 28507176 Guideline
  3. Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial bias in pulse oximetry measurement. New England Journal of Medicine. 2020;383(25):2477–2478. doi:10.1056/NEJMc2029240 PMID 33326721 Cohort analysis (research letter)

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Reviewed by the Physiotherapist India Team. · Human Physiology contents