High-yield map
Key points before the detail
- 01
Oxygen delivery = cardiac output × arterial oxygen content.
- 02
Haemoglobin-bound oxygen dominates arterial oxygen content; PaO2 measures dissolved oxygen tension rather than total oxygen content.
- 03
Right shift promotes unloading; left shift increases affinity.
- 04
Normal oxygen saturation does not exclude inadequate oxygen delivery in anaemia or shock.
Arterial oxygen content
Most oxygen is carried bound to haemoglobin. A small amount is dissolved in plasma and determines PaO2. The familiar oxygen-content equation combines haemoglobin concentration, saturation and a small dissolved-oxygen term; for MRCS reasoning, the key point is that haemoglobin and saturation dominate the result.
From content to delivery
Global oxygen delivery (DO2) is cardiac output multiplied by arterial oxygen content. Cardiac output itself equals heart rate multiplied by stroke volume. This creates two broad ways for delivery to fail: the circulation can move too little blood, or the blood can carry too little oxygen.
| Problem | SpO2 may be | Why DO2 falls |
|---|---|---|
| Severe anaemia | Normal | Too little haemoglobin is available to carry oxygen. |
| Haemorrhagic/cardiogenic shock | Normal | Cardiac output is inadequate despite acceptable saturation. |
| Hypoxaemic respiratory failure | Low | Haemoglobin saturation and therefore oxygen content fall. |
The oxyhaemoglobin dissociation curve
The sigmoid curve reflects cooperative oxygen binding: haemoglobin affinity changes as oxygen molecules bind or leave. Its plateau helps preserve saturation despite moderate falls in alveolar PO2, while the steeper tissue portion allows substantial unloading with relatively small reductions in PO2.
Right and left shifts
P50 is the oxygen tension at which haemoglobin is 50% saturated. A higher P50 indicates lower affinity and therefore a right shift.
| Shift | Typical factors | Effect |
|---|---|---|
| Right | Higher CO2, higher H+ / lower pH, higher temperature, increased 2,3-BPG | Lower haemoglobin affinity and easier tissue unloading. |
| Left | Lower CO2, lower H+ / higher pH, lower temperature, reduced 2,3-BPG, fetal haemoglobin; carbon monoxide also increases remaining-site affinity | Higher affinity and reduced unloading at a given PO2. |
Surgical applications
- In haemorrhage, restoring saturation alone cannot correct the fall in oxygen-carrying capacity from blood loss.
- In sepsis, delivery and utilisation may both be disturbed, so lactate and perfusion trends must be interpreted in context.
- Hypothermia shifts the curve left, potentially impairing unloading even while metabolic demand is reduced.
- Carbon monoxide poisoning can produce misleadingly reassuring conventional pulse-oximetry readings because dyshemoglobins are not distinguished reliably by standard two-wavelength devices.
Active recall
Close the notes and answer these
Try each question from memory before revealing the answer. These public prompts are a small preview of the integrated retrieval system inside SurgAtlas.
01What two major variables determine tissue oxygen delivery?
Cardiac output and arterial oxygen content.
02Why can SpO2 be normal in life-threatening anaemia?
SpO2 is the percentage saturation of haemoglobin, not the haemoglobin concentration. Oxygen content can be very low if there is too little haemoglobin.
03What does a right shift of the dissociation curve do?
It lowers haemoglobin affinity for oxygen and facilitates unloading to tissues at a given PO2.
Sources & editorial basis
References & editorial basis
- SurgAtlas production chapter — Applied Physiology — Respiratory & Cardiovascular Physiology. Primary source for this public lesson. Stable physiological principles have been condensed from the corresponding SurgAtlas production teaching.
- West’s Respiratory Physiology. Reference for stable oxygen transport and dissociation-curve physiology.
This lesson is derived from the corresponding SurgAtlas production teaching material. Where the source makes current management or guideline claims, the public lesson uses the cited contemporary guidance. It is written for education and examination preparation, not as patient-specific clinical advice.
Editorial details
Medical Doctor (MD) · MRCS Part A · Physician · Surgical Educator
SurgAtlas is an educational resource. For patient care, verify current national guidance, local antimicrobial and transfusion policies, specialty pathways and individual patient factors.
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