In one breath
About 98% of the oxygen in arterial blood rides on hemoglobin; only about 2% is dissolved in plasma. One gram of Hb holds 1.34 mL of O₂, so blood with 15 g/dL of Hb can carry about 20 mL of O₂ per 100 mL. How much of that capacity is actually filled (the saturation) depends on the PO₂, which is what the Oxygen–hemoglobin dissociation curve describes.
Builds on: Gas exchange in the lungs · Leads to: Oxygen–hemoglobin dissociation curve · Carbon dioxide transport · Hypoxia
Two forms of oxygen in blood
| Form | Share | In 100 mL of arterial blood |
|---|---|---|
| Bound to hemoglobin (oxyhemoglobin) | ~98% | ~19.7 mL (20.1 mL capacity × 98%) |
| Physically dissolved in plasma | ~2% | 0.3 mL |
Dissolved form and Henry’s law
Henry’s law: at a given temperature, the amount of a gas that dissolves in a liquid is proportional to the partial pressure of that gas.
Dissolved O₂ = solubility × PaO₂ = 0.003 mL/dL per mmHg × 95 mmHg ≈ 0.3 mL per 100 mL of blood
Why this is not enough on its own: with a resting cardiac output of 5 L/min, dissolved O₂ delivers only about 15 mL/min. The body uses about 250 mL/min at rest.
Applied: hyperbaric oxygen
Breathing 100% O₂ in a hyperbaric chamber raises dissolved O₂ to about 6 mL/dL. That delivers roughly 300 mL/min at a cardiac output of 5 L/min, which is enough to keep the tissues oxygenated. This is the basis of hyperbaric oxygen therapy (see Oxygen therapy).
In combination with hemoglobin
Structure and oxyhemoglobin formation
- Hb has four subunits. Each subunit is a heme attached to a polypeptide chain, and the four chains together make up the globin.
- Each heme holds one iron atom in the ferrous (Fe²⁺) state, and each iron binds one O₂. So one Hb molecule carries up to four O₂ (Hb₄O₈).
- Binding is fast (under 0.01 s) and reversible: Hb + O₂ ⇌ HbO₂. Hb carrying O₂ is oxyhemoglobin; Hb without it is deoxyhemoglobin (reduced Hb).
Oxygenation, not oxidation
The iron stays ferrous when O₂ binds, so the reaction is oxygenation. If the iron is oxidized to ferric (Fe³⁺), the pigment becomes methemoglobin, which cannot carry O₂.
- Methemoglobin forms spontaneously, and also under drugs and chemicals such as nitrites and sulfonamides.
- The red-cell enzyme methemoglobin reductase turns it back into Hb, so normally only about 1.5% of Hb is methemoglobin.
- An inherited lack of the reductase raises methemoglobin and lowers O₂-carrying capacity.
Tense and relaxed states: why binding is cooperative
- Deoxyhemoglobin is held in a compact tense (T) state by bonds between its subunits. Its affinity for O₂ is low.
- Binding of the first O₂ loosens the molecule into the relaxed (R) state, which exposes the remaining sites. Affinity rises by a few hundred times (your book quotes 200–500).
- So the amount of HbO₂ formed depends on PO₂. The high PO₂ of pulmonary capillaries favors loading; the low PO₂ of tissue capillaries favors unloading.
- This cooperative behavior is what gives the dissociation curve its S shape.
O₂-carrying capacity of Hb
- 1 g of fully saturated Hb binds 1.34 mL of O₂.
- With Hb at 15 g/dL: 1.34 × 15 ≈ 20.1 mL O₂ per 100 mL. This is the O₂ capacity of blood.
- O₂ content of Hb is the O₂ actually bound. O₂ capacity is the most that could be bound.
- Percentage saturation (SO₂) = (HbO₂ content ÷ HbO₂ capacity) × 100. Example: a content of 15 mL/dL with a capacity of 20 mL/dL gives an SO₂ of 75%, which is typical of mixed venous blood. Arterial blood is normally about 97–98% saturated.
Importance of oxygen saturation, content and extraction
Oxygen saturation
The fraction of heme sites that are carrying O₂. At 100% saturation every heme is occupied.
Oxygen content
The total O₂ in a unit volume of blood: the O₂ bound to Hb plus the O₂ dissolved in plasma. Dissolved O₂ is tiny, so content depends mainly on the Hb concentration and its binding capacity.
Anemia: when saturation misleads
An anemic patient (say Hb 7 g/dL) can have a normal PaO₂ and a normal saturation, because capacity and content fall together. Yet the O₂ content is less than half of normal and the tissues become hypoxic. In anemia, content matters more than PaO₂ or saturation.
Oxygen extraction
- The amount of O₂ the tissues take from the blood. It is an index of how much O₂ they consume.
- It is expressed as the oxygen extraction ratio (OER) = O₂ extracted ÷ O₂ delivered. Your book also calls it the oxygen coefficient ratio; Guyton calls it the utilization coefficient.
- Whole body at rest: about 25% (arterial ~20 mL/dL − venous ~15 mL/dL = 5 mL/dL taken out).
- Metabolically active tissue extracts far more; cardiac muscle extracts up to about 85% even at rest.
Applied physiology
Measurement of O₂ saturation: pulse oximetry
- A noninvasive, continuous measurement of Hb saturation, used routinely on wards and in intensive care.
- A probe on a fingertip or earlobe shines red and infrared light through the tissue. Oxy- and deoxyhemoglobin absorb the two lights differently.
- Only the pulsatile part of the absorbance is used, because it comes from arterial blood. The steady, non-pulsatile part comes from tissue, capillary and venous blood.
- Worth knowing, though not in your pages: the device reads carboxyhemoglobin as if it were oxyhemoglobin, so SpO₂ can look normal in CO poisoning.
Pulmonary damage by free radicals
- In mitochondria, O₂ is normally reduced to water by accepting four electrons from the electron transport chain.
- Leaks in the chain let O₂ take up fewer than four electrons, and this forms free radicals.
- The lung is a frequent victim. Damage to pulmonary capillaries leads to pulmonary edema.
Reactive oxygen species and antioxidants
- Free radical: an atom or molecule with an unpaired electron in its outer orbit.
- Common ones are superoxide (O₂•⁻) and hydroxyl (OH•). Hydrogen peroxide can generate hydroxyl radicals, and superoxide combines with nitric oxide to form peroxynitrite. Together they are the reactive oxygen species (ROS), also called pro-oxidants.
- Antioxidant enzymes neutralize ROS: superoxide dismutase, catalase and the peroxidases.
- When ROS outweigh the antioxidants, the result is oxidative stress: tissue damage and degeneration.
- Other sources of ROS include the respiratory burst of neutrophils in inflammation and reperfusion injury, when blood returns to tissue that was ischemic. Reperfusion injury is one mechanism of refractory shock.
Draw it: the Hb molecule
Draw four chains (α₁, α₂, β₁, β₂) in a square with a heme disc in each and Fe²⁺ at its centre, and put an O₂ on each iron. Label “globin = four chains” and “heme = porphyrin ring + Fe²⁺”. Mark the central cavity between the β chains, where 2,3-DPG binds.
Exam-answer skeleton: "Describe the transport of oxygen in blood" (long essay)
- The two forms and their shares, with the dissolved-O₂ arithmetic (Henry’s law).
- Hb structure; oxygenation vs oxidation; methemoglobin.
- T and R states: cooperative binding.
- Capacity (1.34 mL/g, about 20 mL/dL), content and saturation, with the SO₂ formula.
- The O₂–Hb dissociation curve: draw it with its steep phase, plateau and P50 (see Oxygen–hemoglobin dissociation curve).
- The factors that shift the curve (right vs left table); the Bohr effect.
- Unloading in the tissues; O₂ extraction.
- Applied: CO poisoning, anemia, pulse oximetry.
Asked in exams
- How blood carries oxygen: Nov 2014, 10 marks · Apr 2021, 15 marks · Mar 2024, MCQ