Transport Of Oxygen
The Delivery Problem
Every one of the body's trillions of cells constantly burns fuel and needs a steady oxygen supply, yet oxygen is a gas and the cells are buried deep inside tissues. The body solves this with a dedicated carrier: a protein inside red blood cells called haemoglobin. Oxygen is not simply dissolved in blood — dissolved oxygen alone could barely supply the body — instead it binds chemically to haemoglobin, forming oxyhaemoglobin, which raises blood's oxygen-carrying capacity roughly seventy-fold.
How the Binding Works
When you inhale, oxygen diffuses from the alveoli into the surrounding capillaries and meets red blood cells packed with haemoglobin. Each haemoglobin molecule carries four iron-containing haem groups, and each haem group can bind one oxygen molecule. The reaction is reversible: haemoglobin picks up oxygen (Hb + 4 O₂ ⇌ Hb(O₂)₄) where oxygen is plentiful, in the lungs, and releases it where oxygen is scarce, in the tissues.
Oxyhaemoglobin, the oxygen-loaded form, is bright red; deoxyhaemoglobin, the oxygen-free form, is darker — this is why arterial blood looks bright red and venous blood looks darker.
Cooperative Binding
When the first oxygen molecule binds to haemoglobin, it triggers a shape change that makes it easier for the next three to bind — this is cooperative binding, and it produces an S-shaped (sigmoid) oxygen-haemoglobin dissociation curve rather than a straight line. At the oxygen level found in the lungs, haemoglobin is nearly fully saturated — almost every binding site is filled. At the lower oxygen level found in resting tissues, saturation drops to around 75%, meaning about a quarter of the carried oxygen is unloaded to the cells.
A healthy adult's blood carries about 20 mL of oxygen per 100 mL of blood; only a small fraction of that travels dissolved in plasma, the rest is bound to haemoglobin.
What Shifts the Curve
Haemoglobin's affinity for oxygen is not fixed — it changes with conditions in the tissues, letting the body fine-tune delivery:
| Factor | Change | Effect on oxygen release | Why it helps |
|---|
| Carbon dioxide level | High | Increases release | Active tissues produce more carbon dioxide |
| Acidity (pH) | Lower (more acidic) | Increases release | Active tissues produce lactic acid |
| Temperature | Higher | Increases release | Working muscles run hotter |