Full mechanism, in text

How red cells carry oxygen

1 · Hb tetramer · 4 subunits

Hemoglobin is a tetramer of 2 α and 2 β chains, each holding a heme with an Fe²⁺ at its center, so one molecule carries 4 O₂.Hemoglobin (Hb) is not a single molecule, but a tetramer:
· 2 α chains + 2 β chains (adult HbA1).
· Each chain cradles one heme group at its center.
· Each heme has one Fe²⁺ at its center — the actual O₂-binding site.

So one Hb molecule carries 4 O₂. One red cell holds ~270 million Hb molecules → it transports ~1 billion oxygen molecules per trip.

Key: Fe must be Fe²⁺ (ferrous) for reversible O₂ binding. Fe³⁺ (ferric) cannot bind O₂ at all — an oxidized site is simply out of service, which is methemoglobinemia (triggered by nitrites / certain drugs). And it is worse than merely losing a few seats: the remaining Fe²⁺ sites in the same tetramer are pushed toward high affinity and become reluctant to release oxygen in tissue — carrying capacity falls and unloading gets harder at the same time, which is the real reason metHb starves tissue.

2 · Lung capillary · first O₂ binds

In oxygen-rich alveolar capillaries, one O₂ binds the Fe²⁺ of an α-chain heme and shifts Hb from the low-affinity T state toward the high-affinity R state.In the alveolar capillaries (pO₂ ~ 100 mmHg), a high-oxygen environment:
· An O₂ slips into the heme of one α chain and forms a reversible coordinate bond with Fe²⁺.
· This binding triggers a conformational change in Hb — from the "T state (tense, low-affinity)" toward the "R state (relaxed, high-affinity)."
· A subtle "shift of the Fe atom by ~0.4 Å toward the heme plane" → pulls on the F8 histidine of the α chain → tugs the entire subunit.

This is the textbook example of allostery — a geometric change in one molecule alters the behavior of all four subunits.

3 · Cooperativity: S-curve

Once the first O₂ binds, the other three sites gain affinity, each bound O₂ speeding the next, which gives the Hb oxygen saturation curve its S shape.Because of the conformational change, once the first O₂ binds, the affinity of the remaining three sites rises sharply — each additional bound O₂ makes the next bind faster:

· 1st O₂: low affinity (T state).
· 2nd O₂: medium.
· 3rd O₂: high.
· 4th O₂: highest (R state).

Result: the oxygen saturation curve of Hb is not a straight line but the famous sigmoid (S-curve):
· In the low-pO₂ zone (40 mmHg, tissues): steep release, high O₂-delivery efficiency.
· In the high-pO₂ zone (100 mmHg, lungs): saturation approaches 100%.
· Smooth transition in between.

This is a chemical masterpiece of evolution — the same molecule is most efficient at both pO₂ extremes, with a flexible middle transition. If Hb were non-cooperative like myoglobin, it would either fail to saturate in the lungs or refuse to release O₂ in the tissues.

4 · Tissue · Bohr effect releases

In tissue capillaries, low pO₂, low pH and high CO₂ make Hb release O₂; low pH pushing Hb back to the T state is the Bohr effect.In tissue capillaries (pO₂ ~ 40 mmHg + high CO₂ + low pH):
· Low pO₂ itself drives O₂ off (concentration gradient).
· Low pH (acidic, from CO₂ + lactate) → H⁺ binds histidines on Hb → pushes Hb back to the T state → releases more O₂ = the Bohr effect.
· High CO₂ → directly binds the N-terminus of α chains to form carbamino-Hb → further lowers O₂ affinity.
· 2,3-BPG (bisphosphoglycerate) provides long-term regulation inside red cells: rises at altitude / in chronic hypoxia → pushes Hb toward the T state overall → at the same pO₂, releases more O₂ (the core of altitude adaptation).

This is why muscle gets more oxygen during exercise: exercise → local CO₂ ↑ + lactate ↑ + temperature ↑ → the Bohr effect + temperature effect stack → Hb actively releases more O₂ to the muscles that need it most.

In iron deficiency: total Hb is insufficient → even fully saturated Hb can't carry enough O₂ → the heart is forced to speed up (palpitations) + muscles get inadequate O₂ (poor exercise endurance) — this is the chemical root of the physiological fatigue in iron-deficiency anemia.

Full mechanism, in text

1 · Hb tetramer · 4 subunits

2 · Lung capillary · first O₂ binds

3 · Cooperativity: S-curve

4 · Tissue · Bohr effect releases

Hemoglobin · cooperative O₂ binding

Full mechanism, in text

1 · Hb tetramer · 4 subunits

2 · Lung capillary · first O₂ binds

3 · Cooperativity: S-curve

4 · Tissue · Bohr effect releases

Hemoglobin · cooperative O₂ binding