No oxygen is bound. Hemoglobin sits in the tense T state, where the four hemes have a low affinity for oxygen. This is how it arrives in tissue after delivering its load.
Every cell in your body burns oxygen, but oxygen barely dissolves in water, so blood alone could not carry enough. The fix is hemoglobin, a protein that packs your red blood cells and acts like a taxi: it must pick oxygen up where there is a lot of it (the lungs) and drop it off where there is little (the working tissues). To do that well it cannot simply hold oxygen tightly everywhere — it has to change its grip depending on where it is.
Hemoglobin is a tetramer: four protein subunits (two $\alpha$ and two $\beta$) clipped together. Each subunit holds a heme, a flat ring with one iron atom ($\text{Fe}^{2+}$) at its center, and each iron binds exactly one oxygen molecule. So one hemoglobin carries up to four $\text{O}_2$. The fraction of those seats that are filled is the percent saturation. Plot saturation against the oxygen pressure $P_{O_2}$ and you do not get a straight line — you get an S-shaped curve.
The S-shape comes from cooperative binding: when the first $\text{O}_2$ binds, it tips hemoglobin from the tense T state toward the relaxed R state, and the R state grips the next oxygens more tightly. The saturation follows a Hill relation, $Y=\dfrac{(P_{O_2})^{n}}{(P_{50})^{n}+(P_{O_2})^{n}}$, with $n\approx 2.8$ and $P_{50}\approx 26\ \text{mmHg}$ (the pressure giving half-saturation). The PO₂ slider moves the marker along the curve; the pH and temperature sliders are the Bohr effect — more acid, more CO₂, or more heat raise $P_{50}$ and slide the whole curve right, so hemoglobin lets go of oxygen more easily.
Hit Lungs: high $P_{O_2}$, normal pH — saturation pins near 98%, the flat top of the curve. Now hit Tissue: pressure drops to about 40 mmHg and saturation falls to roughly 75%, so about a quarter of the oxygen is delivered. Hit Exercise and watch the Bohr effect push the curve right so even more unloads. Finally let the binding cycle play and follow oxygens docking onto the irons one at a time while the four subunits pull together — that shape change is the T-to-R switch behind the S-curve.
| Stage | What happens |
|---|---|
| 1. Deoxy (T) | No oxygen bound. The tense T state has low affinity — this is hemoglobin returning from the tissues. |
| 2. First O₂ | One $\text{O}_2$ binds an iron. It is the hardest one to load, which is why the curve starts off shallow. |
| 3. Second O₂ | The first binding nudges hemoglobin toward R, raising affinity, so the second $\text{O}_2$ binds more easily — cooperativity begins. |
| 4. Third O₂ | Affinity is higher still. The curve is now in its steep middle region where small pressure changes load a lot of oxygen. |
| 5. Fully oxygenated (R) | All four hemes are loaded and hemoglobin is in the relaxed, high-affinity R state — the saturated top of the curve, as in the lungs. |
| 6. Unloading in tissue | Low $P_{O_2}$ plus the Bohr effect (acid, CO₂, heat) tips hemoglobin back toward T, and oxygen is released where it is needed. |
If each heme bound oxygen independently, saturation would rise in a simple hyperbola, the way single-site myoglobin does. But hemoglobin’s four subunits talk to each other: binding at one site changes the shape of the whole protein and raises the affinity of the rest. That positive feedback makes the middle of the curve steep, so a modest fall in oxygen pressure between lungs and tissue triggers a large release. The Hill coefficient $n\approx 2.8$ captures that cooperativity — it would be $1$ with no cooperativity and $4$ if all four sites switched together perfectly.
Hard-working tissue produces carbon dioxide, acid (lower pH), and heat. All three stabilize the T state and shift the dissociation curve to the right, raising $P_{50}$ and lowering oxygen affinity — so hemoglobin releases extra oxygen exactly where demand is highest. In the lungs CO₂ is exhaled, pH rises, and the curve shifts back left so hemoglobin reloads. The red-cell molecule 2,3-BPG shifts the curve right as well, and fetal hemoglobin binds oxygen more tightly (a left shift) so a fetus can pull oxygen from the mother’s blood.
Hemoglobin is the protein inside red blood cells that carries oxygen from the lungs to the tissues. It is a tetramer: four protein subunits, two alpha and two beta, fitted together. Each subunit cradles a heme group, a flat ring molecule with a single iron atom (Fe²⁺, the ferrous form) at its center. An oxygen molecule binds directly to that iron, so one heme holds one O₂ and a whole hemoglobin can carry up to four O₂ at once. In the lungs, where oxygen is plentiful, hemoglobin loads up; in the tissues, where oxygen is scarce, it releases the oxygen.
Key takeaway: hemoglobin is a four-subunit protein with four iron-containing hemes, each able to bind one oxygen molecule, so it carries up to four O₂.The S-shape comes from cooperative binding. When the first O₂ binds to one heme, it nudges the whole hemoglobin into a shape that makes the other three hemes bind oxygen more easily, and each further O₂ makes the next one easier still. So at low oxygen levels the curve rises slowly (the first O₂ is hard to load), then steepens sharply in the middle as cooperativity kicks in, then flattens near the top as hemoglobin approaches full saturation. A protein with a single binding site, like myoglobin, has no cooperativity and gives a simple hyperbolic curve instead.
Key takeaway: the curve is S-shaped because binding one oxygen raises the affinity for the next, a property called positive cooperativity.The Bohr effect is the way hemoglobin releases oxygen more readily when conditions signal hard-working tissue. A fall in pH (more acidic), a rise in carbon dioxide, and a rise in temperature all lower hemoglobin’s affinity for oxygen and shift the dissociation curve to the right. Active muscles produce CO₂, acid, and heat, so exactly where oxygen is needed most, hemoglobin lets go of more of it. In the lungs the opposite happens: CO₂ leaves, pH rises, and hemoglobin grabs oxygen tightly again. The molecule 2,3-BPG made in red blood cells also shifts the curve right.
Key takeaway: the Bohr effect is the rightward shift of the oxygen curve caused by lower pH, higher CO₂, and higher temperature, which helps hemoglobin unload oxygen in active tissues.Hemoglobin switches between two overall shapes. The T (tense) state is the deoxygenated form: it has a low affinity for oxygen and its subunits are held in a slightly constrained arrangement. The R (relaxed) state is the oxygenated form: it has a high affinity for oxygen, and the subunits have shifted into a more open arrangement. Binding oxygen pushes hemoglobin from T toward R, and because the R state binds oxygen more tightly, this shift is the molecular basis of cooperativity. Factors of the Bohr effect (acid, CO₂) stabilize the T state, encouraging oxygen release.
Key takeaway: T is the tense, low-affinity, deoxygenated state and R is the relaxed, high-affinity, oxygenated state, and the T-to-R switch drives cooperative binding.In the lungs the partial pressure of oxygen is about 100 mmHg, and hemoglobin becomes roughly 97 to 98 percent saturated, so it loads almost completely. In resting tissues the partial pressure is about 40 mmHg, where hemoglobin is only about 75 percent saturated, so it gives up roughly a quarter of its oxygen on each pass. During exercise, tissue oxygen falls further and the Bohr effect shifts the curve right, so hemoglobin can unload much more. The flat top of the curve also means small drops in lung oxygen barely reduce loading, a useful safety margin.
Key takeaway: hemoglobin is about 98 percent saturated in the lungs and about 75 percent in resting tissues, unloading roughly a quarter of its oxygen, and far more during exercise.P₅₀ is the partial pressure of oxygen at which hemoglobin is exactly 50 percent saturated. For normal human hemoglobin it is about 26 to 27 mmHg at pH 7.4 and 37 degrees Celsius. P₅₀ is a single number that summarizes hemoglobin’s affinity for oxygen: a lower P₅₀ means higher affinity (the curve sits to the left, hemoglobin holds oxygen tightly), and a higher P₅₀ means lower affinity (the curve shifts right, hemoglobin releases oxygen more easily). The Bohr effect raises P₅₀, and conditions like fetal hemoglobin lower it.
Key takeaway: P₅₀ is the oxygen pressure giving 50 percent saturation, and it measures hemoglobin’s affinity, with a right shift raising P₅₀ and a left shift lowering it.Myoglobin is a single-subunit protein found in muscle, with just one heme and one oxygen-binding site. Because it has only one site, it cannot show cooperativity, so its oxygen-binding curve is a simple hyperbola, not an S-shape. Myoglobin also binds oxygen much more tightly than hemoglobin, with a very low P₅₀, so it stays loaded except when oxygen gets very low. This suits its job: myoglobin acts as an oxygen store in muscle, releasing oxygen only when tissue levels drop sharply, while hemoglobin is the transporter that picks up oxygen in the lungs and hands it off in the tissues.
Key takeaway: myoglobin has one site, a hyperbolic curve, and very high affinity for storage, while hemoglobin has four cooperative sites and an S-shaped curve for transport.