About 98% of the oxygen in your blood travels bound to hemoglobin inside red blood cells. The remaining 2% dissolves directly in the liquid portion of blood, called plasma. This two-part system keeps your tissues supplied with oxygen from the moment you inhale to the moment your cells use it.
Hemoglobin: The Primary Carrier
Hemoglobin is a protein packed inside red blood cells, and it does nearly all the heavy lifting when it comes to oxygen transport. Each hemoglobin molecule is made up of four subunits (two alpha, two beta), and each subunit contains an iron atom at its center. Oxygen binds directly to that iron. Since there are four subunits, a single hemoglobin molecule can carry up to four oxygen molecules at once.
One gram of hemoglobin can hold about 1.39 mL of oxygen when fully loaded. With a normal hemoglobin concentration of around 15 grams per 100 mL of blood, that adds up to roughly 20 mL of oxygen in every 100 mL of blood. This is vastly more than what plasma alone could carry.
How Oxygen Binds and Releases
Hemoglobin doesn’t just grab oxygen and hold on. It switches between two shapes: a “tense” form that resists binding oxygen and a “relaxed” form that binds it easily. When no oxygen is attached, hemoglobin sits in the tense state. But once the first oxygen molecule latches on, it triggers a shape change that makes the second, third, and fourth oxygen molecules bind with increasing ease. This behavior is called cooperative binding, and it’s the reason hemoglobin loads up so efficiently in the lungs, where oxygen is plentiful.
The reverse happens in your tissues. As hemoglobin enters areas with lower oxygen levels, it begins releasing oxygen. Losing the first molecule shifts it back toward the tense state, which causes the remaining oxygen molecules to detach quickly. This means hemoglobin doesn’t just trickle oxygen out gradually. It dumps most of its load right where your cells need it.
The Oxygen Dissociation Curve
If you plot how saturated hemoglobin is against the oxygen level in surrounding blood, you get an S-shaped (sigmoid) curve rather than a straight line. That S-shape is a direct result of cooperative binding, and it has real physiological importance.
At the top of the curve, where oxygen levels are high (like in the lungs at around 100 mmHg), the curve flattens into a plateau. This means hemoglobin stays nearly fully saturated even if oxygen levels dip somewhat. You get a built-in safety margin: moderate drops in lung oxygen don’t dramatically reduce how much oxygen your blood picks up. At the steep middle portion of the curve, where oxygen levels match what you’d find in active tissues (around 20 to 40 mmHg), even a small further drop causes hemoglobin to release a large amount of oxygen. This is exactly the behavior your body needs: hold tight in the lungs, let go generously in the tissues.
What Shifts the Curve
Several factors can nudge the dissociation curve left or right, changing how readily hemoglobin grabs or releases oxygen. A rightward shift means hemoglobin releases oxygen more easily. A leftward shift means it holds on tighter.
- Acidity (the Bohr effect): Active tissues produce carbon dioxide, which makes the local environment more acidic. This lower pH pushes hemoglobin toward its tense state, encouraging it to release oxygen precisely where metabolism is highest. Inside red blood cells, an enzyme rapidly converts carbon dioxide into bicarbonate and hydrogen ions. Those hydrogen ions bind to hemoglobin and further promote oxygen release.
- Temperature: Warmer tissues, like exercising muscles, shift the curve to the right. Hemoglobin releases more oxygen in warm areas and holds onto it in cooler ones.
- A metabolic byproduct in red blood cells: Red blood cells produce a compound called 2,3-DPG during their normal metabolism. It binds to the tense form of hemoglobin and stabilizes it, lowering oxygen affinity and boosting oxygen delivery to tissues. Your body increases 2,3-DPG production during endurance training and at high altitude, both situations where squeezing extra oxygen out of hemoglobin helps meet demand. At moderate altitudes (up to about 5,400 meters), this adaptation improves tissue oxygenation. At extreme altitudes, it can actually become counterproductive because hemoglobin struggles to pick up enough oxygen in the lungs.
The Pressure Gradient That Drives It All
Oxygen moves by diffusion, always flowing from higher pressure to lower pressure. In the lungs, the oxygen partial pressure in the air sacs (alveoli) is about 100 mmHg. Blood arriving from the body’s veins has an oxygen partial pressure of only about 40 mmHg. That 60 mmHg gap drives oxygen across the thin alveolar membrane and into the blood, where hemoglobin quickly binds it.
In the tissues, the gradient reverses. Arterial blood arrives at roughly 100 mmHg, while tissue oxygen levels are much lower. Muscles at rest sit around 27 to 31 mmHg. The brain needs between 30 and 48 mmHg. The kidney’s inner medulla runs as low as 10 to 20 mmHg. Skin near the surface drops to just 5 to 11 mmHg. These lower pressures pull oxygen off hemoglobin and into cells. By the time blood leaves the tissues and enters the veins, its oxygen pressure has dropped to about 40 mmHg, ready to be recharged in the lungs on the next pass.
Dissolved Oxygen in Plasma
A small but measurable amount of oxygen dissolves directly in plasma without binding to hemoglobin. This follows a simple physical principle: the amount of dissolved gas is proportional to the pressure of that gas. At an arterial oxygen pressure of 100 mmHg, every 100 mL of plasma holds only about 0.3 mL of dissolved oxygen. That’s far too little to sustain your tissues on its own, which is why hemoglobin is essential.
Still, dissolved oxygen matters. It’s the form that actually diffuses into cells. Hemoglobin releases oxygen into plasma, where it dissolves briefly before crossing into tissues. Dissolved oxygen is also what a pulse oximeter indirectly reflects and what arterial blood gas tests measure directly. A healthy arterial oxygen pressure falls between 75 and 100 mmHg, and a healthy oxygen saturation reading on a pulse oximeter ranges from 95% to 100%.
Fetal Hemoglobin: A Special Case
Before birth, a fetus can’t breathe air, so it pulls oxygen from the mother’s blood through the placenta. Fetal hemoglobin solves this problem by binding oxygen more tightly than adult hemoglobin. Its dissociation curve is shifted to the left, meaning it grabs oxygen at lower partial pressures. This higher affinity lets fetal blood extract oxygen from the mother’s blood even though the oxygen levels in the placenta are much lower than in the lungs. Fetal hemoglobin also binds 2,3-DPG less effectively, which keeps its oxygen affinity high. After birth, the body gradually replaces fetal hemoglobin with the adult form over the first several months of life.