Creator: Dr. Heo Ji-young
Published: 2026-07-25T05:38:12.000Z

Oxygen Is Not the Same Everywhere in the Body

Clip the device onto a fingertip and a number appears in seconds. 98, 99. Oxygen saturation. If it is normal, most people take it to mean oxygen is getting in nicely and move on.

When I look at that number, I hold one thing alongside it: which place this value reflects.

That Number Is Blood That Has Just Left the Lungs

Oxygen saturation shows how fully the oxygen carried in arterial blood is loaded. It is blood just after taking on oxygen in the lungs. So the value tells you very well whether the lungs did their job.

But that blood has not yet arrived. It passes the heart, the great vessels, branches into ever finer paths, and at last hands oxygen over to tissue.

The blood that has handed it over is venous blood. That arterial and venous oxygen differ is not a disease; it means the tissue in between took its share. The saturation commonly measured reads the side before the handover.

Tissue Is Low to Begin With — and Differs by Place

This is where people are surprised. The air we breathe is about 21% oxygen, while tissue in the body generally sits between 2% and 9%. Low is normal.

And it differs by place. The cartilage covering a joint sits at 2–5%, particularly low, and bone marrow measures around 7%. It is not low because it is diseased; those cells settled into work at that much oxygen. No blood vessels enter cartilage at all; it receives what it needs by diffusion.

So the body does not hold oxygen in equal shares. Each place keeps house on a different allotment.

So Saturation Can Be Normal While That Place Is Not

Seen this way, something that looked contradictory resolves.

A 98 at the fingertip means the lungs loaded oxygen well. Whether it reached that place, and whether it was handed over there, is another matter. Where tissue is compressed, where flow is sluggish, where swelling has lingered, the share arriving can fall.

That is how "the oxygen reading is normal, and that spot stays heavy and numb" comes about. The two sentences speak of different places, so both can be true.

I Watch the Arrival More Than the Departure

In the consulting room I watch where it lands more than the number: whether that place is warm, whether colour returns slowly after pressing, whether changing posture changes it. The fingertip number reports the starting point; these things let me estimate the destination.

And helping arrival is usually a modest business: making a compressed place less compressed, getting that place moving, letting the breath deepen. More often than taking in more oxygen, the place to work is letting the oxygen already carried in actually arrive.

That said, if lips or fingertips turn bluish, if slight exertion leaves you breathless, or if a limb goes pale and cold — that is a different story from this one. Having that place examined comes first.

The number at the fingertip reports accurately what the lungs did. But it is the value at the point of departure.

Inside the body, oxygen is divided into different shares by place, and some places live low by design. If the reading is normal and only that spot is different, what the body is speaking of may not be the lungs but the road that reaches it.

Oxygen Is Not the Same Everywhere in the Body

Contents