What are fast and slow tissues?
Short version: a tissue is fast if it takes up and gives up nitrogen in minutes, and slow if it takes hours. Brain and blood are fast. Fat, joints and bone are slow. And the reason comes down to just two things.
Your dive computer tracks sixteen “compartments” with half-times from about 4 to 635 minutes (that's covered in What does my dive computer actually measure?). Those compartments are math. This page is about the real tissues behind them.
What makes a tissue fast or slow
Two things, and only two: how much nitrogen it can hold, and how much blood flows through it to deliver and remove that nitrogen.1,2 Think of a sponge and a hose. A small sponge on a big hose fills and empties in minutes. A big sponge on a small hose takes hours either way. Physiologists write it almost exactly like that:3
half-time ≈ 0.693 × (how much the tissue holds) ÷ (how much blood flows through it)
That one ratio explains every tissue on the list.2,4
| Tissue | Speed | Why |
|---|---|---|
| Lungs, blood | Fastest | The blood is the hose. It matches the gas you're breathing within a breath or two. |
| Brain, spinal cord | Fast | Small sponge, enormous blood supply. The brain gets far more blood per gram than almost anything else. |
| Muscle | It depends | At rest, modest flow. Working hard, flow jumps many times over — so a muscle that's finning against current loads fast, and a muscle that's cold and still on the way up unloads slowly. |
| Fat | Slow | Big sponge: nitrogen dissolves about five times more readily in fat than in water. The blood supply is ordinary, but the sponge it has to fill is huge. |
| Joints, ligaments, cartilage, bone | Slowest | Small hose: very little blood flows through them, and cartilage feeds mostly by slow diffusion. |
The fat myth
One correction to a myth worth killing: fat is not slow because it lacks blood vessels. Fat is full of them — every fat cell sits close to a capillary. Fat is slow because it holds so much nitrogen that its ordinary blood supply takes a long time to fill it, and just as long to empty it. Same sponge, same hose, both directions.3
Why the bends hurts where it does
And notice where the bends classically hurts: the joints. The slowest tissues are the last to let go of their gas, so they're the ones still supersaturated when you surface. That's why the deep ache in a shoulder or knee is the textbook symptom.
Two cautions
Two cautions before you map your own body onto the sixteen compartments. First, the mapping is only an illustration; the model's compartments aren't these tissues, they're numbers chosen to make the predictions come out right. Second, it doesn't follow that carrying more body fat means more risk — the largest real-world study we have found leaner divers at higher risk, not heavier ones.5 (More on that here.) Holding more nitrogen and getting bent are not the same thing.
The one you can change
The muscle row is the one to take into the water. Your computer doesn't know whether you worked hard at depth or shivered on your safety stop — the models don't account for exercise or cold at all.4 Work easy at depth, stay warm on the way up, and you're moving the real tissues in the safe direction while the model isn't looking. It shows up in your gas consumption, too — work harder and your SAC climbs, a number your computer can show you (see the computer FAQ). Heat, cold, exercise and alcohol all work through this same blood flow; that's its own FAQ: Does a hot shower, a heater, exercise or a beer change my off-gassing?
For education only. This page explains the physiology behind decompression and the reasoning behind it. It is not for dive or gas planning. Plan every dive with your training, your dive computer and your gauges, and follow your instructor or dive professional.
References
- Clarke D. The nitrogen saturation myth. Alert Diver. November 2013. Divers Alert Network. ↩
- Divers Alert Network. Introduction to decompression sickness. Chapter 1 in Decompression Sickness, DAN Dive Medical Reference Books. 2020. ↩ab
- Brubakk AO, Neuman TS, editors. Bennett and Elliott's Physiology and Medicine of Diving. 5th ed. Edinburgh: Saunders; 2003. ↩ab
- Pollock NW. Gradient factors. Alert Diver. November 2015. Divers Alert Network. ↩ab
- Marroni A, Kot J, Pieri M, Pelliccia R, Balestra C. Identification of DCS risk factors in recreational diving: a multifactorial model based on the DAN DSL Database 2024. International Maritime Health. 2026;77(1):1–12. doi:10.5603/imh.108038 ↩