🤿 Tissue Loading & Saturation

When you dive, the increased pressure causes nitrogen from the air you breathe to dissolve into your body tissues. The deeper you go and the longer you stay, the more nitrogen accumulates. If you ascend too quickly, this nitrogen can form bubbles in your tissues — causing decompression sickness (DCS), also known as "the bends."

Henry's Law states that the amount of gas dissolved in a liquid is proportional to the pressure of that gas above the liquid. Think of a soda bottle: when sealed under pressure, CO₂ stays dissolved. Open the cap (reduce pressure), and bubbles form as gas comes out of solution.

The same principle applies to your body during a dive. As you descend and pressure increases, more gas dissolves into your blood and tissues. While this applies to all gases you breathe, oxygen (O₂) is not a concern for bubble formation because:

• Your body continuously consumes oxygen through metabolism
• Most oxygen in blood is bound to hemoglobin in red blood cells, not freely dissolved

The gas we must carefully manage is nitrogen (N₂) — it's inert (your body doesn't use it) and makes up 78% of air. For technical divers using trimix, helium (He) follows the same principles but with different solubility characteristics.

Notice that fatty tissues can hold 5× more nitrogen than blood at the same pressure!

Gas flows along the partial pressure gradient — from higher pressure to lower pressure. During a dive, the pathway is:

As you descend, ambient pressure increases. The partial pressure of nitrogen in your lungs becomes higher than in your tissues, so gas flows into tissues. All compartments begin absorbing nitrogen, but fast tissues fill first.

ppN₂_tissue < ppN₂_alveolar → On-gassing

While at your maximum depth, fast tissues approach equilibrium within minutes. Medium tissues continue loading. Slow tissues (like fat) are still far from saturation — they would need hours at depth to fully equilibrate.

As you ascend, the partial pressure of nitrogen in your lungs drops. When tissue nitrogen pressure exceeds alveolar pressure, the gradient reverses and gas flows out — this is off-gassing.

ppN₂_tissue > ppN₂_alveolar → Off-gassing

But there's another threshold: when tissue nitrogen pressure exceeds ambient pressure, the tissue is supersaturated. This is when we need to be careful — the dissolved gas wants to come out of solution and could form bubbles.

ppN₂_tissue > P_ambient → Supersaturation ⚠️

Why Do Bubbles Grow?

A gas bubble in tissue has three pressures acting on it:

• Ambient pressure (P_amb) — pushes the bubble inward
• Surface tension (2γ/r) — the Laplace pressure, also squeezes the bubble. Smaller bubbles have more surface tension.
• Dissolved gas tension (P_tissue) — the total pressure of gases dissolved in surrounding tissue, pushing gas into the bubble

Use the slider to ascend from 30m. Watch how reducing ambient pressure tips the balance — larger bubbles cross the critical radius first.

We've seen what happens during on-gassing and off-gassing — now let's explore how these processes unfold over time. Like radioactive decay and many other natural phenomena, gas diffusion in tissues follows a predictable mathematical pattern.

A tissue's half-time is how long it takes to become 50% saturated (or 50% desaturated) at a given pressure difference. After each half-time period:

A 5-minute compartment reaches 50% saturation in 5 minutes; a 635-minute compartment takes over 10 hours!

A tissue's half-time is constant — it always takes the same time to close 50% of the gap between current tissue pressure and the target. But when that gap is larger, 50% represents more gas in absolute terms.

Example: If a tissue needs to off-gas 2 bar of nitrogen, switching from air (79% N₂) to Nitrox 50 (50% N₂) or pure O₂ (0% N₂) creates a much larger pressure gradient, so each half-time removes more gas — this is why oxygen-rich mixes accelerate decompression!

ℹ️ Both charts use the same depth and the same half-time formula. On-gassing and off-gassing are symmetrical—only the direction changes.

The equations behind the curves. Expand to see how Bühlmann modelled tissue pressure at constant depth and during ascent.

Your body is not homogenous — different tissues have vastly different characteristics that affect how quickly they absorb and release inert gas:

• Blood perfusion — Well-perfused tissues (brain, heart) exchange gas quickly; poorly-perfused tissues (cartilage, fat) exchange slowly
• Gas solubility — Fat dissolves 5× more nitrogen than blood (see table above), so fatty tissues take longer to saturate
• Distance from blood supply — Gas must diffuse from capillaries into tissue; tissues far from blood supply take longer

This is why decompression models use multiple theoretical compartments to simulate the full range of tissue behaviors in your body.

The Bühlmann ZH-L16 model uses 16 theoretical compartments — these are mathematical constructs fit to experimental data, not literal anatomical tissues. They span a range of half-times designed to model the full spectrum of tissue behaviors:

• Fast compartments (5–12.5 min) — saturate in minutes, but also release gas quickly
• Medium compartments (18.5–77 min) — balance between loading and off-gassing
• Slow compartments (109–635 min) — can take hours to fully saturate

At any point during a dive, the controlling compartment is the one closest to its critical supersaturation limit — this compartment determines your decompression ceiling.

Bühlmann developed three variants of the ZH-L16 algorithm, each with different levels of conservatism. The variants differ only in their 'a' coefficients — the half-times and 'b' values remain the same.