---
title: "Tissue compartments"
description: "What a tissue compartment is, which one leads a given dive, and how to read the planner's tissue heat map."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Tissue compartments

import { TissueDemo } from "@/components/react/TissueDemo";
import { trimixDecoWithTissues } from "@/demo/plans";

A tissue compartment is a mathematical fiction with one number attached to it: a
half-time, the minutes it takes to close half the gap between what the
compartment currently holds and what it is breathing. Sixteen of them, together,
are the whole of Bühlmann's model of your body.

The half-times run from a few minutes to several hours. A fast compartment fills
almost as quickly as you descend and empties almost as quickly as you ascend; a
slow one is still loading an hour into the dive and still unloading long after
you are dry. Helium is faster than nitrogen in every compartment, which is why
the two gases are tracked separately rather than as one inert total.

## Loading, and what leading means

Each compartment holds a certain amount of inert gas and has a limit: the most
it is allowed to hold at the depth you are at. **Saturation percentage** is the
first as a percentage of the second, and above 100 % the compartment has passed
its limit.

The **leading compartment** is the one with the highest saturation percentage at
that instant, and it is the one setting your ceiling. Exactly one is flagged as
leading at each step, and it changes hands during a dive: a fast compartment
leads on a short bounce, a slower one overtakes it if you hold the depth, and a
slow compartment usually finishes the job at the shallow end of a long ascent.

A second number per compartment, the **gradient-factor percentage**, says where
its loading sits between the depth you are at and its limit. 100 % is exactly on
the limit; a negative value means the compartment is still on-gassing. It is
what the gradient factors you set are a fraction *of*.

## Reading the tissue view

The planner's tissue view is a heat map. The **GF % trace** across the top
follows the leading compartment's gradient-factor percentage over the dive, with
reference lines at your GF Low and GF High. Below it, one row per compartment,
fastest at the top, and one column per step, with cell colour walking from cool
to hot as a compartment approaches its limit. A trail connects the leading
compartment across the columns, and the axis along the bottom is elapsed
minutes.

Follow the trail: the fast rows light first on descent and are the first to
clear, while the slow rows below are still loading through the decompression
stops and are what holds you at 6 m.

Only Bühlmann produces this view. Thalmann and the hybrid models track their
tissues a different way, one that does not map onto these rows, so a plan from
those models shows no heat map. A GF Calibration plan does show one, because the
schedule it hands you is a Bühlmann schedule.

## What the compartments do not tell you

They are not organs. No compartment is a joint and none is bone; they are rates
chosen so that a set of them, taken together, reproduced the outcomes of the
dives the model was fitted to. A compartment at 40 % saturation is not 40 % of
the way to being hurt: the percentage is a distance to a limit drawn from dive
outcomes, not from any measurement of gas in a person.

Nor does the leading compartment tell you where a bubble would form. The model
has no bubbles and no anatomy, and cannot say that a shoulder is more loaded
than a knee.

## Next

- [Bühlmann ZHL-16C](/theory/buhlmann) is where the limits come from.
- [Tissue saturation](/planning/review-tissues) shows where this view sits in
  the planner.

Source: https://docs.divelogic.ai/theory/tissues/index.mdx