Nitrogen on Blood

Understanding Nitrogen in the Blood: What Happens at 0–30 Meters Underwater?

When divers descend beneath the surface, pressure changes begin affecting the body immediately. One of the most important physiological changes involves nitrogen absorption into the blood and tissues.

Using a simplified decompression model, we can visualize how nitrogen builds up in the body over time at different depths: 0 m, 10 m, 20 m, and 30 m.

This article explains what’s happening, why it matters, and how this connects to real-world diving safety.

The Basic Physics Behind Nitrogen Absorption

When breathing normal air:

  • Air is approximately 79% nitrogen

  • At the surface (1 atmosphere of pressure), nitrogen exerts a partial pressure of about 0.79 atm

As a diver descends:

  • Pressure increases by roughly 1 atmosphere for every 10 meters of seawater

  • The partial pressure of nitrogen increases proportionally

  • The body absorbs more nitrogen into blood and tissues

This relationship is governed by Henry’s Law:

The amount of gas dissolved in a liquid is proportional to the partial pressure of that gas.

The Model Used

To estimate nitrogen uptake over time, we used a simplified version of the classic decompression model developed by John Scott Haldane.

Assumptions:

  • Breathing normal air (79% nitrogen)

  • Pressure increases by 1 atm per 10 meters

  • Single tissue compartment

  • 30-minute time constant (moderate-speed tissue)

  • Starting at surface equilibrium (0.79 atm nitrogen)

Real dive computers use multiple tissue compartments, but this simplified model clearly shows the core concept.

What Happens at Each Depth?

0 Meters (Surface)

Nitrogen remains stable at about 0.79 atm.
No additional absorption occurs because pressure doesn’t increase.

10 Meters (2 atm total pressure)

  • Nitrogen partial pressure ≈ 1.58 atm

  • Blood nitrogen gradually rises toward this value

  • After 30 minutes, tissues are about 63% equilibrated toward the new pressure

This is why even relatively shallow dives still have no-decompression limits.

20 Meters (3 atm total pressure)

  • Nitrogen partial pressure ≈ 2.37 atm

  • Nitrogen loads significantly faster relative to surface levels

  • Supersaturation risk increases during ascent

Bottom time becomes more limited here.

30 Meters (4 atm total pressure)

  • Nitrogen partial pressure ≈ 3.16 atm

  • Nitrogen accumulates rapidly

  • Decompression planning becomes critical

At this depth, no-decompression limits are dramatically shorter.

Why Time Matters: The Exponential Uptake Curve

Nitrogen doesn’t load instantly. It follows an exponential curve:

  • Fast at first

  • Slows as it approaches equilibrium

  • Never truly reaches 100% saturation

This curve shape explains why:

  • The first 10–20 minutes add significant nitrogen

  • Long bottom times dramatically increase decompression stress

  • Slow ascents are essential

What Happens During Ascent?

During ascent:

  • Ambient pressure drops

  • Dissolved nitrogen becomes supersaturated

  • If ascent is too rapid, bubbles can form

This is the mechanism behind decompression sickness (DCS).

Modern dive computers use multi-compartment versions of the original John Scott Haldane model to control ascent rates and required stops.

Important Disclaimer

This model is simplified for educational purposes:

  • Real physiology involves multiple tissues

  • Blood and tissues absorb nitrogen at different rates

  • Dive planning requires certified tables or dive computers

  • Individual factors (hydration, temperature, workload) affect risk

Never use simplified models for actual dive planning.

Why This Matters

Understanding nitrogen uptake helps divers:

  • Respect no-decompression limits

  • Plan bottom time responsibly

  • Ascend slowly and safely

  • Appreciate why repetitive dives increase risk

The deeper you go, the faster nitrogen accumulates.
The longer you stay, the more it builds.

Pressure may be invisible — but its effects are not.

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