Physiology: What you need to know for the Divemaster and Instructor Exams
Will Welbourn explains the respiratory drive, dead air space, hyperventilation, shallow water blackout and CNS oxygen toxicity for the PADI Divemaster and IDC exams.
Greek Word Roots — Your Secret Weapon in the Physiology Exam
Before diving into the topics, learn these five Greek roots. They will help you work out the meaning of words you've never seen before — just by breaking them apart.
| Root | Meaning | Example |
|---|---|---|
| Hyper- | Too much | Hyperactive = too active |
| Hypo- | Too little | Hypoactive = not active enough |
| -capnia | Carbon dioxide | Hypercapnia = too much CO₂ |
| -oxia | Oxygen | Hypoxia = too little oxygen |
| Baro- | Pressure | Barotrauma = pressure injury |
Hyperoxia — too much oxygen | Hypoxia — too little oxygen
Why We Breathe — and What Controls the Urge
We breathe to deliver oxygen to our cells and remove carbon dioxide — the waste product of producing energy. But here is the surprising part: it is rising CO₂, not falling oxygen, that triggers the urge to breathe.
The respiratory reflex centre in the brain monitors CO₂ levels. When CO₂ reaches a certain threshold, it fires the signal to take a breath. Think of it like a gauge — when it hits the red line, you breathe, the CO₂ drops, and the gauge slowly climbs again.
Dead Air Space
Your tidal volume is the total amount of air you move with each normal breath. But not all of that air reaches your alveoli — the tiny air sacs in the lungs where gas exchange actually happens.
The air that fills your mouth, throat, and regulator hose never reaches the alveoli. It plays no part in gas exchange. That is your dead air space.
When you exhale, that dead air stays in your airway. On your next inhale, it is the first air back into your lungs — and it is high in CO₂ and low in oxygen.
Rapid shallow breathing: dead air can be 50%+ of each breath — CO₂ builds up fast.
Deep slow breathing: dead air is a tiny fraction — CO₂ is cleared efficiently.
Involuntary Hyperventilation — Rapid Shallow Breathing
Involuntary hyperventilation is not something you choose to do. It happens when you are stressed, over-exerting yourself, or struggling to breathe from a poorly adjusted regulator.
The pattern is fast, shallow breaths. Because each breath is small, a high proportion of what you inhale is dead air — high in CO₂. This keeps your CO₂ levels elevated, which makes you want to breathe even faster and shallower. It becomes a vicious cycle.
- Cause: over-exertion, anxiety, poorly serviced regulator
- Result: CO₂ levels rise — hypercapnia
- Effect: rapid, shallow breathing gets worse, not better
Voluntary Hyperventilation — Deep Deliberate Breathing
Voluntary hyperventilation is a deliberate choice — taking a series of very deep, full breaths before a breath-hold dive. This is what freedivers sometimes do.
Each deep breath flushes out a large amount of CO₂. After several deep breaths the CO₂ level in the body drops well below normal. The respiratory reflex centre no longer fires. The urge to breathe is delayed.
- Cause: deliberate deep breathing before a breath-hold dive
- Result: CO₂ drops — hypocapnia
- Effect: the urge to breathe is delayed — but oxygen is still being consumed
Shallow Water Blackout
Shallow water blackout is caused by the falling partial pressure of oxygen during ascent. This is where your physics knowledge and your physiology knowledge come together.
Here is what happens step by step:
- The diver voluntarily hyperventilates — CO₂ drops, urge to breathe is suppressed
- They dive to depth — pressure increases the partial pressure of oxygen in their blood, keeping them conscious even as oxygen is consumed
- They start ascending — pressure drops, and the partial pressure of oxygen in their blood drops with it
- Near the surface — typically between 10m and 5m — blood oxygen partial pressure falls below the threshold for consciousness
- The diver loses consciousness without warning — there was no CO₂ signal to make them surface first
Oxygen Toxicity — Too Much Oxygen
CNS Oxygen Toxicity
When the partial pressure of oxygen exceeds 1.6 ata, CNS oxygen toxicity becomes a serious risk. The result is a sudden, uncontrolled convulsion underwater — the diver loses their regulator and drowns. There is often no warning.
Warning signs, when they do occur, are remembered with the mnemonic VENTID-C: Visual disturbances, Ears (tinnitus), Nausea, Twitching, Irritability, Dizziness — followed by Convulsion.
For recreational divers the limit is 1.4 ata as a working maximum, with 1.6 ata as the absolute ceiling. For nitrox divers, the Maximum Operating Depth (MOD) is calculated from these limits.
Pulmonary Oxygen Toxicity
Pulmonary oxygen toxicity results from prolonged exposure to elevated oxygen concentrations over hours or days — not from a single dive. Symptoms include chest pain, coughing, and irritated lungs. This is rare in recreational diving but is relevant for technical divers doing extended decompression on high-oxygen mixes, and for patients receiving oxygen therapy in hospital.