Diving Physiology

Related Subjects: | High Altitude Physiology | Diving Physiology | Trauma Physiology | Exercise Physiology | Gastric Physiology | Rectal Physiology

What is Diving Physiology? ⚓🐠
Diving physiology explains how the body responds to increased pressure, denser breathing gases, immersion, and cold. Understanding these changes is essential for safe diving and for recognising and treating dive-related illness.

Core Physics Underwater 🧮

• Pressure & Depth: In seawater, pressure increases ~1 atmosphere (atm) every 10 m (33 ft). Surface = 1 atm; 30 m ≈ 4 atm.
• Boyle’s law (P×V = constant): As pressure increases, gas volume decreases (lungs, mask, middle ear) → equalise early & often.
• Dalton’s law: Partial pressure of each gas = fraction × total pressure. At depth, rising ppO₂ and ppN₂ drive oxygen toxicity and narcosis risk.
• Henry’s law: Dissolved gas ∝ partial pressure. More nitrogen dissolves at depth; too-rapid ascent forms bubbles → decompression sickness (DCS).
• Gas density & work of breathing 🫁: Denser gas at depth increases resistance; risk of CO₂ retention during exertion.

Physiological Responses to Immersion 🌊

• Mammalian diving reflex:
  • Vagal bradycardia ❤️ conserves oxygen
  • Peripheral vasoconstriction preserves cerebral/cardiac perfusion
  • Blood shift toward the thorax at depth protects alveoli (notably in breath-hold diving)
• Immersion & cold:
  • ↑ Central blood volume → diuresis (cold water diuresis) → dehydration risk 💧
  • Cold stress → shivering, ↑ O₂ consumption; fine-motor decline and afterdrop on exit

Lungs & Ventilation Under Pressure 🫁

• Lung volumes: Gas compression reduces volume; breath-hold descent risks “lung squeeze”.
• Work of breathing: Dense gas, tight suit, regulator load → hypercapnia risk (headache, dyspnoea, panic).
• V/Q changes: Hydrostatic gradients and post-dive microbubbles can increase dead space and alter gas exchange.

Breathing Gas Options 🔬

• Air: 21% O₂ / 79% N₂. Simple, but depth limited by narcosis and ppO₂.
• Nitrox (EANx): ↑ O₂, ↓ N₂ → less N₂ loading and longer NDLs; respect ppO₂ limits (typical working ≤1.4 ata).
• Trimix: Adds helium to reduce narcosis and gas density for deep dives; monitor HPNS and thermal loss.
• Heliox: Helium + O₂, used for very deep/commercial dives; minimal narcosis, low density.

Diving-Related Illnesses & Injuries 🚑

Barotrauma (Boyle’s Law–Driven)

Decompression Sickness (DCS) “The Bends”

• Cause: Inert gas (usually nitrogen) coming out of solution as bubbles during/after ascent.
• Type I: Joint/limb pain, pruritus, skin marbling (cutis marmorata), lymphatic swelling.
• Type II: Neurologic deficits, ataxia/vertigo, pulmonary “chokes”, spinal cord syndromes.
• Immediate actions: 100% O₂, supine, hydrate if able, minimise exertion; arrange hyperbaric recompression ASAP.
• Prevention: Conservative profiles, slow ascents, safety stops, adequate surface intervals, hydration, avoid heavy exertion post-dive, respect “no-fly” intervals.

Arterial Gas Embolism (AGE)

• Mechanism: Over-expansion injury → alveolar rupture → gas enters arterial circulation → stroke-like symptoms within minutes of ascent.
• Management: 100% O₂, airway protection, rapid evacuation for recompression.

Nitrogen Narcosis 🤿

• Features: Euphoria, poor judgement, tunnel vision, slowed reactions (often >30 m on air; dose-dependent with depth).
• Prevention: Limit depth; use helium mixes for deeper dives; ascend if symptomatic.

Oxygen Toxicity (ppO₂ too high) 🧯

• CNS toxicity: Visual/auditory changes, nausea, twitching, seizures (risk ↑ with ppO₂ ≥1.6 ata, CO₂ retention, cold, stress).
• Pulmonary toxicity: With prolonged high FiO₂ exposure → cough, chest tightness, ↓ vital capacity.
• Mitigation: Respect ppO₂ limits (work ≤1.4 ata; deco ≤1.6 ata), manage workload/CO₂, limit exposure time.

CO₂ Retention & Work of Breathing

• Causes: Dense gas at depth, heavy exertion, inadequate ventilation, rebreather scrubber failure.
• Symptoms: Headache, dyspnoea, anxiety/panic, confusion; potentiates O₂ toxicity & narcosis.
• Action: Reduce exertion, improve ventilation, change depth/gas per training; abort if unresolved.

Hypothermia & Thermal Stress ❄️

• Water conducts heat ~25× faster than air → major heat loss risk.
• Prevention: Appropriate wet/dry suit, hood, gloves; nutrition/hydration; warm-up between dives.

Breathing Gas Mixtures

• Nitrox: Higher O₂ than air; reduces N₂ uptake and DCS risk; monitor ppO₂ carefully.
• Trimix: N₂ + O₂ + He; reduces narcosis and gas density for deep/technical dives.
• Heliox: He + O₂; used in deep commercial/medical operations to minimise narcosis and density.

Dive Planning & Safety 🧭

• Dive tables & computers: Plan bottom time, ascent rate, safety/decompression stops; remain within training.
• Ascent discipline: Typical ≤9–10 m/min; add 3–5 min safety stop at 3–5 m when appropriate.
• Buddy system 👥: Pre-dive checks (e.g., BWRAF), gas planning (e.g., rule of thirds), lost-buddy protocol.
• Emergency skills: Controlled emergency swimming ascent (CESA), SMB deployment, rescue towing, 100% O₂ first aid.

Research & Future Directions 🔭

• Refined decompression algorithms (bubble models, personalised risk scoring).
• Lower-density gases and improved rebreather scrubbers to mitigate CO₂ retention.
• Wearables tracking thermal status and perfusion to prevent hypothermia and DCS.

Summary

Diving physiology explores the body’s adaptations to underwater pressure, gas partial pressures, and cold. Key risks—barotrauma, DCS/AGE, narcosis, oxygen toxicity, CO₂ retention, and hypothermia—are largely preventable with prudent dive planning, correct gas choice, disciplined ascent rates, thermal protection, and early use of 100% oxygen when problems arise. 🧡