Understanding Drowning: Physiology, Panic, and What Really Happens

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Snapshot: Drowning is fast, silent, and physiologically complex.The real danger begins long before someone slips beneath the surface. This article breaks down what happens to your heart, lungs, brain, and nervous system, from the first splash to the final moment.

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Drowning doesn’t look like what you see in the movies. It’s not loud, and often, it’s not even visible. It’s one of the most silent ways to die. According to the World Health Organization, drowning is one of the leading causes of accidental death, with over 300,000 cases reported annually. But the true number is likely much higher.

To understand drowning, we need to break it into two main situations:

• Immersion: When the body enters water, but the airways are still above the surface.

• Submersion: When the mouth and nose go underwater, cutting off access to air.

Let’s explore how each of these situations can lead to life-threatening consequences.

Understanding Drowning: Immersion

Hot Water Immersion

In hot water, our capacity to cool off is reduced. Typically, our body depends on the evaporation of sweat to dissipate heat. However, in a hot bath, sweat cannot evaporate, particularly when submerged.

As skin temperature rises, warm receptors send signals to the brain, triggering vasodilation and raising the heart rate. In some cases, especially with dehydration or cardiovascular problems, this can lead to arrhythmias or cardiac arrest.

Leaving a hot tub too quickly can also cause a sudden drop in blood pressure, which can lead to fainting or collapse. This is especially risky for older adults or people with heart conditions. And in extreme heat, reflex seizures have even been reported.

Hyperthermia has played a role in deaths during competitive swimming and warm-water diving. One well-known case is that of swimmer Fran Crippen, who died during a race held in excessively warm conditions.

One of my students is really afraid of feeling cold, so he decided to wear a 7mm open-cell wetsuit even though the water was 21°C. He ended up being extremely uncomfortable throughout the entire session, and we eventually decided to cut the training short.

So no, hot water isn’t always safe. And you must keep that in mind.

Cold Water Immersion & Cold Shock Response

As soon as your skin hits water below 25°C, your body activates the cold shock response, an intense physiological reaction to sudden cold exposure. This response peaks between 10–15°C, and below 5°C, pain receptors in the skin start firing rapidly, making the experience feel almost unbearable.

What happens during cold shock:

• Gasp reflex – an involuntary inhalation that can lead to water entering the lungs

• Hyperventilation – rapid, shallow breathing that reduces breath-hold ability

• Increased heart rate and blood pressure – sudden cardiovascular stress

• Peripheral vasoconstriction – blood vessels narrow in your limbs to preserve core heat

In a controlled setting, like an ice bath, this response can be managed and even trained. But in open water, it's a different story.

These reactions dramatically increase oxygen demand while sabotaging your breath control. If your face is submerged during cold shock, the risk of a heart arrhythmia skyrockets, from 2% to over 80%.

Picture a swimmer diving into early spring waters. The cold hits like a wall. Muscles seize. Lungs gasp. That single reflex can be fatal.

Don’t jump straight into icy water. Ease in. Give your brain time to adjust. And always stay alert for the early signs of hypothermia.

Hypothermia: The Long-Term Threat

The chart below shows how your core body temperature can drop over time during cold water immersion, and what tends to happen at each stage. From the first shivers to unconsciousness and death, hypothermia is a silent, progressive condition. And cold water speeds it up dramatically.

However, the timeline isn’t fixed. How quickly you progress through the stages depends on several things:

• Water temperature

• Time spent in the water

• Body composition

• What you’re wearing

• Your activity level

• Your individual physiology

  
  

For example, someone immersed in 10°C water might begin shivering within minutes, but it could take 30 to 60 minutes or more for their core temperature to reach dangerous levels, especially if they’re calm, floating, and clothed.

In contrast, a thin, active person swimming in 5°C water could develop life-threatening hypothermia in under 15 minutes.

Always be aware of two things:

👉 The water temperature

👉 How long you have been exposed

Cold water doesn’t give many warnings. Know the signs. Know your limits.

And never underestimate the ocean.

This graph shows the drop in core body temperature over time during prolonged cold-water immersion, and what symptoms appear at each stage. From mild shivering to total system failure, hypothermia moves in stages

Understanding Drowning: Submersion

Panic and Sensory Overload

Panic is one of the most dangerous elements in drowning. It hits fast and hard. Swimmers and divers suddenly face cold water, waves, darkness, a difficult dive, or unexpected underwater obstacles, and the fear response kicks in.

This activates the sympathetic nervous system, leading to:

• Tunnel vision

• Poor decision-making

• Muscular shutdown

• Rapid exhaustion

  
  

In fact, around 80% of triathlon-related deaths occur during the swim phase. Often, it's not due to poor fitness, but the body’s overwhelming panic response to unfamiliar and chaotic water conditions.

Breath Holding, Cold Shock, and Autonomic Conflict

Holding your breath triggers the diving response, bradycardia (slowed heart rate), vasoconstriction, and oxygen conservation. But cold water triggers the opposite: a racing heart and panic. Together, they create autonomic conflict, opposing nervous system signals that can lead to dangerous arrhythmias.

Quick Comparison: Cold Shock vs Diving Response

Factor	Cold Shock (Sympathetic)	Diving Response (Parasympathetic)
Trigger	Sudden skin cooling	Apnea, face immersion in cold
Heart rate	Increases	Decreases (bradycardia)
Blood vessels	Constrict peripherally	Constrict peripherally
Risk	Panic, gasp, arrhythmia	Arrhythmia from conflict
Protective effect	None	Oxygen conservation

Lungs, Airway Reflexes, and Aspiration

Laryngospasm

A reflex closure of the vocal cords can sometimes prevent water from entering the lungs, but it also blocks air. It may delay drowning, but won’t prevent it.

Aspiration and Lung Damage

• Seawater: Pulls fluid into lungs, worsening swelling.

• Freshwater: Washes away surfactant (the substance that keeps alveoli open), collapsing the air sacs.

• Both: Impair gas exchange and can produce foam, reducing lung efficiency.

  
  

Swallowing Water & Emesis

Swallowed water can lead to vomiting, which increases the risk of inhaling stomach contents. This causes chemical burns, inflammation, and infection. Interestingly, vomiting can also be triggered not just by nausea, but by hypoxia (low oxygen), ketoacidosis, or elevated CO₂.

Electrolyte Imbalance

Rare but dangerous, especially in seawater drownings. Large-volume aspiration can disrupt electrolyte levels, leading to ventricular fibrillation, hypoxemia, or metabolic acidosis in extreme cases.

Neurophysiology: Understanding Drowning

The brain is extremely sensitive to oxygen deprivation. When oxygen stops flowing:

• Blood vessels dilate to try to compensate

• Neurons begin to die within minutes

• Irreversible damage soon follows

  
  

Cold water can slightly delay this by lowering the brain's metabolic demand—but only if cooling happens fast and deeply. This is part of why some children have survived cold-water drowning with full recovery, but these are rare cases.

Even when victims are revived, long-term neurological damage, from memory loss to permanent cognitive impairments, is common.

Conclusion

Understanding the physiology of drowning helps us recognize the risks and respond more effectively.

• Cold shock destroys breath control.

• Hot water can silently strain your heart.

• Hypothermia shuts you down before you notice.

• Panic isn’t just mental, it’s physiological.

• Autonomic conflict scrambles your heart rhythm.

• Aspiration makes breathing impossible even before unconsciousness.

  
  

There’s a lot more to drowning than just running out of air. By understanding how it works, from your skin to your brain, we’re better equipped to train, protect, and respond. If this article helped you understand the risks better, share it with someone who trains, swims, or dives. The more we know, the better we protect each other.

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Sources

World Health Organization: https://www.who.int/news-room/fact-sheets/detail/drowning

Fran Crippen: https://en.wikipedia.org/wiki/Fran_Crippen

Autonomic conflict: a different way to die during cold water immersion?: https://pmc.ncbi.nlm.nih.gov/articles/PMC3459038/

Physiology Of Drowning: A Review: https://pubmed.ncbi.nlm.nih.gov/26889019/

Therapeutic hypothermia in drowning induced hypoxic brain injury: a case report: https://scispace.com/pdf/therapeutic-hypothermia-in-drowning-induced-hypoxic-brain-kp5qmw0b9t.pdf

Therapeutic hypothermia in drowning induced hypoxic brain injury: a case report: https://casesjournal.biomedcentral.com/articles/10.1186/1757-1626-2-9103/metrics

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To be continued…

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