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Drowning in fresh water is different from drowning in salt water. For one, more people drown in fresh water than salt water. Around 90% of drownings occur in freshwater, including swimming pools, bathtubs, and rivers. This is partly because of the chemistry of the water and how it affects osmosis.
Drowning in Salt Water
Drowning involves suffocating while in water. You don't even need to breathe in the water for this occur, but if you do inhale salt water, the high salt concentration will prevent the water from crossing into your lung tissue. When people drown in salt water, it's usually because they can't get oxygen or expel carbon dioxide. Breathing in salt water creates a physical barrier between the air and your lungs. A person who has inhaled salt water will not be able to breathe again until the salt water is removed.
However, that does not mean there won't be lingering effects. Salt water is hypertonic to the ion concentration in lung cells, so if you swallow it the water from your bloodstream will enter your lungs to compensate for the concentration difference. This will cause your blood to thicken, putting a strain on your circulatory system. Extreme stress on your heart can lead to cardiac arrest within eight to 10 minutes. The good news is that it's relatively easy to rehydrate your blood by drinking water, so if you survive the initial experience, you are well on the road to recovery.
Drowning in Fresh Water
Surprisingly, you can die from breathing in fresh water even hours after you have avoided drowning in it. This is because fresh water is more "diluted" with respect to ions than the fluid inside your lung cells. Fresh water doesn't cross into your skin cells because keratin essentially waterproofs them, but water will rush into unprotected lung cells to try to equalize the concentration gradient across the cell membranes. This can cause massive tissue damage, so even if the water is removed from your lungs there is still a chance you might not recover.
Here's what happens: Fresh water is hypotonic compared to lung tissue. When water enters the cells, it causes them to swell. Some of the lung cells may burst. Because capillaries in your lungs are exposed to the fresh water, water enters the bloodstream, diluting your blood. This causes blood cells to burst (hemolysis). Elevated plasma K+ (potassium ions) and depressed Na+ (sodium ion) levels may disrupt the heart's electrical activity heart, causing ventricular fibrillation. Cardiac arrest from the ion imbalance may occur in as little as two to three minutes.
Even if you survive the first few minutes underwater, acute renal failure may occur from the burst blood cells in your kidneys. If you drown in cold fresh water, the temperature change as the water enters your bloodstream may even cool your heart enough to cause cardiac arrest from hypothermia. On the other hand, in salt water, the cold water does not enter your bloodstream, so the effects of temperature are mainly limited to heat loss across your skin.