Why Can't Humans Drink Seawater?

Summary

• About 71% of Earth’s surface is covered by water, but only 2.5% of Earth’s water is drinkable; much of the rest is seawater.
• Seawater contains too much salt for the kidneys to remove without losing more water than a person takes in.
• Blood carries oxygen and nutrients, supports immune-system function, and helps maintain the right concentration of electrolytes around cells.
• Osmosis moves water across cell membranes in response to differences in dissolved-salt concentration:
  • When blood sodium rises, water moves out of cells and into the blood, causing cells to shrink.
  • When blood sodium falls, water moves into cells, causing them to swell.
• The kidneys help regulate sodium levels. After salty food, they can prompt thirst and remove excess sodium in concentrated urine.
• Seawater overwhelms this system: the kidneys cannot concentrate urine enough to match seawater’s saltiness. The subtitles estimate that urine’s maximum sodium concentration is about half that of seawater. Removing the extra sodium therefore requires the body to lose more water in urine than it gained by drinking seawater, worsening dehydration.

Researchers or Sources Featured

No researchers or specific sources are named. The video refers only to “some estimates” about the maximum sodium concentration in urine.