Fish use this trick too, but for a totally different reason. They breathe water, which is full of salt they don’t always want.
In a fish’s gills, countercurrent exchange allows oxygen to hop into the blood efficiently. Blood flows one way; water flows the opposite. Every bit of oxygen is snatched up.
Fun fact: This system is so efficient that fish can extract up to 80-90% of the oxygen from water. Humans? We’d drown in the same water. Show some respect to a goldfish.
Desert Rats: The Ultimate Thrifters
Some animals take countercurrent exchange to legendary levels. The kangaroo rat never drinks water. Ever.
It gets all its moisture from dry seeds. How? Its kidneys have an ultra-long countercurrent loop. That rat reclaims almost every drop of water from its urine.
It pees crystals. Literal solid pee crystals. That’s not a joke. Its urine is that concentrated. Try doing that after a salty snack.
