are another one of nature's motifs. If you've ever taken chem lab or built a still, you might know about countercurrent exchange even if you don't know the name. You get much better cooling if you run the water in the opposite direction the condensing vapor is going. If you think about it, the reason is obvious -- by the time your moonshine reaches the end of the condenser, it's touching fresh cold water, and (if you're running it right) just as cold. Accordingly, nature uses countercurrent mechanisms whenever possible, notably in the Loop of Henle in the kidney.
The kidney's an interesting piece of equipment. The original kidneys, like the original just-about-everything-else, were segmented. Running down the prototypical chordate's body were a bunch of little "kidneylets", just bundles of blood vessels that leaked fluid into the gut cavity of the critter, where it eventually got flushed out to sea. Think of a bunch of public toilets (and I do mean public, as in no privacy) hanging over a river. Over time these structures developed their own tubes, hooked to a common drain, migrated into their own private locations, and otherwise did the equivalent of discovering indoor plumbing.
From a functional point of view, the kidney doesn't work like you might expect. A kidney consists of gazillions of nephrons, each made up of a renal corpuscle and a renal tubule. Blood flows through a little bundle of capillaries in the corpuscle, and pretty much anything smaller than a protein molecule leaks out. That includes water, salts, sugars, and all sorts of things your body needs to function. Then, in the tubule, 98% of the water and sugar, and a good bit of the salts, is reclaimed. If it weren't for that, you'd be peeing around fifty times as much as you do now (for the few hours you were alive).
Of course, it's all a lot more complicated than that, and by the time we got to the kidneys I could care less about opisthonephros kidneys and archinephric ducts. What started as a head cold had taken a rapid turn for the worse, leaving me with bronchitis, a bacterial infection, and a fever high enough to induce occasional hallucinations. I didn't even know we'd covered the reproductive system until it showed up on a quiz (I got half a point on one question for remembering the parts of the testicle). And I didn't get around to slicing open Gizmo's pubic symphysis (you don't want to know) until a couple weeks before the final.
By then, of course, we were desperately stumbling towards the finish line and covering the nervous system.