Desalination in Kidneys

I recently did a class project on desalination for the Biologically Inspired Design class. We explored several systems and came up with a really elegant design based on bio inspiration from intestines. I had dwelled on kidneys as another viable desalination option for the project. 

Kidneys are hugely complex structures that mix intelligent structure design with a 2 stage process for achieving their multiple purpose of filtering and maintaining the homeostatis balance. 

The great idea that kidneys use is the principle of investing energy on just actively selecting (reabsorbing) the good nutrients and elements back in the blood once it has been freely diffused to the urinary system (where all the work happens)

The idea is that of 'Selection' in favor to active 'Rejection'. Manmade systems (like dialyzers) generally come up with single process designs that either select or reject depending on the task at hand. 

This also helps in dividing the work into different components and not relying on the entire filtration and homeostatis process as a one step process where the kidneys somehow work and reject just the right amount of urinary fluid along with maintaining the ph balance. This rejection depends on how much of urine the body needs  out of its system. 

What is the difference between selection and rejection? If we look deeper in these processes, we may think of selection as the greedy process involving choosing the right amount and checking if the desired chosen quantity has been achieved. Whereas rejection in kidneys is a complementary and dumb process of diffusing out all the molecules that have smaller pores than membrane pores and meet the pressure conditions. A very complicated process would be involved if active selection had to be implemented just as a one step mechanism. 

Overall, I find this an amazing strategy for choice and selection of desired features. There can be several computational and algorithmic applications of this strategy. The vague computational analogy of this system is preprocessing the data and then extracting the desired features without search (as in a sorted index) (The other being just searching and extracting the desired elements to remove)