Discussion:
Analogy of the Renal System for Anatomy Students
Pretend you are cleaning your garage but the big door is stuck. You can only move things through the smaller "people" door. So the cars and riding lawn mower have to stay in the garage. This is analogous to the pores in the glomerulus. They are larger than ordinary capillary pores but still not large enough to let everything out. So, large things like proteins stay in the blood.
You have decided to haul almost everything out that you can fit through the smaller door. Out goes the hoses, garden implements, lawn chemicals, recycling etc., without any sorting. You do this till you run out of energy. (Filtration - what fits goes through filters and it is controlled by size and the pressures.)
After a short rest, you realize that you need some of this stuff. So you exert some more energy (active transport!) and put some of the materials back into the garage. For example, 13 of the 27 hoses are still good so they go back (like tubular reabsorption!!)
The others are put out for the trash pickup (analogous to going to the bladder). After sorting, returning and discarding, you take one last look at what is now in the garage. Do you really need 13 hoses? Isn't that one a little holey? So you take it back out of the garage and put it in the trash pile with the others. (just like tubular secretion, a last chance to excrete something we don't need.)
And now your garage, (and your blood) is clean!