Question

One way to think about urine formation and renal function is to use the analogy of a dirty garage: Imagine you are cleaning your garage, but the big door is stuck. You can only move things through the smaller “people” door, meaning that the cars and riding lawn mower have to stay in the garage. This situation is analogous to the pores in the glomerulus. They are larger than ordinary capillary pores, but still not large enough to let everything out.

Now imagine that you have decided to haul almost everything out that you can fit through the smaller door. Out go the hoses, garden implements, lawn chemicals, recycling etc., without any sorting. You clean until you run out of energy. After a short rest, you realize that you need some of this stuff back. So you exert some more energy and put some of the materials back into the garage. For example, 13 of the 27 hoses are still good so they go back. The others are put out for the trash pickup.

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. And now your garage is clean!

Post an explanation of how the story of the dirty garage relates to urine formation.

Answer 1

Blood pressure forces fluid to move into the glomerular capsule from capillaries through a specialised layer of cells, this layer is called filtration membrane which allows water and small solutes to pass but prevent movement of blood cells and large proteins which is the same condition as in dirty garage in which it was not possible to take out big things.
The resulting filtrate contains waste as well as some other useful substances which our body needs such as ions, glucose, amino acids and smaller proteins therefore they needed substances and some needed substances and water are reabsorbed as we took back the little hoses in our dirty garage.
At the same time waste ions and hydrogen ions move into the renal tubule from the capillaries and hence get secreted.