What is meant by re-absorption?
In the glomerulus of the nephron we have filtration occuring. The subsequent formation of an ultra-filtrate travels down the tubulus from Bowmans space; during this journey, the filtrate will be exposed to various transporters and forces such as solvent drags and diffusion gradients, which leads to the (re)absorption of this filtrate. Reabsorption occurs also in medullary tubules and later collecting ducts, until the filtrate (by now urine) reaches the calyces. (Note that secretion - that tubular cells permit the transportation of various electrolytes for instance to the tubulary lumen - also occurs in the nephron, giving it three ways to manipulate the its luminal contents: filtration, reabsorption and secretion)