In the previous chapter, the term “filtration” was defined, in the context of glomerular filtration. The fluid entering the nephron by glomerular filtration has a composition very similar to that of plasma, except that plasma proteins, which cannot readily cross the glomerular filter, are almost completely absent from the tubular fluid. However, the final urine which leaves the nephrons to enter the bladder and be excreted is very different from the initial glomerular filtrate, because the composition of the filtrate is modified by reabsorption and secretion of specific substances—i.e. there are selective reabsorption and secretion processes in the nephrons (Fig. 4.1).