<P> The mammalian kidney develops from intermediate mesoderm . Kidney development, also called nephrogenesis, proceeds through a series of three successive developmental phases: the pronephros, mesonephros, and metanephros . The metanephros are primordia of the permanent kidney . </P> <P> The microscopic structural and functional unit of the kidney is the nephron . It processes the blood supplied to it via filtration, reabsorption, secretion and excretion; the consequence of those processes is the production of urine . </P> <P> Filtration, which takes place at the renal corpuscle, is the process by which cells and large proteins are filtered from the blood to make an ultrafiltrate that eventually becomes urine . The kidney generates 180 liters of filtrate a day . The process is also known as hydrostatic filtration due to the hydrostatic pressure exerted on the capillary walls . </P> <P> Reabsorption is the transport of molecules from this ultrafiltrate and into the peritubular capillary . It is accomplished via selective receptors on the luminal cell membrane . Water is 99% reabsorbed in the proximal tubule . Glucose at normal plasma levels is completely reabsorbed in the proximal tubule . The mechanism for this is the Na / glucose cotransporter . A plasma level of 350 mg / dL will fully saturate the transporters and glucose will be lost in the urine . A plasma glucose level of approximately 160 is sufficient to allow glucosuria, which is an important clinical clue to diabetes mellitus . Amino acids are reabsorbed by sodium dependent transporters in the proximal tubule . Hartnup disease is a deficiency of the tryptophan amino acid transporter, which results in pellagra . </P>

The initial stage of blood filtration in the kidneys occurs in which portion of the nephron