The physiology of the renal tubule and the diseases that can affect its function are often thought of as complicated and con- fusing. This article will attempt to make tubular disorders slightly easier to under- stand by linking the physiology of the four main nephron segments with the clinical features of the more commonly encoun- tered renal tubular disorders (Fig 1). The proximal tubule The proximal convoluted tubule (PCT) is the main site of active transport and reab- sorption of the majority of solutes present in the glomerular filtrate, as well as the location of the production of the key uri- nary buffer ammonium (NH 4 ): In the early part of the PCT (S1), reab- sorbed solutes include glucose, amino acids, phosphate, bicarbonate and various filtered low molecular weight proteins. In the later part of the PCT (S2) urate is reabsorbed and secreted, and citrate is also reabsorbed.