Some substances aren't filtered by the Bowman’s capsule but enter the nephron further down in the proximal or distal convoluted tubules. This occurs in a process called tubule secretion.
Some drug metabolites are secreted into the tubule in this manner. In addition, metabolites produced by the cells forming the tubules themselves often enter the nephron in this way.
For example, hydrogen, bicarbonate and ammonium ions, products of tubule cell metabolism, are secreted into the lumen of the renal tubule. These processes are important for the regulation of the acid-base balance of the body and it is because of this that urine pH can dramatically vary from being either acidic or alkaline.