A nephron consists of five subunits; this starts with filtration through the glomerulus and the Bowman's capsule. Together, they are known as a corpuscle. This is suceeded by the proximal convoluted tubule, a channel that allows for reabsorption of water and ions. Following the tubule is the Loop of Henle, the generator of the countercurrent multiplier or concentration gradient. The final two subunits are the distal convoluted tubule and the collecting duct. The distal tubule allows for the final regulation of urine pH, while the collecting duct is the final region water can be retained before travelling to the ureter and being excreted. Often, the retaining of water is controlled via vasopressin secretion from the pituitary gland.