These cells work in tandem to reabsorb water, sodium, and urea and secrete acid and potassium. The amount of reabsorbtion or secretion that occurs is related to needs of the body at any given time. These processes are mediated by hormones (aldosterone, vasopressin) and the osmolarity (concentration of electrically charged chemicals) of the surrounding medulla. Hormones regulate how permeable the papillary duct is to water and electrolytes. In the medullary collecting duct specifically, vasopressin upregulates urea transporter A1. This increases the concentration of urea in the surrounding interstitium and increases the osmolarity.