Coloured transmission electron micrograph (TEM) of a kidney collecting duct. The epithelium shows two cell types, the principal (light blue) and intercalated (green) cells. The epithelial basement membrane (pink) and a blood capillary of fenestrated endothelium (red) are also seen. The collecting duct participates in electrolyte and fluid balance regulated by the hormones aldosterone and vasopressin.