Kidney tubules. Coloured scanning electron micrograph (SEM) of proximal convoluted tubules (dark brown) in the kidney. The proximal convoluted tubules function to reabsorb water, glucose and other small molecules from the filtrate, producing a concentrated urine. The tubules consist of a lumen (interior) lined with cuboidal epithelial cells. These cells are covered in many finger-like projections known as microvilli, which serve to increase the cell's surface area for reabsorption. The reabsorbed molecules move into the interstitial fluid of the kidney and are absorbed back into the blood. The liquid in the tubules passes to the bladder. Magnification: x420 when printed 10cm wide.