Kidney tubules. Fluorescent light micrograph of a section through kidney tissue showing numerous tubules (black/green). The tubules are seen in cross-section. Their walls are black, with cell nuclei stained purple. The inner lining of the tubules is green. Tubules are part of the filtration system in a kidney. Blood passes through capillaries in the kidney, and fluid and molecules of waste products pass into the tubules. During the passage through the tubules, much of the water is reabsorbed, leaving a concentrated solution of waste products. This drains to the ureters, which pass it to the bladder as urine.