A kidney uriniferous filtration tubule (nephron). The main filtering units in the kidney are called glomeruli. Filtered fluid passes into the specialized tubules seen here. Water and useful substances are absorbed, concentrating the fluid. The concentrated fluid passes to the bladder as urine. The tubules have two portions. The first is called the nephron, and is concerned with urine production. The second, called collecting tubules, carry out the final concentration of urine.