Kidney. Light micrograph of a section through the medulla (central region) of a healthy human kidney, showing the loops of Henle (white tubes). These form part of the nephrons, the functional units of the kidney that filter waste from the blood to form urine. Each nephron possesses a loop of Henle, a long tube surrounded by blood capillaries (seen here containing red blood cells, red). Blood filtered by the glomeruli (not seen) in the outer layer of the kidney passes into the loops of Henle, from where water and selected substances are re-absorbed into the bloodstream. Each loop is covered with flat squamous epithelial cells (black). Magnification unknown.