Each kidney comprises over a million of these microscopic units, which regulate and control the formation of URINE. A tuft of capillaries invaginates the Bowman's capsule, which is the blind-ending tube (GLOMERULUS) of each nephron. Plasma is filtered out of blood and through the Bowman's capsule into the renal tubule. As the filtered fluid passes along the tubule, most of its water and electrolytes (sodium, potassium, bicarbonate, etc.) are reabsorbed into the blood stream. The tubules eventually empty the filtrate, which by now is urine, into the renal pelvis from where it flows down the ureters into the bladder. (See KIDNEYS.)