The kidneys remove urea from the blood through tiny filtering units called nephrons. Urea, water and other waste substances make the urine as they pass through the nephrons and down the renal tubules of the kidney.

• The two ureters are narrow tubes that carry urine from the kidneys to the bladder. Muscles in the ureter walls continually tighten and relax forcing urine downward, away from the kidneys. If urine backs up, or is allowed to stand still, it can lead to kidney infection, damage, or failure. About every 10 to 15 seconds, small amounts of urine are emptied into the bladder from the ureters.
• The bladder is a triangle-shaped, hollow organ located in the lower abdomen. It is held in place by ligaments that are attached to other organs and the pelvic bones. The bladder’s muscle walls relax and expand to store urine, and contract and flatten to empty urine through the urethra.
• Two, circular sphincter muscles help keep urine from leaking out of the body by closing tightly like a rubber band around the opening of the bladder.
• Nerves in the bladder alert a person when it is time to urinate, or empty the bladder.
• The urethra is the tube that allows urine to pass outside the body. The brain signals the bladder muscles to tighten, which squeeze urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax to let urine exit the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs.