The bladder, or urinary bladder, is a hollow, muscular, balloon-shaped sac that serves as a storage container for urine. The bladder is located behind the pubic bone. It sits within and is protected by the pelvis (hipbones). The bladder is held in place by ligaments, bands of tough, fiber-like tissue that connect it to the pelvis and other organs. In a healthy person, the bladder can comfortably store up to 16 ounces (2 cups) of urine for 2 to 5 hours. Nerves in the bladder signal the body when it is time to urinate.

The narrow, internal opening of the urethra within the bladder is called the bladder neck. In this region, the bladder's bundled muscular fibers help the sphincter muscles to close the urethra.

The urethra is the tube that passes urine from the bladder out of the body. In women, the urethra is roughly 4 centimeters long. It is composed of smooth muscle fibers and sphincter muscle fibers, a thin layer of elastic tissue, and collagen tissue that binds all of the fibers together. The urethra is lined with mucous membrane. The female urethra starts at the bladder neck and exits the body directly above and in front of the opening of the vagina (female reproductive canal).

In men, the urethra is roughly 8 to 9 inches in length. It extends from the bladder neck to the end of the penis. The male urethra is composed of three portions: the prostatic, membranous and spongy portions. The prostatic portion is the widest part of the tube, which passes through the prostate gland. Its walls are made up of fibrous tissue, muscle fibers and tiny glandular openings that connect to the prostate. The membranous portion is approximately three-quarters of an inch long and lies between the triangular ligaments of the male pelvis. The spongy portion is the longest part of the urethra, which extends through the body of the penis and exits at the glands (tip of the penis). The corpus spongiosum is the lower area of the penis that directly surrounds and protects the male urethra.

How does the urinary system work?
The urinary tract is a system composed of organs that help to form, store and remove liquid waste, or urine, from the body. When food is eaten, the body uses the nutrients in the food to provide energy, sustain health and perform activities such as self-repair. Moisture in the food or drink is absorbed, and the urinary system removes the waste products that are left behind in the bodily fluids.

Specifically, the urinary system rids the blood of a waste product called urea, which is formed when protein-rich foods, such as meat, poultry and certain vegetables, are digested. Urea travels in the bloodstream to the kidneys -- the bean-shaped organs located in the mid-back, just below the rib cage -- which filter out the urea and combine it with water and other waste products to form urine.

Roughly a quart and a half of urine is made and eliminated daily in adults. How much urine is produced depends upon a variety of factors. Such factors include the amount of fluids and foods that are consumed, the amount of fluid lost through sweat and breathing, and the use of medications (for example, diuretics, or water pills), all of which can affect urine production.

After it is formed, the urine then passes out of the kidneys and flows down the ureters, two thin tubes which enter the bladder. Every 10 to 15 seconds, tiny amounts of urine are delivered to the bladder from the ureters. As it expands, the bladder swells into a round shape. When the bladder is full, the body is ready to get rid of the urine. The brain sends a signal that tells the urinary system to relax so that the urine can pass out of the bladder through a tube called the urethra.

At this point, a person who is continent (able to control urine flow voluntarily) feels the urge to pass the urine, and two special circular muscles -- the urethral sphincter muscles -- act like rubber bands to keep the urethra closed until the person makes a conscious decision that it's time to urinate. Another group of muscles, the pubococcygeus (PCG) muscles, surround the urethra to help keep the urine in and prevent unwanted leakage. During the process of urination (emptying the bladder), the brain signals the bladder muscles to tighten and contract (squeeze out the urine). If the sphincter and PCG muscles are allowed to relax, then the urine can exit the urethra and normal urination occurs. If you are unable to hold your urine until you reach a toilet, you may have a form of urinary incontinence.