The bladder is the container in the body that stores urine. The other term for bladder is “vesical,” which is derived from the Latin word vesicular. The bladder is a soft, round structure that is located in the pelvis. The pubic bone is in front of the bladder; the rectum in men or the uterus in women is behind the bladder. Urine drains into the bladder through an opening on each side at the bottom of the bladder. Urine is stored in the bladder until a person is ready to urinate. In order to urinate, the muscle in the bladder wall squeezes, pushing the urine out of the bladder through the urethra.In women, the urethra is short, only approximately 1 inch long. In men, it is much longer because it has to pass through the prostate and then the penis before finally opening at the tip of the penis.
In the middle of the abdomen, just beneath the lower ribs, are the kidneys. The kidneys filter the blood to produce urine. The urine that the kidneys produce exits the kidney through the renal pelvis and flows into the ureters. The ureters are soft, muscular tubes that are about the width of a pencil. They carry the urine from the kidneys down to the bladder, where they open into the base of the bladder. The adult bladder normally holds approximately 400 ml of urine. The bladder wall has three separate layers. The innermost layer that is in contact with the urine is a thin layer called the urothelium.
The middle layer is made of muscle fibers that can squeeze. When the muscles contract, they increase the pressure inside the bladder, squeezing the urine out of the bladder. The outermost layer is a thin but protective layer called serosa .The bladder has two functions. The first is the storage of urine, and the second is the emptying of urine. In an infant, the bladder constantly fills and empties without any control by the brain.
During toilet training, the brain learns to control the bladder, enabling it to hold (store) the urine until a time when it is socially accept-able to urinate. Emptying is the second function that the bladder must perform. In infancy, before toilet training, this is actually the most important function of the bladder. Although most of us take these two processes for granted, either one or both can malfunction. If the storage function fails, the bladder can become very small and contracted, holding just a tiny amount of urine before it needs to empty.
In contrast, it may become floppy and dilated, holding several liters of urine before it is ready to empty. It can also become “overactive,” causing feelings of urgency and the need to urinate more than eight times per day. When the actual emptying function goes wrong,the bladder may only partially empty each time, leaving a high remaining amount of urine (the so-called postvoid residual). The bladder muscle may also weaken to the point where one is completely unable to urinate. This is called urinary retention. When storing urine, the bladder must do so at a low pressure.
This allows the new urine made in the kidneys to flow downward into the bladder. A safe bladder pressure is less than 40 cm H2O. When the pressures are higher than this, the urine may “back up” in the kidneys. High pressures in the kidneys over a long period of time may damage the kidneys. During urination, the bladder must squeeze to force the urine out. The pressure in the bladder at these times may be much higher than 40 cm H2O, but it does not usually damage the kidneys. This is because the pressure is elevated for just a short time and then quickly returns to normal.