The bladder, a hollow sac located behind the pubic bone, is an integral organ of the urinary system. The main functions of the bladder are to store urine that is produced by the kidneys and excrete it from the body. To fulfill these functions the bladder wall consists of three layers of smooth muscle that collectively make up the detrusor muscle lined with a stretchable mucus membrane and connected to the central nervous system by three sets of nerves. The nerves and muscles work together in perfect coordination to control the storage of urine, signal the brain and void urine.
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Urine is produced in the kidneys and continually drains into the bladder through the two ureters, one on each side. The ureters, which are 8- to 10-inch-long tubes, are lined with muscles that contract and relax to help move the urine from the kidney to the bladder.
The bladder has a third tube, the urethra, through which the urine is excreted from the body. This tube is controlled by the internal urethral sphincter, a circular muscle located between the neck of the bladder and the urethra. This sphincter is very important because without it the bladder would not be able to store urine; instead urine would just continually pass through the bladder and out the body.
The bladder wall is able to stretch, making it a perfect storage area. As urine enters the bladder, the bladder distends to allow more volume. As it gets full, stretch receptors in the bladder wall signal the brain.
The nerves in the bladder are peripheral nerves, meaning they branch from the spinal cord that then connects to the brain. When the bladder is full, the nerves send a signal through the nerves, up the spinal cord and to the brain. The brain then sends a signal back to the bladder that instructs the internal urethral sphincter to relax and the detrusor muscle to contract.
As the detrusor muscle of the bladder contracts, the pressure inside the bladder becomes higher than the pressure in the urethra, allowing urine to flow out the relaxed internal urethral sphincter. All of these signals must be precisely coordinated for the urine to be completely voided from the bladder. Although the brain automatically regulates the voiding of urine, people have the ability to delay voiding--thus giving them time to reach a restroom.