The urinary bladder is an organ of the urinary system that stores urine, which is sent from the kidneys and transported through the ureters. The bladder, which is located in the lower abdomen will slowly fill with urine, until full, at that point, the brain will send sensory signals to the bladder, informing it to release the urine when ready.  The bladder is capable of holding up 500 cc's (similar to a pint) of urine. It has the capacity to be stretched almost limitlessly, while empty, without rupturing because of it's make-up of various stretchy materials (see anatomy).  The bladder is what controls the process of urination; it controls the muscles located in the bladder, and while contracting, it allows two valves to open up and let the urine travel through the urethra. The size of the urethra varies, depending on the sex status of female or male. The urethra will then excrete the urine out of the body. 
The shape, size, and position of the bladder, all varies depending on the amount of fluid the bladder is containing. When full, the bladder is completely different in size, shape, and position than when it is empty.  While the amount of urine increases, the urinary bladder is stretching, in result of it's composition of mucosa. The mucosa is composed of stratified transitional epithelium, making it easier to stretch outwards. After the bladder is emptied, the mucosa that possesses several layers called rugae, create folds. These folds called rugae, and the transitional epithelium, let the bladder enlarge as it fills with urine. The bladder has a shape that is approximately similar to a sphere. It can be found down in the abdomen, towards the end of both ureters, located directly behind the pubic bone.  A ureter is responsible for removing the fluid from the kidneys and transporting it to the bladder.  Both ureters are connected to the bladder from both sides, going diagonally through the wall of the bladder. This happens so the ureters are being somewhat squeezed while the bladder is filling, resulting in a valve that provides a one-way flow of urine. This prevents a reflux flow of the urine from returning back to the kidneys. Both openings of the ureters are found close to the bladder outlet. This creates a triangle between the two ureter openings and the bladder outlet; this is the beginning of the urethra. The bladder is capable of holding up to 400 cc. of urine. However, while empty, it is no larger than a tennis ball. Despite the location of the bladder, it has no real connection to the lower abdomen. There are a few blood vessels attached to the sides of the bladder that provide a substantial blood supply. Several nerve vessels also exist, providing a network of nerve-bundles. 
There is a mucous membrane lining the interior of the bladder. This is loosely connected to the muscular coat and can appear to be wrinkled or folded when the bladder is shortened. The bladder is made up of four different layers: serous, muscular, submucous, and mucous coats. The serous layer or coat, is a partial layer that is acquired from the peritoneum. It helps connect the bladder to the abdominal and pelvic walls. The muscular layer possesses three sub-layers of smooth muscular fibers. The first layer which is the external layer, is made up of fibers and has a longitudinal alignment. The middle layer is where the fibers tend to be arranged in a circular manner. Then the internal layer, where the fibers are typically in a longitudinal alignment. 
The bladder's main function is to act as a repository for urine.  Once the bladder is filled with urine, impulses are sent to the brain by the sensory nerves, telling the brain that the bladder is full. Then the sensory nerves will connect with other nerves located in the spinal cord, who will transfer this information. Once the information is relayed, the brain will dispatch impulses down to the bladder, with direct orders to empty its substances. This urine is produced by the kidneys, and is moved through the body via ureters, to be kept in the bladder. 
Following is the link to a video that gives a graphic display of how the nerves are stimulated and sent to the brain, in order to indicate to the brain to empty the bladder.(Video)
Other than just being a storage space for the urine, the bladder is also in charge of the muscles that allow the bladder to fill with urine and excrete urine out of the body. When the bladder muscles contract, two valves are able to be opened up, which lets the urine travel from out of the bladder, into the urethra. From that point, the urethra will carry the urine out of the body. 
Diseasescancer is a type of cancer when a tumor is found somewhere either on the interior or exterior of the bladder.  It is the sixth most recurrent type of cancer in America. The tumors can either be papillary or nonpapillary. Papillary, meaning the tumors appear wart-like and are attached to some stem. Nonpapillary, meaning the tumors are more rare, yet more dangerous; often resulting in an unfortunate outcome. In America, most bladder cancers begin forming from the transitional cells (cells lining the bladder). Similar to most cancers, the cause of bladder cancer is undetermined. Although, it has been shown that several factors can assist in contributing to the development of bladder cancer. Things like smoking, specific cancer treatments like chemotherapy, and radiation, chemical exposure, parasitic infection, and different forms of bladder infections. It is possible that if left untreated, the cancer will spread to other organs such as: the prostate, rectum, ureters, uterus, vagina, various bones, liver, and lungs.
Several of the symptoms that come with bladder cancer, can also happen with non-cancerous circumstances. Symptoms like: blood tainted urine, painful urination, frequency to urinate, urgency to urinate, pain in the abdominal area, anemia, fatigue, weight loss and more. There are various kinds of treatment that are available to help bladder cancer. Treatments such as: surgery, chemotherapy, immunotherapy, bladder removal, and more. Treatments vary on the specific stage of cancer. 
Urinary Retention: Urinary retention is the bladder's inability to excrete urine. There are two kinds of urinary retention, acute and chronic urinary retention. Chronic urinary retention allows one to be able to urinate, but only slightly and for a short amount of time, limiting one from completely emptying the bladder. Although this may not appear life-threatening, it can cause harmful circumstances and should be receive assistance from a doctor. Acute urinary retention is the inability to urinate whatsoever, even if the bladder is completely full. If this happens, this will require immediate and emergency medical treatment. Almost anyone can experience either chronic or acute urinary retention. However, it is most common in men who are between the ages of fifty and sixty. This is the result of an enlarged prostate. A woman may also suffer from urinary retention if her bladder sags or shifts from it's normal spot; this is a condition called cystocele. This can also happen if a lower part of the colon is sagging and catches on to the bladder. This will result in movement of position and sagging, resulting in urinary retention. This condition is called rectoceles. 
Urinary Incontinence: Urinary incontinence is the bladder's inability to hold urine in. It signifies almost total loss of bladder control. Symptoms vary, some being considered as little to mild leaking, and ranging to irrepressible wetting. This condition can occur in anyone, however, it is more likely to happen with age. This can result in weak bladder muscles, or muscles that are overactive. If the muscles that keep the bladder closed are weaker, then accidents may occur in situations where sneezing, laughing, or lifting heavy objects take place. These are examples of stress incontinence. When bladder muscles are too active, this can cause a constant urge to go to the bathroom, when there's only a small amount of urine in the bladder. This is known as urge incontinence or an overactive bladder. Other conditions such as prostate issues and nerve damage can also result in incontinence. 
Cystitis:Cystitis is known as another term for swelling of the bladder. This inflammation can occur in result of a bacterial or viral infection, which can also be connected to a urinary tract infection. Cystitis is known to be very painful and cumbersome, yet can cause serious problems to the kidneys, if left untreated. It also can occur in response to various drugs, radiation, hygiene spray, and can also be a result of other medical conditions. The treatment for specific bacterial cystitis is typically just antibiotics, however other serious cases require other and more in depth treatments. Some symptoms of cystitis are: a constant feeling to urinate, burning pain while urination, blood in the urine, soreness or pain in the lower area, a slight fever, and more. It is necessary to call a doctor immediately if blood is found in the urine or excessive back pain, vomiting, or fever occurs. 
- Bladder (Human Anatomy) Author unknown, Web MD, July 2009
- The Bladder By Gwilym G. Davis, http://chestofbooks.com/health/anatomy, October 22
- The Urinary Bladder- Gray's Anatomy of the Human Body Author unknown, 2009
- The Human Body: Fearfully and Wonderfully Made! By Dr. Jay L. Wile and Marilyn M. Shannon, Apologia Educational Ministries, 2008.
- Urinary Bladder Author unknown, National Cancer Institute
- Overview of the Bladder By Dr. Eugene Rajaratnam, www.drrajamd.com, 2001.
- Urinary Retention NIH Publication, October 2007, American Urological Association Foundation
- Cystitis By Mayo Clinic Staff, March 2010, Mayo Clinic