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What is the Bladder?

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  • Written By: Tricia Ellis-Christensen
  • Edited By: O. Wallace
  • Last Modified Date: 19 November 2016
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The human bladder can be seen as the final way station before urine is voided from the body. Its function is fairly singular, to hold urine until we can get to the bathroom. Despite its simple function, it has some unique and interesting features.

The bladder is located in the lower abdomen. In men it is directly in front of the prostate, which is why an enlarged prostate can lead to problems urinating. In women, the bladder is placed between the vagina and the uterus, which explains why a pregnant woman is often very bladder sensitive as her unborn child grows larger.

In both men and women, the bladder is connected to the kidneys by two tube-like structures called ureters. These are the essential passageways for urine to exit the kidneys and enter the bladder. Unlike the control we can exert over urine voiding from the bladder, the movement of urine into the bladder from the kidneys is internally controlled.

At the bottom of each bladder, we have what is called a urethra. This is how urine passes from our bladders to outside our bodies. In women, the urethra is relatively short, not even extending to the length of the vagina. In men, the urethra is much longer, about 8 inches (20.32 cm) and opens at the tip of the penis.

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An interesting feature of the bladder is that it can expand and contract. When the bladder is empty, it’s about the size of a medium adult fist. But it can expand in adults to accommodate a little over two cups (.47 liters) of fluid. However, people often begin to feel the urge to go when they hold about one cup (.23 liters) of fluid in their bladders. Sometimes even as little as 5 ounces of urine (.14 liters) can stimulate the need to use the bathroom.

With infants, a certain amount of fluid in the bladder simply opens up the bottom muscle of the bladder, called the sphincter, allowing for urination. Older children and adults become aware of the need to void urine when a certain portion of the bladder near the ureters, called the trigone, becomes stretched. This message is translated to the brain, and people learn to recognize it as they age as a signal to head for the nearest bathroom.

If this signal is ignored, as it is in babies, the bladder will take over and do the work for you. The sphincter will open completely and void urine, once the bladder reaches a certain level of fullness. Children who sleep heavily may have trouble with bedwetting long after they can control bladder urges during the day, simply because their bodies still don’t recognize the brain’s signals when they’re asleep.

Overall, the bladder is a very useful organ. It performs the important function of secreting urine from the body, and signals our brains with the appropriate time to find a toilet. Just as we flush urine down the toilet, the bladder is nature’s way of flushing toxins, extra fluids and waste materials out of our systems.

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