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The ureters

They are two hollow ducts that connect the kidneys with the posterior part of the bladder. Each one has a length close to 30 centimeters that develops from the inferior part of each kidney, continuing through the lower part of the abdomen and first portion of the pelvis. The wall of the ureters is made up of three layers. The outermost is the adventitia, made up of connective tissue with abundant blood and lymph vessels and nerves. This lining covers, at the same time, the middle lamina or muscle layer (formed by fibers of smooth muscle) and the innermost layer or mucosa (made up of lining epithelium).

The ureters act in a way that is similar to the esophagus (of the digestive system). Both are only passage ducts, but that doesn’t make them passive. Though a series of contractions and relaxations of its walls, the tubular structure makes its contents advance.

In its junction with the urinary bladder (the next section of the urinary apparatus) we find the ureteral orifices, which allow the passage of urine. They act as real valves that regulate the passage of content in a single direction; however, they don’t work like other bodily sphincters, preventing backflow. This way, if there is any defect or anomaly in these conductive tubes and in the ureteral orifices, the most probable outcome will be the urine going back towards the kidneys, generating complications.

Urinary bladder

Urine continues its way through the urinary ducts to a muscle storage pouch called urinary bladder.This organ, protected by the osseous walls of the pelvis, has the capacity to inflate just like a balloon as it receives urinary waste, until it reaches a limit for its evacuation. When it is empty, it has an appearance similar to a prune, stretched it resembles a grapefruit.
It has three important layers. The first of them is a mucosa highly adapted to withstand the great acidity of urine. This layer also houses some cells (of cylindrical appearance, others are flattened) in charge of letting us know when the bladder has reached its storage limit. 

The intermedial lining corresponds to a sub-mucous layer, while the exterior of the bladder is formed by muscle fibers that crisscross in different directions. The job of this storage organ completely depends on the muscles that form it.
When the urinary bladder is empty, its muscles stay relaxed and their inner lining presents numerous wrinkles or folds. And when the bladder slowly fills with urine, it also stretches its walls and wrinkles until the receptors in its wall detect a degree of such extension that the evacuation of urine begins.

The urethra

The last stretch urine passes through and through which it will be expulsed to the outside is the urethra. This conductive tube that covers from the internal urethral orifice to the urinary meatus or external urethral orifice, is made up of two layers (one mucous and another muscular), which ease the exit of waste liquid from the body.

Anatomically, the urethra has noticeable differences between men and women. Men’s is longer, nearing 15 centimeters. It travels through the penis, and in its route, from up to down, three sections are recognized: the prostatic urethra (which is located relative to the prostate, receiving its excretory ducts), the membranous urethra (smaller portion, around one centimeter) and the spongy urethra (route of the duct through the penis). The male urethra, besides transporting urine, is the exit duct of semen, vital fluid of reproduction that contains the male sexual cells (spermatozoa).

The female urethra, on the other hand, has a considerably lower length (approximately, four centimeters). It only makes up the final portion of the urinary ways, reason for which it is in charge of only transporting urine to the outside. Its exit orifice is found between the clitoris and vagina.

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