It's tempting to think of the urinary system as nothing more than a mechanism for fluid disposal. After all, the more a person drinks, the more they urinate. In reality, however, the urinary system is much more complex than it appears, and its true purpose is filtration and cleaning of the bloodstream rather than disposal of fluid. Achieving these purposes requires that its various components work together seamlessly.
Function
The major function of the urinary system is to filter toxins and waste products out of the bloodstream while maintaining as much fluid in the blood as possible and without eliminating useful molecules dissolved in the blood. According to Dr. Gary Thibodeau in his text, "Anatomy and Physiology," the blood is a complex soup of molecules and cells. Some of the molecules, like blood sugar, are useful, and the body needs to maintain them. Others, like urea, are waste products, and it's the job of the urinary system to eliminate them.
Gross Features
In medicine, the word "gross" means macroscopic, in reference to large-scale anatomy. The gross features of the urinary system, therefore, are the visible organs. The major organs of the urinary system include the kidneys, which perform the complex task of filtering the blood. The ureters carry urine produced by the kidneys to the bladder, while the bladder stores urine until it's eliminated. Finally, the urethra carries urine from the bladder to the outside of the body. Notes Dr. Thibodeau, while urinary systems are largely similar in men and women, men have significantly longer urethras because of their penises.
Microanatomy
The kidneys are possessed of some complex microscopic features that allow them to act as filters. They're each composed of more than a million functional units called nephrons, which are convoluted tubes through which fluid passes as it's gradually turned into urine. Explains Dr. Lauralee Sherwood in her book, "Human Physiology," the shape of the nephron helps it act as an efficient filter. Early portions of the nephron remove useful molecules, like blood sugar, from the urine, while later portions conserve water and useful salts. Harmful molecules stay in the nephron and are eventually eliminated.
Expert Insight
While the urinary system ostensibly eliminates fluid and plays an important role in fluid balance, Dr. Sherwood points out that in no way is fluid elimination the primary purpose of the system. In fact, the urinary system maintains fluid to a much greater degree than it eliminates fluid. The entire volume of blood in the human body passes through the kidneys hundreds of times a day. If the kidneys didn't resorb and maintain most of that liquid volume, an individual would urinate out their entire blood volume in less than four minutes.
Warning
Although you may think that the kidneys as the only "real" functional structures of the urinary system, the ureters, bladder, and urethra also play vital roles. Dr. Thibodeau explains that releasing urine at will depends upon conscious control of a circular muscle, or sphincter, at the base of the bladder. Trauma, including that of childbirth, can weaken the urinary sphincter and reduce an individual's ability to hold their bladder. Further, holding urine for long periods of time can lead to bacterial colonization of the bladder, which results in painful urinary tract infections.
References
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
