Urinary System

Anatomy of the female urinary system. Source: National Cancer Institute.

The urinary system is a group of organs, tubes, muscles, and nerves whose main function is to keep the volume and composition of body fluids within normal limits. One aspect of this function is to rid the body of waste products that accumulate as a result of cellular metabolism. Because of this, the urinary system is sometimes also referred to as the excretory system.

Other Names

Synonyms

• Excretory System
• Uropoietic Organs
• Urinary Tract

Related terms

• Genitourinary System (also includes the Genital Systems)

Description

The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The two kidneys form the urine and account for the other functions attributed to the urinary system. The ureters carry the urine away from kidneys to the urinary bladder, which is a temporary reservoir for the urine. The urethra is a tubular structure that carries the urine away from the urinary bladder for elimination out of the body.

Role of the Urinary System in the Body

Although the urinary system has a major role in excretion, other organs contribute to the excretory function. The lungs in the respiratory system excrete some waste products, such as carbon dioxide and water. The skin is another excretory organ that rids the body of wastes through the sweat glands. The liver and intestines excrete bile pigments that result from the destruction of hemoglobin. The major task of excretion still belongs to the urinary system. If it fails, the other organs cannot take over and compensate adequately.

How It Works

The urinary system maintains an appropriate fluid volume by regulating the amount of water that is excreted in the urine. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood.

In addition to maintaining fluid homeostasis in the body, the urinary system controls red blood cell production by secreting the hormone erythropoietin. The urinary system also plays a role in maintaining normal blood pressure by secreting the enzyme renin.

The urinary system removes a waste called urea from the bloodstream. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

The kidneys are a pair of bean-shaped organs about the size of a fist. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule. Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

From the kidneys, urine travels down two thin tubes called ureters to the bladder. The ureters are about 8 to 10 inches long. Muscles in the ureter walls constantly tighten and relax to force urine downward away from the kidneys. If urine is allowed to stand still, or back up, a kidney infection can develop. Small amounts of urine are emptied into the bladder from the ureters about every 10 to 15 seconds.

The bladder is a hollow muscular organ shaped like a balloon. It sits in the pelvis and is held in place by ligaments attached to other organs and the pelvic bones. The bladder stores urine until nerves inside the bladder signal the urge to urinate. At that time, the brain signals the bladder muscles to tighten, squeezing urine out of the bladder. At the same time, the brain signals the sphincter muscles to relax. As these muscles relax, urine exits the bladder through the urethra. When all the signals occur in the correct order, normal urination occurs. If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours.

Diseases of the Urinary System

Problems in the urinary system can be caused by aging, illness, or injury. As they age, changes in the kidneys’ structure cause them to lose some of their ability to remove waste from the blood. Also, the muscles in the ureters, bladder, and urethra tend to lose some of their strength. Urinary infections may occur more frequently because the bladder muscles do not tighten enough to empty the bladder completely. A decrease in strength of muscles of the sphincters and the pelvis can also cause incontinence, the unwanted leakage of urine. Illness or injury can also prevent the kidneys from filtering the blood completely or block the passage of urine.

Conditions

Diseases of the urinary system include the following:

• Cancer
  • incontinence
  • Bladder cancer
• Kidney failure
• Kidney stones
• Overactive bladder
• Urinary tract infection

Symptoms

Symptoms of disease of the urinary system include the following:

• Dysuria (burning with urination)
• Hematuria (blood in the urine)
• Incontinence

Procedures

Procedures to diagnose and treat diseases of the urinary sytem include:

• Cystoscopy (examining the bladder with a camera)
• Ureteroscopy (examining the ureter with a camera)
• Urodynamics (a test to study the function of the bladder)

Surgery

Surgeries that are performed on the urinary system include the following:

• Cystectomy (removal of the bladder)
• Lithotripsy (breaking up and removing kidney stones)
• Nephrectomy (removal of a kidney)

Radiographic studies

Radiologic studies commonly performed to evaluate the urinary system include the following:

• Computed tomography (CT) scan
• Magnetic resonance imaging (MRI)
• Ultrasound

Related Professions

• A urologist is a physician who is trained to provide medical and surgical care for disorders of the urinary tract of men and women and the reproductive tract in men.
• A nephrologist is a medical doctor trained to treat diseases of the kidney.

History

In 1877, Maximilian Nitze introduced the first usable version of a cystoscope for looking into a patient's bladder through the urethra. He used a wire loop as a light source. The field of endoscopy grew after Thomas Edison developed light bulbs that could be used in these instruments for viewing the inside of the human body.^[1]

Research

Future research

• The field of urinary proteomics is rapidly growing. Researchers hope to identify proteins easily measured in the urine that can help identify disease in the body, both within the urinary tract and in other parts of the body.^[2]

Recent discoveries

• There is evidence that kidney disease may be able to be used as a marker for co-existing cardiovascular disease. ^[3]
• Magnetic resonance urography was compared to ultrasonography as an imaging tool to evaluate fetal abnormalities of the urinary system. ^[4]
• The nervous system control of the bladder is discussed and the impact of nervous system dysfunction is discussed and ways of testing for such dysfunction is discussed. ^[5]
• Because the urinary system and alimentary (digestive tract) system are in close proximity, a disorder in one system may affect the other. This relationship is discussed. ^[6]

References

1. ↑ William P. Didusch Center for Urologic History. Cystoscopy.
2. ↑ Barratt J, Topham P. Urine proteomics: the present and future of measuring urinary protein components in disease. CMAJ. 2007 Aug 14;177(4):361-8. Abstract | Full Text
3. ↑ deZeeuw D. Renal disease: a common and a silent killer. Nat Clin Pract Cardiovasc Med. 2008 Jul;5 Suppl 1:S27-35. Abstract
4. ↑ Kajbafzadeh AM, Payabvash S, Sadeghi Z. Comparison of magnetic resonance urography with ultrasound studies in detection of fetal urogenital anomalies. J Pediatr Urol. 2008 Feb;4(1):32-9. Epub 2007 Sep 20. Abstract
5. ↑ Roberts MM. Neurophysiology in neurourology. Muscle Nerve. 2008 Jul;38(1):815-36. Abstract
6. ↑ Halachmi S, Farhat WA. Int J Adolesc Med Health. 2008 Jan-Mar;20(1):17-22. Abstract

External Links

American Urological Association

UrologyHealth.org: A patient information Web site.

American Kidney Fund

National Kidney Foundation