The urinary system consists of organs, muscles, tubes, and nerves that are responsible for producing, transporting, and storing urine. The major structures of the urinary system include the kidneys, the ureters, the bladder, and the urethra.
The two kidneys are located lateral (to each side) to the spinal column, along the posterior (back) wall of the abdominal cavity. Each kidney is bean-shaped and approximately the size of one's fist (4 to 5 in, or 10 to 13 cm in length). The hilus is the indentation found along the medial side (the side closest to the midline of the body) of the kidney and is the point at which blood vessels (the renal artery and renal vein), nerves, and the ureter enter and exit the organ. The outer layer of the kidney is called the renal cortex, and the inner region of the organ is called the renal medulla.
The individual filtering unit of the kidney is called a nephron, of which there are approximately one million in each kidney. Each nephron extends from the renal cortex into the renal medulla and empties into the funnel-like reservoir of the kidney called the renal pelvis. There are three major components of the nephron: Bowman's capsule, the glomerulus (plural, glomeruli), and the renal tubule. Bowman's capsule is a structure that contains the glomerulus, a cluster of capillaries that is the main filtering device of the nephron. The afferent arteriole brings blood from the branches of the renal artery into Bowman's capsule, where fluid is filtered through the glomerulus. Blood exits the glomerulus by way of the efferent arteriole, passing through the persitubular capillaries and eventually entering the renal vein. The renal tubule has four main sections: the proximal tubule, the loop of Henle, the distal tubule, and the collection tubule. The end closest to Bowman's capsule is called the proximal tubule. The loop of Henle extends from the proximal tubule in the renal cortex to the medulla and back to the cortex, into the distal tubule. The distal tubule empties into a collecting duct which in turn empties into the renal pelvis.
Urine is transported from the renal pelvis of each kidney to the urinary bladder by way of a thin muscular tube called the ureter. The ureter of an adult is typically 8 to 10 in. (21 to 26 cm) long and approximately 0.25 in. (0.75 cm) in diameter. The walls of the ureter are muscular and help to force urine toward the bladder, away from the kidneys.
The urinary bladder is a hollow organ with flexible, muscular walls; it is held in place with ligaments attached to the pelvic bones and other organs. Its primary function is to store urine temporarily until urination occurs, when urine is discharged from the body. When the bladder is empty, its inner wall retracts into many folds that expand as the bladder fills with fluid. The bladder of a healthy adult can typically hold up to 2 cups (0.5 l) of urine comfortably for two to five hours. Circular muscles called sphincters are found at bladder openings—from the ureters and to the urethra—and control the flow of urine out of the bladder by closing tightly around the opening.
The urethra is a tube that leads from the bladder to the body's exterior. In females, the urethra is typically about 1.5 in. (4 cm) in length and carries only urine; its opening is found anterior (in front of) the opening to the vagina. In males, however, the urethra is much longer—approximately 8 in. (20 cm) in length—and extends from the bladder to the tip of the penis. It passes through the prostate gland; semen is directed into the urethra via the ejaculatory ducts of the prostate. The male urethra therefore alternately transports urine (during urination) and semen (during ejaculation).
Production and transport of urine
Urine is a fluid composed of water and dissolved substances that are in excess of what the body needs to function, as well as various wastes that are by-products of metabolism, such as urea, a nitrogen-based waste. These substances are transported into the bloodstream, which enters the kidney by way of the afferent arteriole, a branch of the renal artery.
The blood is filtered from there through the glomerulus, where glucose, minerals, urea, other soluble substances, and water pass through to the renal tubule. This fluid is called filtrate. Filtered blood leaves the glomerulus through the efferent arteriole, which branches into the renal vein. The filtrate is transported through the renal tubule where, under normal circumstances, most of the water (about 99%), glucose, and other substances are reabsorbed into the bloodstream through the peritubular capillaries. Urine is what remains at the distal end of the renal tubule.
The urine is transported from the distal and collections tubule to a collection duct and into the renal pelvis. It enters the ureter and is transported to the bladder; a small amount of urine is carried from the renal pelvis to the bladder via the ureter every 10 to 15 seconds. As the bladder fills with urine, pressure from the accumulating fluid stimulates nerve impulses causing the muscles in the wall of the bladder to tighten. Simultaneously, the sphincter muscle at the opening to the urethra is signaled to relax, and urine is forced out of the bladder through the urethra.
Cystitis— Inflammation of the urinary bladder.
Dialysis— A medical procedure in which waste products are filtered from the bloodstream by a machine.
Filtrate— The fluid that results when blood is filtered through the glomerulus; a precursor to urine.
Hilus— The indentation found along the medial side of the kidney; the point at which blood vessels (the renal artery and renal vein), nerves, and the ureter enter and exit the organ.
Nephritis— Inflammation of the kidney.
Nephron— The individual filtering unit of the kidney; consists of Bowman's capsule, the glomerulus, and the renal tubule.
Renal cortex— The outer layer of the kidney.
Renal medulla— The inner region of the kidney.
Renal pelvis— The funnellike reservoir of a kidney that empties to the ureter.
Sphincters— Circular muscles that control the flow of urine in/out of openings to/from the bladder.
Role in human health
Kidney diseases and other urinary system disorders affect millions of Americans to some degree. An estimated 8.4 million new urinary conditions occur each year, including infections of the kidneys, urinary tract, bladder, and others. Urinary tract stones prompt over 1.3 million visits annually to the doctor's office with over 250,000 hospital stays. Urinary incontinence is estimated to affect 13 million adults in the United States. In 1998, approximately 398,000 individuals were diagnosed with end-stage renal disease (ESRD), of which over 63,000 died. In that same year, 245,910 patients utilized dialysis services—a medical procedure in which waste products are filtered from the bloodstream by a machine.
Common diseases and disorders
- Nephritis (also called glomerulonephritis): Nephritis is an inflammation of the kidneys. It may be caused byabacterial infection (pyelonephritis) or an abnormal immune response. Chronic nephritis may result in extensive damage to the kidneys and eventual kidney failure.
- Urinary tract infection (UTI): This broad term includes infections of the urethra and/or bladder (lower UTI) or the kidneys and/or ureters (upper UTI). UTIs may be caused by bacteria, fungi, viruses, or parasites.
- Cystitis: More commonly known as a bladder infection, cystitis is common in women and may be caused by bacteria introduced into the urethra from the vagina. Cystitis in males may result from a prostate infection. It can be treated successfully with antibiotics.
- Urinary incontinence: This is defined as involuntary urination. Urinary incontinence may involve an urgent desire to urinate followed by involuntary urine loss (urge incontinence); an uncontrolled loss of urine following actions such as laughing, sneezing, coughing, or lifting (stress incontinence); loss of small amounts of urine from a full bladder (overflow incontinence); continual leakage of urine (total incontinence); or a combination of problems (mixed incontinence).
- Kidney/urinary tract cancers: Cancer may develop in any of the structures of the urinary system. Kidney cancer accounts for approximately 2% of cancers diagnosed in adults, more often affecting males than females. Bladder cancer may also occur, with smoking being the most significant risk factor.
- Urinary tract stones: Urinary calculi or urinary tract stones may also be called kidney stones or bladder stones, depending on the site of their formation. They may form because of an excess of salts or a lack of stone-formation inhibitors in the urine. Urinary tract stones may cause bleeding, pain, urine obstruction, or infection.
Tortora, Gerard, and Sandra Grabowski. Chapter 26. In Principles of Anatomy and Physiology, 8th ed. New York: John Wiley, 1996.
Johnson, Sarah T. "From incontinence to confidence." American Journal of Nursing (February 2000): 69-74.
American Foundation for Urologic Disease. 1128 N. Charles Street, Baltimore, MD 21201. (800) 242-2383. 〈http://www.afud.org〉.
National Institute of Diabetes and Digestive and Kidney Diseases. 31 Center Drive, MSC 2560, Bethesda, MD 20892-2560. 〈http://www.niddk.nih.gov〉.
Berkow, Robert, Mark H. Beers, Andrew J. Fletcher, and Robert M. Bogin, eds. "Section 11: Kidney and Urinary Tract Disorders." The Merck Manual: Home Edition. 2000. 〈http://www.merck.com/pubs/mmanual_home/sec11/122.htm〉.
Fadem, Stephen Z. "How the Kidney Works." The Nephron Information Center. 2000. 〈http://nephron.com/htkw.html〉.
"Kidney and Urologic Disease Statistics for the United States." National Kidney and Urologic Diseases Information Clearinghouse. June, 1999. 〈http://www.niddk.nih.gov/health/kidney/pubs/kustats/kustats.htm〉.
"Your urinary system and how it works." National Kidney and Urologic Diseases Information Clearinghouse. 1 May, 1998. 〈http://www.niddk.nih.gov/health/urolog/pubs/yrurinar/index.htm〉.