All human cells bathe in an internal sea of fluid with which they exchange nutrients and other molecules essential for life.
—Rita Mary King
Topics in This Chapter
The human urinary system, along with the lungs and the skin, works to expel the wastes produced in metabolic activities. The organs, tubes, muscles, and nerves that work together to create, store, and carry urine are the urinary system. The urinary system includes 2 kidneys, 2 ureters, the bladder, 2 sphincter muscles, and the urethra.
• Afferent arteriole—carries blood into the glomerulus of the nephron.
• Bowman’s (glomerular) capsule—a sac where fluids from blood in the glomerulus are collected and further processed to form urine.
• Calyx—propels urine through the pelvis and ureters to the bladder.
• Efferent arteriole—carries blood away from the glomerulus.
• Glomerulus—a small cluster or mass of blood vessels or nerves that filters waste products form the blood and this initiates urine formation.
• Juxtaglomerular apparatus—a renal structure that is the site of rennin secretion.
• Loop of Henle—portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Its primary function is to reabsorb water and ions from the urine.
• Macula densa—an area of closely packaged cells lining the distal convoluted tubule.
• Proximal convoluted tubule—the most proximal segment of the renal tubular portion of the nephron. It is responsible for the reabsorption of glucose, amino acids, various ions and water.
• Renal capsule—a tough fibrous layer surrounding the kidney. It provides some protection from trauma and damage.
• Renal pelvis—the funnel-like dilated proximal part of ureter. It acts as a funnel for urine flowing to the ureter.
• Renal pyramids—cone-shaped tissues of the kidney.
• Ureter—long, narrow duct that conveys urine from the kidney to the urinary bladder.
The principal function of the urinary system is to maintain 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, it is sometimes referred to as the excretory system.
In addition to the urinary system’s major role in excretion, it is important to point out that other organs contribute to the excretory function. For example, 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.
By regulating the amount of water that is excreted in the urine, the urinary system maintains an appropriate fluid volume. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood.
Important Point: 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 rennin.
The urinary system consists of the kidney, ureters, urinary bladder, and urethra. The 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 from the urinary bladder to the outside.
Important Point: Normal urine is sterile. It contains fluids, salts, and waste products, but is free of bacteria, viruses, and fungi.
The kidneys are the primary organs of the urinary system. The kidneys are the organs that filter the blood, remove the wastes, and excrete the wastes in the urine. They are the organs that perform the functions of the urinary system. The other components are accessory structures to eliminate the urine from the body.
The paired kidneys are located between the 12th thoracic and 3rd lumbar vertebrae, 1 on each side of the vertebral column. The right kidney usually is slightly lower than the left because the liver displaces it downward. The kidneys, protected by the lower ribs, lie in shallow depressions against the posterior abdominal wall and behind the parietal peritoneum. This means they are retroperitoneal. Each kidney is held in place by connective tissue, called renal fascia, and is surrounded by a thick layer of adipose tissue, called perirenal fat, which helps to protect it. A tough, fibrous, connective tissue renal capsule closely envelops each kidney and provides support for the soft tissue that is inside.
Important Point: In the adult, each kidney is approximately 3 cm thick, 6 cm wide, and 12 cm long. It is roughly bean-shaped with an indentation, called the hilum, on the medial side. The hilum leads to a large cavity, called the renal sinus, within the kidney. The ureter and renal vein leave the kidney, and the renal artery enters the kidney at the hilum.
Each kidney contains over a million functional units, called nephrons, in the parenchyma (cortex and medulla). A nephron has 2 parts: a renal corpuscle and a renal tubule. The renal corpuscle consists of a cluster of capillaries, called the glomerulus, surrounded by a double-layered epithelial cup, called the glomerular capsule. An afferent arteriole leads into the renal corpuscle and an efferent arteriole leaves the renal corpuscle. Urine passes from the nephrons into collecting ducts then into the minor calyces.
Important Point: The juxtaglomerular apparatus, which monitors blood pressure and secretes rennin, is formed from modified cells in the afferent arteriole and the ascending limb of the nephron loop.
Each ureter is a small tube, about 25 cm long, that carries urine from the renal pelvis to the urinary bladder. It descends from the renal pelvis, along the posterior abdominal wall, behind the parietal peritoneum, and enters the urinary bladder on the posterior inferior surface (SEER 2006).
The urinary bladder is a temporary storage reservoir for urine. It is located in the pelvic cavity, posterior to the symphysis pubis, and below the parietal peritoneum. The size and shape of the urinary bladder vary with the amount of urine it contains and with pressure it receives from surrounding organs.
Important Point: Adults pass about a quart and a half of urine each day, depending on the fluids and foods consumed.
The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae. The rugae and transitional epithelium allow the bladder to expand as it fills (SEER 2006).
The final passageway for the flow of urine is the urethra, a thin-walled tube that conveys urine from the floor of the urinary bladder to the outside. The opening to the outside is the external urethral orifice. The mucosal lining of the urethra is transitional epithelium. The wall also contains smooth muscle fibers and is supported by connective tissue.
The internal urethral sphincter surrounds the beginning of the urethra, where it leaves the urinary bladder. This sphincter is smooth (involuntary) muscle. Another sphincter, the external urethral sphincter, is skeletal (voluntary) muscle and encircles the urethra where it goes through the pelvic floor. These 2 sphincters control the flow of urine through the urethra.
Important Point: The flow of urine through the urethra is controlled by an involuntary internal urethral sphincter and voluntary external urethral sphincter.
In females, the urethra is short, only 3 to 4 cm (1.5 inches) long. The external urethral orifice opens to the outside just anterior to the opening for the vagina. In males, the urethra is much longer, about 20 cm (7 to 8 inches) in length, and transports both urine and semen. The first part, next to the urinary bladder, passes through the prostate gland and is called the prostatic urethra. The second part, a short region that penetrates the pelvic floor and enters the penis, is called the membranous urethra. The third part, the spongy urethra, is the longest region. This portion of the urethra extends the entire length of the penis, and the external urethral orifice opens to the outside at the tip of the penis (SEER 2006).
Problems in the urinary system can be caused by aging, illness, or injury. As we get older, changes in the kidneys’ structure cause them to lose some of their ability to remove wastes from the blood. Also, the muscles in our ureters, bladder, and urethra tend to lose some of their strength. We may have more urinary infections because the bladder muscles do not tighten enough to empty our 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.
The National Institutes of Health (2004) points out that disorder of the urinary system range in severity from easy-to-treat to life-threatening:
• Kidney stones—is the term commonly used to refer to stones, or calculi, in the urinary system. Stones form in the kidneys and may be found anywhere in the urinary system. They vary in size. Some stones cause great pain while others cause very little. The aim of treatment is to remove the stones, prevent infection, and prevent recurrence. Both nonsurgical and surgical treatments are used. Kidney stones affect men more often than women.
• Benign prostatic hyperplasia (BPH)—is a condition in men that affects the prostate gland, which is part of the male reproductive system. The prostate is located at the bottom of the bladder and surrounds the urethra. BPH is an enlargement of the prostate gland that can interfere with urinary function in older men. It causes blockage by squeezing the urethra, which can make it difficult to urinate. Men with BPH frequently have other bladder symptoms including an increase in frequency of bladder emptying both during the day and at night. Most men over age 60 have some BPH, but not all have problems with blockage. There are many different treatment options for BPH.
• Interstitial cystitis (IC)—is a chronic bladder disorder also known as painful bladder syndrome. In this disorder, the bladder wall can become inflamed and irritated. The inflammation can lead to scarring and stiffening of the bladder, decreased bladder capacity, pinpoint bleeding, and, in rare cases, ulcers in the bladder lining. The cause of IC is unknown at this time.
• Prostatitis—is inflammation of the prostate gland that results in urinary frequency and urgency, burning or painful urination (dysuria), and pain in the lower back and genital area, among other symptoms. In some cases, prostatitis is caused by bacterial infection and can be treated with antibiotics. But the more common forms of prostatitis are not associated with any known infecting organism. Antibiotics are often ineffective in treating the nonbacterial forms of prostatitis.
• Proteinuria—is the presence of abnormal amounts of protein in the urine. Healthy kidneys take wastes out of the blood but leave in protein. Protein in the urine does not cause a problem by itself. But it may be a sign that your kidneys are not working properly.
• Renal (kidney) failure—results when the kidneys are no able to regulate water and chemicals in the body or remove waste products from your blood. Acute renal failure (ARF) is the sudden onset of kidney failure. This can be caused by an accident that injures the kidneys, loss of a lot of blood, or some drugs or poisons. ARF may lead to permanent loss of kidney function. However, if the kidneys are not seriously damaged, they may recover. Chronic kidney disease (CKD) is the gradual reduction of kidney function that may lead to permanent kidney failure, or end-stage renal disease (ESRD). You may go several years without knowing you have CKD.
• Urinary tract infections (UTIs)—are caused by bacteria in the urinary tract. Women get UTIs more often than men. UTIs are treated with antibiotics. Drinking lots of fluids also helps by flushing out the bacteria.
Important Point: The name of the UTI depends on its location in the urinary tract. An infection in the bladder is called cystitis. If the infection is in one or both of the kidneys, the infection is called pyelonephritis. This type of UTI can cause serious damage to the kidneys if it is not adequately treated.
• Urinary incontinence (loss of bladder control)—is the involuntary passage of urine. There are many causes and types of incontinence, and many treatment options. Treatments range from simple exercises to surgery. Women are affected by urinary incontinence more often than men.
• Urinary retention (bladder-emptying problems)—is a common urological problem with many possible causes. Normally, urination can be initiated voluntarily and the bladder empties completely. Urinary retention is the abnormal holding of urine in the bladder. Acute urinary retention is the sudden inability to urinate, causing pain and discomfort. Causes can include an obstruction in the urinary system, stress, or neurologic problems. Chronic urinary retention refers to the persistent presence of urine left in the bladder after incomplete emptying. Common causes of chronic urinary retention are bladder muscle failure, nerve damage, or obstructions in the urinary tract. Treatment for urinary retention depends on the cause.
1. The urinary system is sometimes referred to as the _____________ system.
2. The organs of the urinary system are not the only organs that contribute to the excretory system.
• True
• False
3. The kidneys are the organs that filter the _____________, remove the _____________, and _____________ the wastes in the urine.
4. The cortex and medulla make up the parenchyma, or functional tissue, of the kidney, which contains over a million _____________.
5. The renal pelvis is a large cavity that collects the _____________as it is produced.
6. The internal and external urethral _____________control the flow of urine through the urethra.
7. Urine passes from the nephrons into collecting ducts then into the minor _____________.
8. _____________ rids the body of wastes through the skin.
9. _____________ maintains normal blood pressure.
10. The major task of excretion belongs to the _____________ system.
King, R.M. 2003. Biology Made Simple. New York: Made Simple Publishers.
Marieb, E.N., and K. Hoelin. 2006. Human Anatomy and Physiology, 7th ed. New York: Benjamin Cummings.
NIH. 2004. Your Urinary System and How It Works. NIH Publication No. 04–3195. Accessed August 8, 2006, at http://kidney.niddk.nih.gov/kudiseases/pubs/yoururinary/.
Saladin, K.S. 2006. Anatomy and Physiology: The Unity of Form and Function. New York: McGraw-Hill.
Seeley, R.R., T.D. Stephens, and P. Tate. 2004. Anatomy and Physiology, 7th ed. New York: McGraw-Hill.
SEER. 2006. Anatomy and Physiology. In SEER Cancer Training. Accessed August 9, 2006, at http://training.seer.cancer.gov.
