22.4: Urinary Tract
- Page ID
- 63510
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- Identify the ureters, urinary bladder, and urethra, as well as their location, structure, histology, and function
- Compare and contrast male and female urethras
- Describe the micturition reflex
The urinary system has a urinary tract , tubular structures through which urine passes, that includes the ureters, urinary bladder, and urethra. This section will start with urine excretion, tracing the flow of urine from the renal pelvis to ureter, urinary bladder, and urethra. Urine is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently. Blood is filtered, and the filtrate is transformed into urine at a relatively constant rate throughout the day. This processed liquid is stored until a convenient time for excretion. All structures involved in the transport and storage of the urine are large enough to be visible to the naked eye. This transport and storage system not only stores the waste, but it protects the tissues from damage, prevents infection by foreign organisms, and for the male, provides reproductive functions.
Ureters
As urine is formed it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The pelvis narrows to become the ureter of each kidney (Figure \(\PageIndex{2B}\)). As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. As the ureters enter the pelvis, they sweep laterally, hugging the pelvic walls (Figure \(\PageIndex{2A}\)). As they approach the bladder, they turn medially and pierce the bladder wall obliquely. This is important because it creates an one-way valve that allows urine into the bladder but prevents reflux of urine from the bladder back into the ureter. Children born lacking this oblique course of the ureter through the bladder wall are susceptible to “vesicoureteral reflux,” which dramatically increases their risk of serious UTI. Pregnancy also increases the likelihood of reflux and UTI.
The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium containing dome (or umbrella) cells that flatten as urine passes (Figure \(\PageIndex{3}\)). The lamina propria is loose adjacent to the epithelium and becomes denser as it approaches the muscularis externa. The muscular layer of the ureter consists of longitudinal and circular smooth muscles that create the peristaltic contractions to move the urine into the bladder without the aid of gravity. Finally, a loose adventitial layer composed of collagen and fat anchors the ureters between the parietal peritoneum and the posterior abdominal wall.
Urinary Bladder
The urinary bladder collects urine from both ureters (Figure \(\PageIndex{4}\)). The ureters enter on the inferior-posterior wall on either side of the urethra. The three openings form a structure called the trigone which is smooth unlike the rest of the mucosal lining of the bladder. The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine.
The urinary bladder is a highly distensible organ because the middle layer is comprised of irregular crisscrossing bands of smooth muscle. The interior surface is made of transitional epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty it resembles cuboidal epithelia, but when stretched it “transitions” (hence the name) to a squamous appearance (Figure \(\PageIndex{4}\). Similar to the stomach, most of the mucosal layer has rugae which aid in expansion. Volumes in adults can range from nearly zero to 500–600 mL. The bladder’s strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying.
Urethra
The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (Figure \(\PageIndex{5}\)).
Figure \(\PageIndex{5}\): Comparison of the Female and Male Urethra. The urethra transports urine from the urinary bladder to the outside of the body. The female urethra (A) is short, as it only passes through the pelvic floor before reaching the external surface. The majority of the male urethra (B) is shared by the urinary and reproductive systems and, thus, is longer because it passes through both the prostate gland and the penis. (Image credit: "Urethra Comparison - Male and Female" by Jennifer Lange is licensed under CC BY-NC-SA 4.0. Modification of originals "Organs of the Urinary System" by NIH Medial Arts and "Cenveo - Drawing Male and female urinary tract - English labels" by Cenveo.)
The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis. In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.
Female Urethra
The external urethral orifice is embedded in the anterior vaginal wall inferior to the clitoris, superior to the vaginal opening, and medial to the labia minora. Its short length, about 4 cm, is less of a barrier to fecal bacteria than the longer male urethra and the best explanation for the greater incidence of UTI in women. Voluntary control of the external urethral sphincter is a function of the pudendal nerve that arises in the sacral region of the spinal cord.
Male Urethra
The male urethra passes through the prostate gland immediately inferior to the bladder before passing below the pubic symphysis. The length of the male urethra varies between men but averages 20 cm in length. It is divided into four regions: the preprostatic urethra, the prostatic urethra, the membranous urethra, and the spongy or penile urethra. The preprostatic urethra is very short and incorporated into the bladder wall. The prostatic urethra passes through the prostate gland. The membranous urethra passes through the deep muscles of the perineum, where passage through is controlled by the external urethral sphincter. The spongy urethra exits at the tip (external urethral orifice) of the penis after passing through the corpus spongiosum. Mucous glands are found along much of the length of the urethra and protect the urethra from extremes of urine pH. Innervation is the same as in females.
Micturition Reflex
Micturition is a less-often used, but proper term for urination or voiding. It results from an interplay of involuntary and voluntary actions by the internal and external urethral sphincters. When bladder volume reaches about 150 mL, an urge to void is sensed but is easily overridden. Voluntary control of urination relies on consciously preventing relaxation of the external urethral sphincter to maintain urinary continence. As the bladder fills, subsequent urges become harder to ignore. Ultimately, voluntary constraint fails with resulting incontinence, which will occur as bladder volume approaches 300 to 400 mL.
Normal micturition is a result of stretch receptors in the bladder wall that transmit nerve impulses to the sacral region of the spinal cord to generate a spinal reflex. The resulting parasympathetic neural outflow causes contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter. At the same time, somatic motor neurons are inhibited, resulting in the relaxation of the skeletal muscle of the external urethral sphincter. The micturition reflex is active in infants but, with maturity, children learn to override the reflex by asserting external sphincter control, thereby delaying voiding (potty training). This reflex may be preserved even in the face of spinal cord injury that results in paraplegia or quadriplegia. However, relaxation of the external sphincter may not be possible in all cases, and therefore, periodic catheterization may be necessary for bladder emptying.
Urinary Tract
Difficulties voiding may require catheterization so that urine does not back up and cause damage to the kidneys. A urinary catheter is a flexible tube inserted into the bladder through the urethra to allow urine to drain from the bladder for collection in a bag. A health professional usually performs the procedure, but self-catheterization is also possible for those with chronic conditions. A catheter may be in place for long periods of time (indwelling catheter) or removed after each use (intermittent catheterization). The most common type of indwelling catheter is a Foley catheter that is retained in place by means of a balloon tip. The balloon is deflated for Foley catheter removal. Intermittent catheters do no have the balloon and cannot be anchored in place.
Specific reasons a urinary catheter may be used include:
- urethral obstruction because of scarring or prostate enlargement, for example
- bladder weakness or nerve damage that affects your ability to urinate
- to drain the bladder during childbirth if you have an epidural anaesthetic
- to drain the bladder before, during, and/or after some types of surgery
- a last resort treatment for urinary incontinence when other types of treatment have been unsuccessful
Concept Review
The ureters are retroperitoneal and lead from the renal pelvis of the kidney to the trigone area at the base of the bladder. A thick muscular wall consisting of longitudinal and circular smooth muscle helps move urine toward the bladder by way of peristaltic contractions.The bladder is largely retroperitoneal and can hold up to 500–600 mL urine. The urethra is the only urinary structure that differs significantly between males and females. This is due to the dual role of the male urethra in transporting both urine and semen. The urethra arises from the trigone area at the base of the bladder. Urination is controlled by an involuntary internal sphincter of smooth muscle and a voluntary external sphincter of skeletal muscle. The shorter female urethra contributes to the higher incidence of bladder infections in females. Micturition is the process of voiding the urine and involves both involuntary and voluntary actions. Loss of control of micturition is called incontinence and results in voiding when the bladder contains about 250 mL urine.
Review Questions
Q. Peristaltic contractions occur in the ________.
A. urethra
B. bladder
C. ureters
D. urethra, bladder, and ureters
- Answer
-
Answer: C
Q. What is the epithelium of the urinary bladder?
A. stratified squamous
B. transitional
C. stratified cuboidal
D. simple columnar
- Answer
-
Answer: B
Q. Which part of the urinary system is not completely retroperitoneal?
A. kidneys
B. ureters
C. bladder
D. nephrons
- Answer
-
Answer: C
Critical Thinking Questions
Q. Why are females more likely to contract bladder infections than males?
- Answer
-
A. The longer urethra of males means bacteria must travel farther to the bladder to cause an infection.
Q. How can the bladder fill with urine while we sleep? Gravity is not helping this process.
- Answer
-
A. Peristalsis will move the urine to the bladder, without the effect of gravity. The one-way valve allows the urine to enter the bladder but prevents reflux back into the ureter. Thus, the bladder fills up with urine over night.
Glossary
- anatomical sphincter
- smooth or skeletal muscle surrounding the lumen of a vessel or hollow organ that can restrict flow when contracted
- detrusor muscle
- smooth muscle in the bladder wall; fibers run in all directions to reduce the size of the organ when emptying it of urine
- external urinary sphincter
- skeletal muscle; must be relaxed consciously to void urine
- internal urinary sphincter
- smooth muscle at the juncture of the bladder and urethra; relaxes as the bladder fills to allow urine into the urethra
- incontinence
- loss of ability to control micturition
- micturition
- also called urination or voiding
- physiological sphincter
- sphincter consisting of circular smooth muscle indistinguishable from adjacent muscle but possessing differential innervations, permitting its function as a sphincter; structurally weak
- retroperitoneal
- outside the peritoneal cavity; in the case of the kidney and ureters, between the parietal peritoneum and the abdominal wall
- sacral micturition center
- group of neurons in the sacral region of the spinal cord that controls urination; acts reflexively unless its action is modified by higher brain centers to allow voluntary urination
- trigone
- area at the base of the bladder marked by the two ureters in the posterior–lateral aspect and the urethral orifice in the anterior aspect oriented like points on a triangle
- urethra
- transports urine from the bladder to the outside environment
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology