13.5: Urine Transport and Elimination

The urinary tract transports, stores, and releases urine through a coordinated system of tubes, muscles, and reflexes.

Master this section and you'll be able to

• Describe the path of urine flow from the renal pelvis to the exit from the body. 
• Explain how the structure of the ureters, bladder, and urethra supports one-way transport, storage, and controlled release of urine.
• Summarize how the micturition reflex and sphincters regulate urination and how these processes relate to common clinical situations.

Besides the all important kidneys, the urinary system also includes the urinary tract, a set of tubular structures — the ureters, urinary bladder, and urethra — through which urine travels. This section follows urine as it leaves the kidneys, moving from the renal pelvis into the ureters, then the bladder, and finally the urethra. Although urine varies in composition, it must be removed safely and efficiently, so the body uses specialized structures to transport and store it. Blood is filtered continuously, and the resulting filtrate is processed into urine at a fairly steady rate. The urine is then stored until it can be excreted at an appropriate time. All organs that transport or store urine are visible to the naked eye, and together they not only move and hold this waste fluid but also protect tissues from damage, help prevent infection, and in males contribute to reproductive function.

Figure \(\PageIndex{1}\): Urinary Tract Organs. The organs of the urinary tract transport, store, and void urine.  (Image credit: "Organs of the Urinary System" by NIH Medial Arts is licensed under CC-BY-NC 2.0.)

Ureters

As urine is produced, it first drains into the minor and major calyces of the kidney, which merge to form the funnel-shaped renal pelvis located in the hilum. The ureters are two narrow tubes, about 30 cm long. They are made of smooth muscle and lined with transitional epithelium, a special tissue that can appear columnar when stretched and squamous when relaxed. Each kidney has its own ureter, and these tubes carry urine from the renal pelvis down to the urinary bladder.

Along their length, ureters have a few natural points of narrowing where kidney stones or other obstructions are more likely to lodge. Urine does not simply drip into the bladder. Instead, peristaltic waves move urine along the ureters.

The smooth muscle in the ureter wall contracts rhythmically. These waves of contraction — just like in the GI tract called peristalsis — push urine toward the bladder in small spurts rather than allowing it to simply drain by gravity. When urine enters the bladder, small transverse folds of the bladder mucosa act as one-way valves (the ureteral valves) and prevent urine from flowing backward into the ureters. Pregnancy can increase the chance of urine flowing backward (reflux) into the ureters, which raises the risk of a UTI. As the uterus grows, it can press on the ureters and partially block the normal flow of urine into the bladder. When urine cannot drain easily, it may back up toward the kidneys, creating conditions where bacteria can grow more easily and cause infection.

Urinary Bladder

The urinary bladder collects urine arriving from the two ureters. Each ureter enters the bladder on the inferior posterior wall, one on each side of the urethral opening. These three openings define the trigone, a smooth triangular region that lacks the folds found in the rest of the bladder’s mucosal lining (Greek tri = “three,” as in “trigonometry”).

In females, the bladder lies anterior to the uterus, posterior to the pubic bone, and anterior to the rectum. Late in pregnancy, the enlarging uterus compresses the bladder and reduces its capacity, which increases urinary frequency. In males, the bladder is positioned similarly but without the uterus and with the prostate gland located just inferior to it. The bladder is partially retroperitoneal, and its dome is covered by peritoneum, which allows the dome to rise into the abdominal cavity when the bladder becomes full.

The wall of the bladder is highly distensible because its middle layer is composed of the detrusor muscle, a thick layer of smooth muscle arranged in irregular, crisscrossing bundles. When stimulated during the micturition reflex, the detrusor muscle contracts to generate the force needed to expel urine. The interior surface of the bladder is lined with transitional epithelium, which changes shape depending on bladder volume. When the bladder is empty, these epithelial cells appear more cuboidal or columnar; when stretched, they become squamous and allow the lumen to expand. Most of the bladder’s mucosa contains rugae that flatten out during filling, similar to the stomach. Adult bladder capacity varies but generally ranges from nearly zero when empty to about 500 to 600 mL when full. Although bladder muscle tone decreases with age, contraction of abdominal skeletal muscles can increase intra-abdominal pressure and assist with more forceful bladder emptying.

Micturition Reflex

Micturition is a technical term for urination or voiding. It depends on both involuntary and voluntary control of two sphincters: the smooth-muscle internal urethral sphincter and the skeletal-muscle external urethral sphincter. As the bladder fills to about 150 mL, stretch receptors begin sending signals that create the first urge to urinate, but this urge can be consciously suppressed by keeping the external sphincter contracted. As bladder volume increases, the sensation becomes stronger and more difficult to ignore. Once the bladder reaches roughly 300 to 400 mL, voluntary control may fail, and involuntary leakage (incontinence) can occur.

The basic mechanism of micturition is a spinal reflex.

Stretch receptors in the bladder wall send signals to the sacral spinal cord, which responds with parasympathetic output that contracts the detrusor muscle and relaxes the internal sphincter. At the same time, somatic motor neurons to the external sphincter are inhibited, causing that sphincter to relax. In infants, this spinal reflex operates without conscious control, which is why urination is automatic early in life. As the nervous system matures, children learn to override the reflex by keeping the external sphincter contracted until an appropriate time to void.

In certain spinal cord injuries, the micturition reflex may remain intact even though voluntary control is lost. In these cases, the bladder may still contract reflexively, but the person may not be able to relax the external sphincter voluntarily. When coordinated emptying is not possible, intermittent catheterization may be required to remove urine safely from the bladder.

 

Catherization

Difficulties voiding may require catheterization so that urine does not back up into the urinary tract and potentially damage the kidneys. A urinary catheter is a flexible tube inserted through the urethra into the bladder to allow urine to drain into a collection bag. Catheter placement is usually performed by a trained health professional, although some individuals with chronic conditions learn to self-catheterize. A catheter may remain in place for extended periods (indwelling catheter) or be inserted and removed each time it is needed (intermittent catheterization).

The most common type of indwelling catheter is the Foley catheter, which is held in place by a small balloon at the tip. For removal, the balloon is simply deflated. Intermittent catheters do not have a balloon and therefore cannot be anchored; they are designed for temporary, single-use drainage.

Common reasons for urinary catheterization include:

• Urethral obstruction, such as scarring or prostate enlargement, that prevents normal urine flow
• Bladder weakness or nerve injury that interferes with voluntary voiding
• Bladder drainage during childbirth when an epidural anesthetic reduces bladder sensation
• Bladder drainage before, during, or after certain surgeries

Figure \(\PageIndex{2}\): Foley Catheter.  A urinary catheter is placed in the bladder to provide passage for urine.  Foley catheters have a balloon below the intake opening that is inflated to keep the catheter in place within the urinary tract.  (Image credit: "Urinary Catheter" by Cancer Research UK is licensed under CC BY-SA 4.0, via Wikimedia Commons.)

Urethra

The urethra transports urine from the bladder to the outside of the body for disposal. It is the only structure in the urinary tract that differs significantly between males and females; all other urine-transport structures are essentially identical. The most noticeable difference between the two urethras is length, but their course and function also differ in ways that have important clinical consequences.

In males, the urethra is long, about 18 to 20 cm. It passes through the prostate gland, the pelvic floor, and the penis, and its course is curved. It has a dual role because it carries both urine from the bladder and semen from the reproductive tract.

In females, the urethra is much shorter, about 3 to 5 cm. It follows a relatively straight path from the bladder to its opening in the vulva, just anterior to the vaginal opening, and its only function is to conduct urine.

Clinical Implications

Because the female urethra is short and its opening lies close to the vagina and anus, bacteria have a shorter and more direct route to the bladder. This helps explain the higher incidence of urinary tract infections (UTIs) in people with female anatomy.

In contrast, the male urethra is longer and its opening is more distant from potential sources of bacteria. UTIs are therefore less common in males, but other conditions such as urethral strictures or obstruction from prostate enlargement are more common concerns.

Figure \(\PageIndex{3}\): 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.)

In both males and females, the proximal (upper) part of the urethra is lined with transitional epithelium, the same stretchable tissue found in the bladder. As the urethra approaches the body’s exterior, this changes to a nonkeratinized, stratified squamous epithelium, which is better suited to protect against friction at the urethral opening.

Voiding is controlled by two sphincters stacked vertically:

• The internal urinary sphincter sits at the bladder–urethra junction. It is made of smooth muscle and is regulated involuntarily by the autonomic nervous system.

• Just below it is the external urinary sphincter, made of skeletal muscle. This sphincter is under voluntary control, allowing a person to consciously delay urination.

 

Can you assign the appropriate terminology to each number?

Figure \(\PageIndex{4}\): Labeling Practice. Can You Assign a Name to All of these Numbers? 

_{^{1) Human urinary system. 2) Kidney. 3) Renal pelvis. 4) Ureter. 5) Urinary bladder. 6) Urethra. 7) Adrenal gland. 8) Renal artery and vein. 9) Inferior vena cava. 10) Abdominal aorta. 11) Common iliac artery and vein. 12) Liver. 13) Large intestine (colon);14) Pelvis.}}