13.7 Urine Composition, Transport, and Elimination

Composition of Urine

As previously mentioned, the kidneys filter the entire plasma volume about 60 times each day. Once filtrate is formed, substances that need to be reclaimed back into the blood are reabsorbed, and substances that need to be eventually excreted in the urine stay in the filtrate and/or are added to the filtrate by secretion. Urine composition is variable (Table 13.7.1), depending on water intake and losses, nutrient intake, and other factors; however, blood cells and proteins are not normally found in the urine. Urine characteristics such as color and odor are rough descriptors of a person’s state of hydration. For example, if you exercise or work outside and sweat a great deal, your urine will be darker in color due to an increased urine concentration. Alternatively, a well hydrated person will have light or clear colored urine due to a more dilute urine  (Figure 13.7.1).

The pH of the urine can vary more than 1,000-fold, from a normal low of 4.5 to a maximum of 8.0, depending on actions of specific cells of the kidney and on diet. Urine osmolarity ranges from a low of 50 to 100 mOsM to as high as 1,200 mOsM. The color of urine is determined mostly by the breakdown products of red blood cell destruction. The “heme” of hemoglobin is converted by the liver into water-soluble forms that can be excreted into the bile and indirectly into the urine. This yellow pigment is urochrome and its urine levels depend on several factors, namely hydration status. Urobilinogen is another byproduct of heme breakdown and normal levels range from 0.08 to 1.8 mg/dL.

Urine color may also be affected by certain foods like beets, berries, and fava beans. Dehydration produces darker, concentrated urine that may also possess the slight odor of ammonia. Ammonia (NH₃) is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH₂, is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes. Urea is not only less toxic but is also one of the main solutes contributing to the vertical osmotic gradient in the kidney interstitial fluid.

Urine volume varies considerably. The normal range is 800 to 2,000 mL per day. The kidneys must produce a minimum urine volume of about 400 mL per day to rid the body of wastes. Output below this level may be caused by severe dehydration or renal disease.

Nitrogenous Wastes in Urine

Nitrogenous wastes are produced by the breakdown of proteins during normal metabolism. Proteins are broken down into amino acids, which in turn are deaminated by having their nitrogen groups removed. Deamination converts the amino (NH₂) groups into ammonia (NH₃), ammonium ion (NH₄⁺), urea, or uric acid (Figure 13.7.2). Ammonia is extremely toxic, so most of it is very rapidly converted into urea in the liver. Human urinary wastes typically contain primarily urea with small amounts of ammonium and very little uric acid.

Urine Transport, Storage, and Elimination

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 due to the wide range of pH and osmolarity of the urine.

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 renal pelvis narrows to become the ureter of each kidney. 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 pass laterally, hugging the pelvic walls. As they approach the bladder, they turn medially and join with the bladder wall obliquely (Figure 13.7.4.) This is important because it creates a physiological sphincter 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 (urinary tract infection.) Pregnancy also increases the likelihood of reflux and UTI.

The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium (Figure 13.7.3) and scattered goblet cells that secrete protective mucus. 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.

Bladder

The urinary bladder collects urine from both ureters (Figure 13.7.4). The bladder lies posterior to the pubic bone and anterior to the rectum. 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. When empty, the region of the bladder that does not collapse is called the trigone (Greek “tri-” meaning “triangle”), which is delineated by the opening of the ureters and the urethra, forming a triangular area.

External Website

View the University of Michigan WebScope website here to explore the bladder tissue sample in greater detail.

 

The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it “transitions” (hence the name) to a squamous appearance  Figure 13.7.4). Normal bladder volumes in adults can range from about 50 mL when empty to 500 to 600 mL when full.

The detrusor muscle contracts with significant force in the young. 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. Such voluntary contraction is also used in forceful defecation and childbirth.

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. The urethra in females is about 3 to 4 cm in length, while in males it is approximately 20 cm long. All other urine transport structures are identical between females and males (Figure 13.7.5).

The urethra in both males and females begins inferior and central to the trigone. The urethra tracks posterior and inferior to the pubic symphysis (Figure 13.7.5). In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urethral sphincter consisting of smooth muscle, and by the external urethral sphincter, which is comprised of skeletal muscle. Voluntary control of the external urethral sphincter is a function of the pudendal nerve. It arises in the sacral region of the spinal cord, traveling via the S2 to S4 nerves of the sacral plexus.

Disorders of the Urinary Tract – Urinary Tract Infection (UTI)

Urinary tract infections (UTIs) are bacterial infections of the kidneys, bladder, ureters, or urethra. They are usually caused by E. coli and other bacteria from the bowel or the bladder that enter the tract and proliferate. While UTIs do occur in males, they are more common in females for two reasons: 1) the shorter urethra in females means a shorter distance of travel for bacteria to reach the bladder; and 2) the female urethral opening is closer to the anus than in males, making it easier for fecal bacteria to enter the urinary tract. In addition, sexual activity can increase the risk of UTI in females since bacteria that collect in the vagina during intercourse can gain entry into the urinary tract through the nearby urethral opening. Nearly 50% of females will experience a UTI in their lifetime, a greater likelihood than in males.

Additionally, catheterization and any other condition that obstructs the flow of urine can lead to a UTI. For example, males with a swollen prostate have an increased likelihood of UTI: the enlarged prostate constricts the urethra and this can result in insufficient voiding of the bladder. The urine remaining in the bladder can accumulate bacteria and this growth can lead to a UTI.

Micturition Reflex

Micturition is the physiological 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.

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, the spinal cord inhibits somatic motor neurons, 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).

Nerves involved in the control of urination include the hypogastric, pelvic, and pudendal nerves. Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center (Figure 13.7.6). Since the external urethral sphincter is voluntary skeletal muscle, actions by somatic motor neurons maintain contraction (and thereby continence) during filling of the bladder. At the same time, sympathetic nervous activity via the hypogastric nerves suppresses contraction of the detrusor muscle. With further bladder stretch, afferent signals traveling over sacral pelvic nerves activate parasympathetic neurons. This activates efferent neurons to release acetylcholine at the neuromuscular junctions, producing detrusor contraction and bladder emptying.

Incontinence refers to the inability to prevent discharge of urine from the bladder. Causes of incontinence range from disruption of neural pathways mediating voluntary micturition, to less serious and more frequent causes such as sudden increases in bladder pressure. Examples of the latter include coughing or sneezing combined with impaired sphincter function. Incontinence can range from occasional, mild leakage to more frequent and severe loss of control.

Section Review

Urine composition varies depending on water intake and losses, nutrient intake, and other factors; however, blood cells and proteins are not normally found in the urine. Urine color is predominantly due to urochrome, a yellow pigment resulting from heme breakdown.

Nitrogenous wastes produced in protein metabolism are excreted in urine, with urea being the primary nitrogenous waste. Ammonium and uric acid are excreted in smaller amounts.

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.

Micturition is the process of voiding the urine and involves both involuntary and voluntary actions. Voluntary control of micturition requires a mature and intact sacral micturition center. It also requires an intact spinal cord.

Review Questions

Critical Thinking Questions

Glossary

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 urethral sphincter

skeletal muscle forms this sphincter; must be relaxed consciously to void urine

internal urethral sphincter

smooth muscle sphincter 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

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; does not collapse when the bladder is empty

urethra

transports urine from the bladder to the outside environment

Glossary Flashcards

References

Cleveland Clinic. (2025, February 10). Urethra: Location, anatomy, Function & Conditions. Cleveland Clinic.

Cleveland Clinic. (2025c, June 2). What are kidney stones?  Cleveland Clinic.

Image Descriptions

Figure 13.7.2. This diagram displays the molecular structures of three important nitrogen-containing compounds involved in waste excretion: ammonia, urea, and uric acid. From left to right, the first structure shows ammonia (NH3) as a simple molecule with one blue nitrogen atom bonded to three white hydrogen atoms in a pyramidal arrangement. The second structure shows urea (CO(NH2)2), which contains a central black carbon atom double-bonded to a red oxygen atom at the top, with two blue nitrogen atoms attached to the sides, each bonded to two white hydrogen atoms. The third and largest structure shows uric acid (C5H4N4O3), a more complex heterocyclic compound featuring two fused ring structures containing multiple blue nitrogen atoms (four total), red oxygen atoms (three total), black carbon atoms, and white hydrogen atoms. These three molecules represent the primary forms of nitrogenous waste excreted by different animals, with ammonia being the simplest and most toxic, urea being moderately toxic and water-soluble, and uric acid being the least toxic and least water-soluble form. The ball-and-stick models use standard color coding with white for hydrogen, blue for nitrogen, red for oxygen, and black for carbon atoms. [Return to Figure 13.7.2]

Figure 13.7.3. This histological cross-section of a ureter shows the three main tissue layers of this muscular tube that transports urine from the kidney to the bladder. The image is stained with hematoxylin and eosin (H&E), giving it characteristic pink and purple coloration. At the center is the lumen, the hollow space through which urine flows, appearing as white irregular star-shaped or clover-shaped areas. The innermost layer surrounding the lumen consists of darker purple-stained mucosal tissue with numerous folds that allow the ureter to expand. The middle layer, labeled as “Smooth muscle layer,” appears as thick pink bands of tissue surrounding the mucosa, composed of smooth muscle cells arranged in circular and longitudinal orientations that enable peristaltic contractions to propel urine toward the bladder. The outermost layer, labeled as “Adventitia,” is the fibrous connective tissue layer that appears as lighter pink tissue at the periphery, anchoring the ureter to surrounding structures. The overall circular to oval shape of the cross-section demonstrates the tubular nature of the ureter, with the folded mucosal lining creating the characteristic stellate appearance of the lumen when the ureter is not distended. [Return to Figure 13.7.3]

Figure 13.7.4. This illustration consists of two parts: (a) an anatomical cross-section of the urinary bladder and (b) a magnified histological view of the bladder wall. Part (a) shows the bladder as a hollow, muscular organ with labeled structures including the two ureters entering from above at the top corners, the peritoneum covering the superior surface like a protective cap, the thick detrusor muscle layer forming the bladder wall, the ureteral openings where the ureters connect to the bladder, the internal urethral sphincter at the bladder neck, and the external urethral sphincter below where urine exits through the urethra. The bladder is depicted in shades of tan and pink, with the muscle layers shown in deeper orange tones. Part (b) displays a microscopic histology image (H&E stain) showing a magnified section of the bladder wall with three distinct layers labeled: the transitional epithelium (also called urothelium) forming the innermost lining that contacts urine, appearing as a darker pink layer with multiple cell layers capable of stretching; the lamina propria, a connective tissue layer beneath the epithelium shown in lighter pink; and the submucosa, a deeper connective tissue layer containing blood vessels and nerves. An arrow connects a small boxed region in the anatomical diagram to the corresponding histological detail, demonstrating how the microscopic structure relates to the bladder’s overall anatomy. [Return to Figure 13.7.4]

Figure 13.7.5. This illustration presents side-by-side sagittal (side view) cross-sectional anatomical diagrams comparing (a) female and (b) male pelvic anatomy, with emphasis on the urinary bladder and surrounding structures. Both diagrams are detailed line drawings with the urinary bladder highlighted in peach/tan color. In diagram (a), the female anatomy shows labeled structures including the uterus positioned posterior to the bladder, the urinary bladder in the anterior pelvic cavity, the ureter entering the bladder from above, the pubic bone forming the anterior boundary, the urethra exiting the bladder inferiorly, the cervix at the lower end of the uterus, the rectum posterior to the reproductive organs, the vagina between the urethra and rectum, the clitoris at the anterior junction of the labia, the labium minora (inner vaginal folds), the labium majora (outer vaginal folds), and the anus as the terminal opening of the digestive tract. In diagram (b), the male anatomy displays the urinary bladder in a similar anterior position, the ureter entering from above, the seminal vesicle positioned posterior-superior to the bladder, the prostate gland surrounding the urethra at the bladder neck, the ductus deferens (vas deferens) connecting to the reproductive system, the pubic bone anteriorly, the urethra extending through the penis, the penis as the external organ, the epididymis attached to the testis, the testis (testicle) in the scrotum, the rectum positioned posteriorly, and the anus. Both diagrams effectively illustrate how the urinary bladder sits in the pelvic cavity superior to the pelvic floor, anterior to the reproductive organs in females and integrated with the male reproductive tract, demonstrating the anatomical differences in urethral length and the relationship between urinary and reproductive structures in each biological sex. [Return to Figure 13.7.5]

Figure 13.7.6. This sagittal cross-sectional diagram illustrates the female pelvic anatomy with emphasis on the urinary bladder and pudendal nerve. The urinary bladder is shown in peach color in the anterior pelvis, with wavy lines indicating the internal folds that allow it to expand. Key labeled structures include the uterus positioned behind the bladder, the pubic bone at the front of the pelvis, the urethral sphincter that controls urine release, the clitoris, labium minora, and labium majora forming the external genitalia, the sacrum at the upper back of the pelvis, and the anus. The pudendal nerve is highlighted in yellow, showing its path from the sacral region as it branches through the pelvic floor to supply the perineum, external genitalia, and anal region. This nerve provides sensory and motor function important for urinary and fecal continence as well as sexual sensation. The diagram demonstrates how the bladder sits anterior to the uterus and how the pudendal nerve innervates the pelvic floor structures. [Return to Figure 13.7.6]

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