The distal farther from the bladder third of the urethra is lined with mucus-secreting epithelium. The mucus helps protect the epithelium from urine, which is corrosive. Below the epithelium is loose connective tissue, and below that are layers of smooth muscle that are continuous with the muscle layers of the urinary bladder.
When the bladder contracts to forcefully expel urine, the smooth muscle of the urethra relaxes to allow the urine to pass through. In order for urine to leave the body through the external urethral orifice, the external urethral sphincter must relax and open. This sphincter is a striated muscle that is controlled by the somatic nervous system , so it is under conscious, voluntary control in most people exceptions are infants, some elderly people, and patients with certain injuries or disorders.
The muscle can be held in a contracted state and hold in the urine until the person is ready to urinate. Following urination, the smooth muscle lining the urethra automatically contracts to re-establish muscle tone, and the individual consciously contracts the external urethral sphincter to close the external urethral opening. Original: Male anatomy blank. Betts, J. Figure In Anatomy and Physiology Section 7. What happens when you hold your pee?
The taboo secret to better health Molly Winter. A secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to impact the behavior of the receiving individuals.
The central nervous system organ inside the skull that is the control center of the nervous system. A muscular, tube-like organ of the urinary system that moves urine by peristalsis from a kidney to the bladder. A distinctive pattern of smooth muscle contractions that propels foodstuffs distally through the esophagus and intestines.
A liquid waste product of the body that is formed by the kidneys and excreted by the other organs of the urinary system.
One of a pair of organs of the excretory and urinary systems that filters wastes and excess water out of blood and forms urine. A tube-like organ of the urinary system that carries urine out of the body from the bladder and, in males, also carries semen out of the body. The smaller population of type II, fast twitch, fibers are likely recruited for assistance during periods of increased abdominal pressure i.
The number and diameter of these fibers decrease with age [ 72 ]. There are no reported differences between females and males in the proportions of these fibers. A group of German investigators analyzed the pelvic floor muscles of young, healthy, female and male cadavers. Due to small sample size, no quantitative analyses were performed, but they reported no histomorphological differences between female and male specimens [ 73 ].
In , Tobin et al. Levator ani LA muscles were removed from day-old female and male embryos and subjected to morphological analysis. The investigators found that female LA muscles contained significantly fewer motor units MU versus and each unit had comparatively smaller cross-sectional areas Conversely, the number of motor units in males increased rapidly to by postnatal day 6, while in females, the number of MUs only increased modestly to The authors posit that exposure to testosterone is responsible for this differential LA muscle development [ 74 ].
Satellite cells, a population of myogenic stem cells residing at the periphery of muscle fibers, seem to be influenced by androgens and may play a role in this sexual dimorphism. Niel et al. With prenatal exposure to testosterone, the number of satellite cells in the female group increased as well as the size of the LA muscle. The authors conclude that the sex differences found in the developing rat LA are attributed to sexual dimorphism in satellite cells, which may be androgen-sensitive [ 75 ].
These animal studies are in concordance with another study that investigated sex differences in the levator ani muscle of human fetuses [ 76 ]. They show that at the same point in development, the male levator ani muscle constitutes a thick muscular layer, while in the female, the LA muscle is thin and its bundles are integrated with connective tissue [ 76 ].
Further studies are necessary to elucidate the developmental differences between female and male pelvic floor musculature in humans. Urinary tract infections UTIs affect nearly million individuals each year [ 77 ], and nearly half of women experience recurrence of UTIs [ 78 ].
Such sex-related vulnerability has been commonly attributed to anatomical differences involving the female lower urinary tract. Observations in support of this view include the fact that the female external urethra is in close proximity to the entrance of the vagina, which houses large numbers of microbes [ 79 ]. The current view is that bacteria migrate from the gut to the vagina and then to urethra, evidenced by the prevalence of native bowel flora that become uropathogens [ 57 ].
The mucosal layer of the urothelium is consistently exposed to countless microbes. Micturition is a passive mechanism for removing threats of infection. However, using single-molecule atomic force microscopy, Miller et al. The mucin layer provides a secondary defense, inhibiting bacterial attachment to the mucosal wall [ 81 , 82 ]. While the mucosal layer is an important anatomical line of defense, more recent evidence supports the existence of nonstructural contributors rendering women more vulnerable to UTI.
For the most part, this vulnerability is thought to stem from the reticuloendothelial system RES , which provides the host immunity against microbes [ 83 ]. In addition, neutrophils and monocytes can enter the bladder upon UPEC infection [ 84 , 85 ]. Mora-Bau et al. Depletion of monocytes had little effect on bacterial clearance. Macrophages have been hypothesized to adapt an immune response via cytokine secretion or antigen sequestration. When depleting mice of macrophages, no change was observed for effecter cell infiltration or cytokine secretion; however, there was an increase in the number of dendritic cells containing UPEC cells [ 83 ].
Although there was an increase in immune response, it did not alter reinfection nor remove the bacteria. Also, mast cell-derived IL enhanced immune tolerance and decreased response to infection in the bladder, enhancing the risk of chronic infection [ 86 ]. Interestingly, these increased numbers were found in the urothelium and not the detrusor.
A thin layer of glycosaminoglycans populate the urothelium, and no difference was observed for either sex or by region of the rabbit bladder [ 88 ]. More research is needed to determine if the differences in rate of UTI between women and men is related to differences in the innate ability of the RES to fight infection. Clinicians often equate the presence of bacteria in urine with infection.
This concept is based on the long-held dogma that urine is sterile. Recently, however, researchers have used next-generation sequencing and enhanced culture methods to detect communities of bacteria, fungi, and viruses microbiota in catheterized urine collected directly from the female bladder [ 89 — 95 ]. Other studies have revealed associations between bladder bacteria and post-operative and post-instrumentation UTIs [ 96 , 97 ], urgency urinary incontinence [ 98 , 99 ], and response to overactive bladder treatment [ ].
These findings suggest that the urinary tract possesses its own protective microbiota and that disruption dysbiosis of this community results in LUT symptoms for recent reviews, see [ — ]. The bacteria most commonly detected in the bladders of adult women are members of the genera Lactobacillus , Gardnerella , Streptococcus , and Staphylococcus [ 98 — ]. Typical uropathogens e. Indeed, the conventional view that UTI is almost always caused by UPEC is suspect, as the standard urine culture protocol performed by clinical microbiology laboratories worldwide misses the vast majority of non- E.
These results argue against the conventional E. While almost all bladder microbiota research has been performed in peri-menopausal women, preliminary studies of pregnant women and men have been reported. Intriguingly, the bladder microbiota of women whose pregnancies went to full term resemble those of older peri-menopausal women [ ].
Live bacteria have also been detected in urine obtained by a catheter from older men. The study sample size is too small to conclude much, except that the male urinary bladder is probably also not sterile [ ]. Thus, the discovery of the bladder microbiota offers an exciting opportunity to advance our understanding of LUT health and disease. Receptors for estrogen, progesterone, and testosterone can be found in the LUT of both sexes across species and across developmental time periods [ — ].
Reproductive hormones have also been shown to play a role in LUT structure and integrity. Female rats that have undergone oophorectomy have a thinner bladder wall and widened spaces between detrusor fascicles compared to controls [ ].
Injections with either estrogen or testosterone reversed these effects [ ]. In males, following castration either before or after puberty, pelvic neurons are significantly smaller than those in noncastrated control male rats [ ]. Testosterone replacement following surgery remediated these effects, signifying that testosterone is important for maturation and maintenance of autonomic pelvic neurons in males [ ]. In contrast, estrogen replacement had no effect on male rat pelvic ganglia [ ].
In female rabbits, injections of either progesterone or testosterone slightly reduce bladder capacity and compliance, while treatment with estrogen increases bladder capacity and compliance [ , ]. In male rabbits, injections of either testosterone or estrogen significantly increases bladder capacity but progesterone treatment has no effect [ ].
The cyclical nature of estrogen and progesterone is associated with worsening LUT symptoms prior to menstruation [ ]. Furthermore, urethral length is increased midcycle of the menstrual cycle, at a time of high estrogen, compared to other phases [ ].
Vaginal estrogen has been shown to reduce LUT symptoms associated with genitourinary syndrome of menopause, such as UTIs and overactive bladder [ ].
However, other studies show conflicting results [ ]. It is clear that reproductive hormones have a role in LUT function and that these hormones can mediate different effects in females compared to males. What remains unknown is the mechanisms underlying the role of these hormones in the LUT of both sexes. Indirect effects of reproductive hormones, such as the role of estrogen in regulating adrenergic receptors in the LUT or its involvement in urothelial cell proliferation through different ER subtypes, have been demonstrated [ ].
However, the presence of reproductive hormone receptors in both sexes suggests direct effects may also be possible [ ]. Sex differences in LUT cells influence form and function. For example, variations in development of the rhabdosphincter result in the observed sexual dimorphism in voiding patterns, while differential expression of immune cells in the bladder mucosa or the differential responses of the urothelium may amplify the rate of urinary tract infections in women.
Along with anatomical and functional variations between women and men, developmental and morphologic disparities on the cellular level need to be taken into account when treating pathology. Research in this field is still immature, and further investigation is needed to answer questions such as the following: How do sex differences in urinary microbiome impact susceptibility to urinary tract infections?
How does the complex interplay between sex hormones and the lower urinary tract mediate development, physiology, and susceptibility to malignancy?
Do sex differences in native reticuloendothelial system cell populations mediate variations in susceptibility to urinary tract infections? Can improved understanding of the complex development of male and female LUT stimulate breakthroughs in the diagnosis and management of LUT dysfunction? This review should serve as a guideline to current knowledge regarding LUT sex differences, as well as a catalyst for investigators wishing to contribute to the field. All other authors contributed to the writing of the manuscript.
All authors read and approved the final manuscript. All other authors declare that they have no competing interests. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. National Center for Biotechnology Information , U. Journal List Biol Sex Differ v. Biol Sex Differ. Published online Oct Author information Article notes Copyright and License information Disclaimer.
Margot S Damaser, Phone: , Email: gro. Corresponding author. Received Jun 20; Accepted Sep This article has been cited by other articles in PMC. Abstract Females and males differ significantly in gross anatomy and physiology of the lower urinary tract, and these differences are commonly discussed in the medical and scientific literature. Keywords: Sex differences, Lower urinary tract, Cell biology, Urology.
Cell populations in or associated with the LUT Detrusor smooth muscle The bladder provides a continent, neurologically controlled, compliant reservoir for urinary storage and a method to consciously void when appropriate. Urothelium The urothelium is a specialized epithelium that lines the lumen and is five to seven cells thick, divided into three layers: an apical layer comprised of umbrella cells only, one cell layer thick that is in contact with urine , an intermediate layer comprised of intermediate cells, two to three cells thick , and a basal layer comprised of basal cells two to three cells thick.
Trigone The mechanistic basis of urinary continence involves relaxation of the detrusor and simultaneous contraction of the bladder neck. Urethra Female urethra The adult female urethra is 3—4 cm in length [ 36 , 37 ] Fig. Open in a separate window. Male urethra The male urethra has a diameter of 8—9 mm, is approximately 18—20 cm long, and is divided into the anterior and posterior urethra [ 37 ] Fig.
Urethral smooth muscle The SM layer of the urethra contains oblique and longitudinal muscle fibers surrounded by circular fibers in both women and men [ 43 ]. Table 1 Urethral smooth muscle sex differences summary. Urethral striated muscle Striated muscle in the LUT is essential for providing support to the pelvic floor and coordinating the initiation of micturition, the emptying of urine from the bladder.
Table 2 Urethral striated muscle sex differences summary. Species Sex Histologic studies Functional studies References Rat Female Striated muscle is thin and circular, prominently occupying the middle urethra.
Type II fibers are predominant. Luminal muscle contains a mix of type I and II fibers. Benoit et al. Pelvic floor striated muscle The pelvic floor, or pelvic diaphragm, is a bowl-shaped structure that is composed of complex interconnected ligaments and striated muscles. Table 3 Pelvic floor striated muscle sex difference summary.
Reticuloendothelial system e. Microbiome Clinicians often equate the presence of bacteria in urine with infection. The role of hormones in sex differences in the LUT Receptors for estrogen, progesterone, and testosterone can be found in the LUT of both sexes across species and across developmental time periods [ — ]. Conclusions Sex differences in LUT cells influence form and function. Notes Ethics approval and consent to participate Not applicable.
Consent for publication Not applicable. Competing interests Dr. References 1. Institute of Medicine. Exploring the biological contributions to human health: does sex matter? Policy: NIH plans to enhance reproducibility. Expert panel recommendations on lower urinary tract health of women across their life span. J Women's Health Larchmt ; 25 — Policy: NIH to balance sex in cell and animal studies. Andersson KE, Arner A. Urinary bladder contraction and relaxation: physiology and pathophysiology.
Physiol Rev. Gross anatomy and cell biology of the lower urinary tract. Second international consultation on incontinence. Plymouth: Health Publication; 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 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 to 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, 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.
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. Nerves involved in the control of urination include the hypogastric, pelvic, and pudendal.
Voluntary micturition requires an intact spinal cord and functional pudendal nerve arising from the sacral micturition center. Since the external urinary sphincter is voluntary skeletal muscle, actions by cholinergic 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.
The kidneys and ureters are completely retroperitoneal, and the bladder has a peritoneal covering only over the dome. 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 hilum 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 sweep laterally, hugging the pelvic walls. As they approach the bladder, they turn medially and pierce the bladder wall obliquely.
What Are the Parts of the Urinary Tract? What Do the Kidneys Do? To help keep your kidneys and urinary tract healthy: Get plenty of exercise.
Eat a nutritious diet. Stay hydrated. For girls: Wipe from front to back after pooping so germs don't get into the urethra. Avoid bubble baths, sitting in the tub after shampoo has been used, and scented soaps. These can irritate the urethra. Wear cotton underwear.
Promptly change out of wet bathing suits. Go for regular medical checkups.
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