Urinary Tract

The urinary tract is located in the abdomen and pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis and consists of the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys through the ureters to the urinary bladder and out through the urethra. The bladder acts as a reservoir for urine until micturition is appropriate. Regulation of micturition relies on the CNS and the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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Embryology

Kidneys and ureters

Kidneys go through 3 embryologic stages:

  • Pronephros: 
    • A functionless, segmented structure made out of a duct from which a series of tubules branch off toward the midline (1 on each side)
    • Regresses and disappears on or right after the 4th week
  • Mesonephros: 
    • Mesonephric tissue forms by the 4th week and is connected to the pronephric duct through the mesonephric duct.
    • The mesonephric (wolffian) duct connects to the cloaca, and the ureteric bud grows from the duct.
  • Metanephros: 
    • The ureteric bud meets the metanephric blastema, forming the metanephros.
    • Development from the ureteric bud: ureter, renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis, 5 major calyces, minor calyces, collecting ducts
    • Development from the metanephric blastema: Bowman capsule, proximal and distal tubules, loop of Henle
    • The kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys ascend, accompanied by the elongation of the ureters.

Bladder and urethra

  • When the mesonephric duct fuses with the cloaca, a portion of the duct becomes a part of the posterior bladder.
  • During gestational week 7, the cloaca is divided by the urogenital membrane/septum into ventral (urogenital sinus) and dorsal ( rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal) components.
  • The urogenital sinus gives rise to the bladder (cranially) and the urethra (caudally).
Development of the kidney and bladder

Development of the kidney and bladder
A: The metanephros forms by the 5th week, with the ureteric bud meeting the metanephric blastema, and the mesonephros regresses.
B: The mesonephric duct fuses with the cloaca, with a portion of the duct becoming part of the posterior bladder. During gestational week 7, the cloaca is divided by the urogenital membrane/septum into ventral (urogenital sinus) and dorsal ( rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal) components. The urogenital sinus gives rise to the bladder (cranially) and the urethra (caudally).

Image by Lecturio.

Ureters

Anatomy

Description: 

  • Collapsible, muscular S-shaped tubes
  • Approximately 25 cm in length
  • Start: ureteropelvic junction
    • At the level of the second lumbar vertebra on the left
    • Right slightly lower
  • End: ureterovesicular junction

Function: 

The function of the ureters is to transport urine from the renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis to the urinary bladder.

Route of ureters:

  • Leave the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys posterior to the renal artery and vein
  • Descend anterior to the psoas major muscle, where gonadal vessels cross anteriorly. 
  • The right ureter travels behind the duodenum.
  • The left ureter travels lateral to the inferior mesenteric vessels.
  • Ureters then enter the pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis, crossing the iliac vessels (usually at the common iliac artery bifurcation into the internal and external iliac arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries).
  • Ureters go into the bladder on the posterior aspect in the trigone.

3 sites of narrowing:

  • Ureteropelvic junction 
  • Crossover of the common iliac arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Ureterovesicular junction

Vasculature and lymphatic drainage:

  • Upper or abdominal ureter arterial blood supply:
    • Renal artery
    • Gonadal arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
    • Ureteric branch of abdominal aorta
    • Common iliac arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Distal ureter: common iliac branches and internal iliac branches (uterine and vesical arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Venous drainage follows arterial supply.
  • Lymphatic drainage:
    • Internal, external, and common iliac nodes
    • Left ureter mainly to the left paraaortic nodes
    • Right ureter mainly to the right paracaval and interaortocaval nodes

Innervation:

  • Intrinsic pacemaker: peristalsis is generated by pacemaker cells in the renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis to facilitate movement of urine. 
  • Sympathetic:
    • Preganglionic: T10–L1
    • Postganglionic: fibers from the aorticorenal, superior, and inferior hypogastric autonomic plexuses 
  • Parasympathetic: S2–S4
Gross anatomy of the urinary system

Diagram of the gross anatomy of the urinary system, showing the kidney, renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis, ureter, urinary bladder, and urethra.

Image: “Illu urinary system” by Arcadian. License: Public Domain

Histology

Ureteral wall structure from the lumen toward the outside:

  • Tunica mucosa: consists of a layer of transitional epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium (urothelium)
  • Lamina propria: loose connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue
  • Tunica muscularis: thickest layer, with spirally organized muscular tissues that are responsible for peristalsis.
    • Longitudinal layer (stratum longitudinale)
    • Circular layer (stratum circulare) 
  • Tunica adventitia: 
    • Connective-tissue layer 
    • Site of blood and nerve vessels
Cross-sectional histologic view of the ureter

Cross-sectional histologic view of the ureter:
Various tissue types within the ureter are illustrated. Note the outermost adventitial layer, deep to which are found the muscular layer, loose connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue, and interior lumen lined with urothelium.

Image: “2607 Ureter” by OpenStax College. License: CC BY 3.0

Urinary Bladder

Anatomy

Description: 

  • Hollow, muscular organ that sits on the pelvic floor and accepts urine via the ureters.
  • Musculature connects with that of the urethra, functioning as an internal urethral sphincter.
  • Connected: 
    • To the umbilicus via the medial umbilical ligament 
    • To the pelvic walls by thickened areas of the pelvic fascia
  • The average bladder capacity is 400 mL.

Parts:

  • Apex: located anterosuperiorly, toward the public symphysis
  • Body: main part of the bladder
  • Fundus: base of the bladder, located posteriorly
  • Trigone: triangular area within the fundus, defined by the 2 ureteral orifices 
  • Bladder neck: 
    • Formed by the convergence of the fundus and the 2 inferolateral surfaces of the bladder
    • Continuous with the urethra

Spatial relations:

  • In females, the urinary bladder is anterior to the uterus and anterior vaginal wall. 
  • In males, the urinary bladder is anterior to the rectum Rectum The rectum and anal canal are the most terminal parts of the lower GI tract/large intestine that form a functional unit and control defecation. Fecal continence is maintained by several important anatomic structures including rectal folds, anal valves, the sling-like puborectalis muscle, and internal and external anal sphincters. Rectum and Anal Canal.

Vasculature and lymphatic drainage:

  • Arterial supply: 
    • Superior and inferior vesical arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries (indirect branches of the internal iliac arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries)
    • Also from the obturator artery and the inferior gluteal artery
  • Venous drainage: most drain into the internal iliac vein
  • Lymphatic drainage: most via external iliac nodes

Innervation:

  • Sympathetic innervation:
    • The lower thoracic and upper lumbar spinal levels (T10–L2) in the hypogastric plexuses and nerves
    • Produces detrusor relaxation → urine retention
  • Parasympathetic innervation:
    • The sacral spinal nerves (S2–S4) form the pelvic splanchnic nerves.
    • Produces detrusor contraction → urination
  • Somatic innervation:
    • The pudendal nerve (S2–S4), a branch of the sacral plexus
    • Voluntary control to the external urethral sphincter
  • Mechanoreceptors (bladder wall):
    • Sense bladder fullness or distention 
    • Signals are transmitted to CNS via the general visceral afferent fibers of the hypogastric and pelvic splanchnic nerves.

Histology

Microscopic description of the wall structure (from the lumen toward the outside):

  • Transitional epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium (urothelium):
    • The urothelium forms a series of rugae, thick mucosal folds that flatten out when the bladder becomes full.
    • The area that is smooth (or without rugae) is the trigone.
    • The trigone consists of:
      • Apical (umbrella cell) layer: innermost layer acting as a barrier for the lumen
      • Intermediate layer: 2–3 layers of polygonal cells
      • Basal layer: 2–3 layers of cuboidal cells
  • Lamina propria: loose connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue supporting separation of the urothelium from the muscularis propria
  • Muscularis propria (detrusor muscle of the bladder wall): 
    • Consists of:
      • Inner longitudinal layer
      • Intermediate circular layer
      • Outer longitudinal layer
    • The internal urethral orifice is surrounded by the internal urethral sphincter, which is under sympathetic autonomic control.
    • Parasympathetic signals cause contractions of the detrusor muscle, leading to urination.
  • Serosa: thin layer of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue that continues with the peritoneal layer
  • Adventitia: layer connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue where serosa is not present

Urethra: Male and Female

Male urethra

Description:

  • Extends from the internal urethral orifice in the bladder neck to the external urethral orifice of the glans penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis
  • Much longer than the female urethra (up to 25 cm long)
  • Responsible for passage of both urine (from the bladder) and semen (from the ejaculatory ducts)

Structure segments:

  • Preprostatic urethra: 
    • Intramural
    • Extends from bladder neck to the prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. Prostate and other Male Reproductive Glands
  • Prostatic (within the prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. Prostate and other Male Reproductive Glands) urethra:
    •  3–4 cm long
    • Receives the prostatic ducts and ejaculatory ducts (from the testes and the seminal vesicles)
  • Membranous urethra: 
    • 2 cm long
    • Runs through urogenital diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm
    • Surrounded by the external urethral sphincter, which voluntarily controls micturition
  • Penile (spongy) urethra:
    • Passes through the corpus spongiosum of the penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis 
    • Ends at the external urethral orifice (meatus)
    • Receives the ducts of the bulbourethral (Cowper) glands

Neurovasculature and lymphatic drainage:

  • Arterial blood supply:
    • Prostatic urethra: inferior vesical artery (branch of the internal iliac artery)
    • Membranous urethra: bulbourethral artery (branch of the internal pudendal artery)
    • Penile urethra: branches of the internal pudendal artery
  • Venous drainage: follows the arterial supply
  • Lymphatic drainage:
    • Internal iliac nodes
    • Deep and superficial inguinal nodes
  • Innervation: prostatic plexus (sympathetic, parasympathetic and visceral afferent fibers)

Histology:

  • Urothelium: 
    • Urothelium from the preprostatic urethra to proximal prostatic urethra 
    • Pseudostratified or stratified columnar epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium form the membranous urethra to the penile urethra.
    • Stratified squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium distally (glans)
  • Lamina propria: connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue with venous plexus 
  • Muscularis: longitudinal and circular layers

Female urethra

Description:

  • Much shorter than the male urethra (approximately 5 cm long)
  • Extends from the internal urethral orifice in the bladder neck to an area between the labia minora; embedded in the anterior wall of the vagina Vagina The vagina is the female genital canal, extending from the vulva externally to the cervix uteri internally. The structures have sexual, reproductive, and urinary functions and a rich blood supply, mainly arising from the internal iliac artery. Vagina, Vulva, and Pelvic Floor
  • Runs behind the pubic symphysis and pierces through the perineal membrane and pelvic floor 

Parts:

  • Proximal area (about ⅔) is surrounded by the external urethral sphincter muscle (circular fibers)
  • Distal end is encircled by the compressor urethrae muscle.
  • The external urethral orifice is located: 
    • Anteriorly to the vaginal opening 
    • 2–3 cm posteriorly to the clitoris
  • 2 mucous glands (Skene glands) lie on either side of the orifice.
Female urethral opening

Female urethral opening, in relation to the vaginal area

Image by Lecturio.

Neurovasculature and lymphatic drainage:

  • Arterial blood supply: internal pudendal artery, vaginal artery
  • Venous drainage: internal pudendal veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins
  • Lymphatic drainage: 
    • Internal iliac nodes (proximal urethra) 
    • Superficial inguinal nodes (distal urethra)
  • Innervation: pudendal nerve, vesical plexus

Histology:

  • Urothelium 
    • Multilayered transitional epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium
    • Has nonkeratinized squamous epithelium Epithelium The epithelium is a complex of specialized cellular organizations arranged into sheets and lining cavities and covering the surfaces of the body. The cells exhibit polarity, having an apical and a basal pole. Structures important for the epithelial integrity and function involve the basement membrane, the semipermeable sheet on which the cells rest, and interdigitations, as well as cellular junctions. Surface Epithelium distally
  • Lamina propria: connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue with venous complex
  • Muscularis: consists of a longitudinal and a circular layer (stratum longitudinale and circulare)

Micturition

Regulation

  • The bladder has 2 states of function: 
    • Urine storage (bladder filling) phase
    • Emptying phase
  • Control mediated by the CNS and spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord:
    • Pontine micturition center (PMC): 
      • Controls bladder/detrusor contraction and sphincter relaxation
      • Allows micturition (emptying) when suppression from higher brain centers is withdrawn
    • Periaqueductal gray (PAG):
      • Receives signals of bladder distention (from sacral cord)
      • Sends information to higher brain centers (e.g., prefrontal cortices) and PMC, thus serving as a relay station
    • Higher brain centers:
      • Play a crucial role in inhibiting PMC if voiding is deemed inappropriate
      • Reduce urge to urinate and allows urine storage
      • Also, the motor cortex controls the external urethral sphincter (voluntary muscle).

Urination

  • The filling phase is characterized by: 
    • Sympathetic contraction of the inner urethral sphincter
    • Distention of the detrusor without reflex contractions under sympathetic stimulation
    • Increased tone of the intrinsic urethral muscles 
  • As the bladder fills to capacity, the contractility of the detrusor increases, sending afferent signals to the brain (PMC and higher brain centers) via the PAG:
    • If micturition is not possible, the cortex overrides these signals and urination is delayed.
    • If micturition is possible, signals are sent from the pontine center to the spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord to initiate the micturition reflex.
    • Beyond maximal capacity filling of the bladder, the micturition reflex overrides cortical control.
  • The micturition phase is characterized by:
    • Relaxation of the external urethral sphincter (somatic innervation)
    • Relaxation of the internal urethral sphincter and opening of the bladder neck (sympathetic innervation)
    • Detrusor muscle contraction (parasympathetic innervation)
    • The abdominal wall and pelvic floor muscles assist by increasing the force on the bladder to help achieve complete emptying.
Events of micturition

Events of micturition involves:
Detrusor muscle contraction (ANS, particularly the parasympathetic system), opening of the internal urethral sphincter (sympathetic innervation) and opening of the external urethral sphincter.

Image by Lecturio.

Clinical Relevance

  • Bladder exstrophy: congenital anomaly involving protrusion of the urinary bladder through a defect in the abdominal wall. The condition is occasionally associated with other anomalies of the genitalia, urinary tract, or pelvic wall. The diagnosis is obvious at birth. The treatment is surgical reconstruction of the bladder and should be undertaken at experienced medical centers. 
  • Disorders of the penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis and male urethra: congenital conditions, which include epispadias (urethra opens not at the tip of the penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis but on the dorsal penile surface) and hypospadias (urethra opens not at the tip of the penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis but on the ventral surface of the penis Penis The penis is the male organ of copulation and micturition. The organ is composed of a root, body, and glans. The root is attached to the pubic bone by the crura penis. The body consists of the 2 parallel corpora cavernosa and the corpus spongiosum. The glans is ensheathed by the prepuce or foreskin. Penis). 
  • Urinary tract infection: inflammatory condition of infectious etiology that involves the urinary tract. Anatomically, these infections can be divided into upper and lower urinary tract infections Urinary tract infections Urinary tract infections (UTIs) represent a wide spectrum of diseases, from self-limiting simple cystitis to severe pyelonephritis that can result in sepsis and death. Urinary tract infections are most commonly caused by Escherichia coli, but may also be caused by other bacteria and fungi. Urinary Tract Infections.
  • Nephrolithiasis Nephrolithiasis Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis: clinical condition characterized by the presence of stones in the kidney or along the urinary tract. Nephrolithiasis Nephrolithiasis Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis presents with acute flank pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain, dysuria, and occasionally hematuria. Noncontrast CT is the diagnostic method of choice. Management depends on the size of the stone. Small stones that are likely to pass on their own are managed conservatively with hydration and analgesics. Large stones that are unlikely to pass spontaneously are managed with extracorporeal shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock-wave lithotripsy, ureterorenoscopy, or percutaneous nephrolithotomy.
  • Urinary incontinence Urinary incontinence Urinary incontinence (UI) is involuntary loss of bladder control or unintentional voiding, which represents a hygienic or social problem to the patient. Urinary incontinence is a symptom, a sign, and a disorder. The 5 types of UI include stress, urge, mixed, overflow, and functional. Urinary Incontinence: involuntary loss of bladder control or unintentional voiding. There are 5 types of urinary incontinence: stress, urge, mixed, overflow, and functional. The etiology of urinary incontinence is multifactorial. For women, risk factors include prior vaginal deliveries and menopause Menopause Menopause is a physiologic process in women characterized by the permanent cessation of menstruation that occurs after the loss of ovarian activity. Menopause can only be diagnosed retrospectively, after 12 months without menstrual bleeding. Menopause; for men, the main risk factor is prior prostate Prostate The prostate is a gland in the male reproductive system. The gland surrounds the bladder neck and a portion of the urethra. The prostate is an exocrine gland that produces a weakly acidic secretion, which accounts for roughly 20% of the seminal fluid. Prostate and other Male Reproductive Glands surgery. Diagnosis is clinical. Management depends on the type(s) of incontinence and the etiology.
  • Urinary tract obstruction Urinary tract obstruction Urinary tract obstruction (UTO) refers to the blockage of the urinary tract, which can occur anywhere in the urinary tract. Urinary tract obstruction can be acute or chronic, partial or complete, and unilateral or bilateral. Urinary tract obstruction can cause acute or chronic kidney disease. Urinary Tract Obstruction: manifests as an inability of urine to pass through the ureters, bladder, or urethra due to a partial or complete blockage. Benign prostatic hyperplasia Benign prostatic hyperplasia Benign prostatic hyperplasia (BPH) is a condition indicating an increase in the number of stromal and epithelial cells within the prostate gland (transition zone). Benign prostatic hyperplasia is common in men > 50 years of age and may greatly affect their quality of life. Benign Prostatic Hyperplasia, malignancies, and kidney stones are among the etiologies of obstruction. Presentation and management is dependent on the underlying condition.
  • Vesicoureteral reflux Vesicoureteral Reflux Vesicoureteral reflux (VUR) is the retrograde flow of urine from the bladder into the upper urinary tract. Primary VUR often results from the incomplete closure of the ureterovesical junction, whereas secondary VUR is due to an anatomic or physiologic obstruction. Vesicoureteral Reflux: retrograde flow of urine from the urinary bladder to the ureter and the kidney. Vesicoureteral reflux Vesicoureteral Reflux Vesicoureteral reflux (VUR) is the retrograde flow of urine from the bladder into the upper urinary tract. Primary VUR often results from the incomplete closure of the ureterovesical junction, whereas secondary VUR is due to an anatomic or physiologic obstruction. Vesicoureteral Reflux can be asymptomatic and resolve spontaneously, or may need to be corrected surgically. The condition is associated with frequent urinary tract infections Urinary tract infections Urinary tract infections (UTIs) represent a wide spectrum of diseases, from self-limiting simple cystitis to severe pyelonephritis that can result in sepsis and death. Urinary tract infections are most commonly caused by Escherichia coli, but may also be caused by other bacteria and fungi. Urinary Tract Infections, leading to pyelonephritis Pyelonephritis Pyelonephritis is infection affecting the renal pelvis and the renal parenchyma. This condition arises mostly as a complication of bladder infection that ascends to the upper urinary tract. Pyelonephritis can be acute or chronic (which results from persistent or chronic infections). Typical acute symptoms are flank pain, fever, and nausea with vomiting. T Pyelonephritis and Perinephric Abscess and nephropathy.

References

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