BIOL1421: Chapter 24 (Urinary System)

functions of the urinary system
-filtration, reabsorption, secretion
-blood plasma filtered through kidneys; unwanted substances eliminated through urination

urinary system filters blood, to remove waste through urine
-voided through urethra
-kidneys produce urine, bladder stores urine
-ureter carries urine from kidneys → bladder
-urethra carries urine from bladder → outside of body

-aorta → kidney produces urine → ureter → urethra/bladder

-kidney is located posterior of peritoneum of digestive system; located between T1 and L3 vertebrae

each kidney has outer renal cortex and inner renal medulla, renal pyramids
-renal pelvis is connected to ureter → bladder, urethra

blood supply to the kidneys
-aorta gives oxygenated blood through renal arteries → kidneys
-inferior vena cava w renal veins

-acruate artery surround each renal pyramid, some blood goes to cortex,
-renal artery → segmental artery → interlobular artery → arcuate artery → renal pyramids

nephron structure
-nephron is head of tube that collects urine; where filtration is done
-glomerulus - proximal convul

-reabsorption: first part of tube (nephron → blood)
-secretion: end part of tube (wastes from blood → renal tube)

urine content: 95% water, 2% urea, 2.*% dissolved salts and other ions

function of adh
-produced by hypothalamus, stored in posterior pituitary gland, in involved with nephron’s secretion in distal convulated tube, adh decreases urine excretion

pathway of urine
cortex → blood filtered in nephron → renal pyramid → renal pelvis → ureter → bladder → urethra → urethra orifice

ureter connects renal pelvis to bladder, contains smooth muscle

female bladder is located in front of vagina, and below the uterus
male bladder is located in front of rectum, and above the prostate gland

bladder anatomy
-surrounded by smooth muscle, detrusor
-2x ureters enter the trigone area
-ureteral orifice

micturition reflex: responsible for releasing urine
-internal urethral sphincter works involuntarily
-external urethral sphincter works voluntarily
-stimulated by sacral spinal nerves → thalamus → cerebral cortex

female urethra is shorter than male urethra
-bladder neck → prostate, prosthetic urethra →

INTRODUCTION TO URINARY SYSTEM
-urinary system + respiratory system = excrete non-useful metabolic products
-kidneys maintain homeostasis by:
1.) FILTRATION filtering blood plasma (specifically nitrate); contributes to urine production
2.) REABSORPTION returning most water + useful solutes → bloodstream; contributes to bloodstream content
3.) SECRETION selectively eliminating unneeded substances (through filtrate; excess water); contributes to urine production
-resulting urine is stored and transported for excretion, by other organs of urinary system

KIDNEY FUNCTIONS
1.) regulate various properties of the blood
(eg. ionic composition, pH, volume, pressure, glucose levels)
*our body wants to keep Na+ in bloodstream, through osmotic pressure
*excreting H+ ions through urine ensures body isn’t too acidic
*glucose should ideally be reabsorbed into body

2.) produce hormones
(eg. calcitriol which produces calcium, erythropoietin which produces red blood cells)

3.) excrete waste + foreign substances

LOCATION AND EXTERNAL ANATOMY OF KIDNEY
kidneys are paired, retroperitoneal organs
*retroperitoneal: located behind the peritoneum
-includes renal hilum, which is indentation where ureter exits and nerves, blood, lymphatic vessels enter and exit
-includes 3 layers of connective tissue
1.) renal capsule (deep layer), which maintains shape and forms barrier
2.) adipose capsule (middle layer), which cushions and supports
3.) renal fascia (superficial layer), which anchors to abdominal wall

INTERNAL ANATOMY OF THE KIDNEY
1.) renal cortex
-outer layer

2.) renal medulla
-inner layer
-includes renal pyramids (papillary duct of collecting duct)
-includes renal columns (renal cortex tissue extending between pyramids)

3.) renal sinus
-includes minor calyx, major calyx, renal pelvix, branches of renal blood vessels + nerves, adipose

PATHWAY OF URINE DRAINAGE:
collecting duct (in renal pyramid) → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urethra

BLOOD SUPPLY OF THE KIDNEY
1.) renal artery, which are afferent vessels receiving 20-25% of resting cardiac output from abdominal aorta (of the heart)
*cardiac output pushes blood to different parts of the body
2.) segmental arteries, which are branches within renal sinus
3.) interlobar arteries and veins, which are located in renal column between adjacent renal pyramids
4.) arcuate arteries and veins , which are located at pyramid base between renal medulla and cortex
5.) cortical radiate arteries and veins, which are located within renal cortex
6.) afferent arteriole, which are found as one per nephron
7.) glomerulus, which are network of capillaries that filter blood for urine production
(unique because their location between two arterioles)

8.) efferent arteriole , which are found one per nephron
9.) peritubular capillaries, which some form loop of Vasa recta in renal medulla
10.) renal vein, which efferent vessels carry blood through → to inferior vena cava

PATHWAY OF BLOOD SUPPLY TO KIDNEY:
renal artery (at hilum) → segmental artery → interlobular artery → arcuate artery → cortical radiate artery → afferent arteriole → nephron (where filtrate is found and filtration occurs); glomerulus

PATHWAY OF BLOOD SUPPLY FROM KIDNEY TO HEART:
efferent arteriole → peritubular capillaries, vasa recta→ interlobular vein → arcuate vein → renal vein → inferior vena cava → right atrium of heart

NEPRON STRUCTURE
nephrons consists of two parts
1.) renal corpuscle
-includes glomerulus, which is the capillary network
-includes glomerular capsule, which is also known as Bowman’s capsule that is the head that surrounds the glomerulus

2.) renal tubule
-includes proximal convoluted tubule (PCT); which is close to glomerulus head
-includes Loop of Henle, which extends into renal medulla (has descending limb of nephron loop + ascending limb of nephron loop)
-includes distal convoluted tubule (DCT)

*glomerulus head → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct (reznal hilum)

nephrons also consists of two types
1.) cortical nephron (80-85% of nephrons)
-incudes corpuscles in outer cortex w/ short loops of Henle

2.) juxtamedullary nephron (15-20% of nephrons)
-includes corpuscles deep in cortex w/ long loops of Henle
-includes thin and thick ascending limb

HISTOLOGY OF NEPHRON STRUCTURE
1.) nephron
-includes single layer of epithelial cells in both parts of nephron
-each region has distinctive features, reflecting function

2.) glomerular capsule
-includes simple squamous epithelial cells (outer parietal layer)
-includes capsular space, which contains filtrate
-includes podocytes (inner visceral layer, which appears as foot or hand)
-includes mesangial cells, which increases surface area when relaxed and decreases surface area when contracted

JUXTAGLOMERULAR APPARATUS (JGA)
-includes two layers (macula densa + juxtaglomerular cell)
consists of three main types of cells:
1.) macula densa, which is part of distal convoluted tubule and detects sodium levels
-responds by signaling changes in blood flow and concentration

2.) juxtaglomerular cell, which is part of afferent arteriole
-produces enzyme renin, in response to low blood pressure

3.) extraglomerular mesangial cells

3.) proximal convoluted tubule
-includes cuboidal epithelial cells w/ brush border microvilli

4.) Loop of Henle
-includes descending + first part of ascending simple squamous
-includes thicker part of ascending limb cuboidal → columnar

5.) juxtaglomerular apparatus
-includes macula densa cells of ascending limb
-includes juxtaglomerular (JG) cells of afferent arteriole
-function: involved in blood pressure regulation within kidneys

6.) distal convoluted tubule and collecting duct
-includes principal cells and intercalated cells
-function: involved in blood volume, composition, pH regulation

filtration: glomerulus → capsular space
reabsorption: materials from urine (tubular) → blood
secretion: materials from blood → urine (tubular)

FORMATION OF URINE
1.) glomerular filtration
-from blood plasma in glomerulus → glomerular capsule space
2.) tubular reabsorption
-from filtrate in renal tubule lumen → peritubular and vasa recta capillaries
3.) tubular secretion
-from blood in peritubular capillary → renal tubule lumen

GLOMERULAR FILTRATION MEMBRANE
filtration: pressure drives movement across membrane; size of pores determines permeability (what enters)

three layers forming the pores include:
1.) glomerular endothelial cells
-their fenestrations prevent filtration of blood cells and platelets
(red blood cells + platelets = stay in blood, not filtered)

2.) basal lamina
-the basement membrane of endothelial and podocyte cells prevent filtration of larger plasma proteins
(larger plasma proteins = stay in blood, not filtered)

3.) pedicles of podocytes intertwin
-forms filtration slits w/ thin slit membrane prevents filtration of albumin plasma protein)
(albmumin = stay in blood, not filtered)

FILTRATION VOLUME
large volume of filtration through glomerulus. compared to other body capillaries, due to:
1.) large surface area of glomerular capillaries
-mesangial cells contract to change surface area available for filtration

2.) thin and more porous due to fenestra of endothelial cells

3.) higher glomerular capillary blood pressure
-afferent arteriole is larger diameter than efferent arteriole, which creates outflow resistance

NET FILTRATION PRESSURE (NFP)
NFP = GBHP - CHP - BCOP
-where GBHP is glomerular blood hydrostatic pressure, which promotes filtration
(blood pressure in afferent arteriole → glomerulus)
-where CHP is capsular hydrostatic pressure, which opposes filtration
(blood pressure within capsular space)
-where BCOP is blood colloid osmotic pressure, which opposes filtration
(pressure created when colloids in blood absorb water)

GLOMERULAR FILTRATION RATE (GFR)
mechanisms affecting filtration PRESSURE include:
1.) adjusting blood flow
-occurs when there is coordinated control of the diameter of renal arterioles

2.) altering available glomerular capillary surface area
-occurs through mesangial cells

glomerular filtration RATE controlled by three mechanisms including:
1.) renal autoregulation, which is the ability to maintain renal blood flow (despite normal changes in systemic blood pressure)
2.) neural regulation
3.) hormonal regulation
(increase in renin, angiotensin, aldosterone, ADH causes increase in filtration + high blood pressure)

RENAL AUTOREGULATION OF GFR
1.) myogenic mechanism:
-as blood pressure rises, smooth muscle fibers in afferent contract and blood flow to glomerulus decreases
-as blood pressure drops, smooth muscle fibers in afferent relax and blood flow to glomerulus increases

2.) tubuloglomerular feedback is negative feedback by macula densa cells
-detects changes in Na+ and Cl- levels in filtrate
-responds by controlling diameter of afferent arteriole, sends signal to juxtaglomerular cells
(increased Na+ levels → constriction of afferent arteriole → decreases blood flow to glomerulus)

renal autoregulation also inhibits release of renin, from juxtaglomerular cells

NEURAL REGULATION OF GFR (generally not as strong as renal autoregulation)
neural regulation of GFR is achieved through autonomic nervous system
1.) normally, sympathetic stimulation to kidney is low and autoregulation continues
(sympathetic stimulation is fight or flight)
2.) sympathetic stimulation of kidney increases w/ exercise or hemorrhage (where sympathetic neurons release norepinephrine, afferent arterioles vasoconstrict)
3.) GFR drops (resulting in reduced urine output to conserve blood volume, permitting greater blood flow to other parts of the body; like muscles where it is needed)

HORMONAL REGULATION OF GFR
1.) angiotensin II
-constricts both afferent + efferent arterioles
-reduces renal blood flow + GFR; thus blood pressure increases
-part of renin-angiotensin-aldosterone system (so linked to hormonal regulation of tubular secretion and reabsorption)

2.) atrial natriuetic peptide (ANP)
-from atria of heart, as blood pressure rises
(results in decreasing blood pressure)
-relaxes mesangial cells
-increased surface area for filtration, which increases GFR (more filtrate)

TUBULAR REABSORPTION
tubular reabsorption reclaims 99% of filtered water + many other substances from filtrate (mainly occurs in descending limb of PCT + last part of DCT + collecting duct w/ ADH that increases pore size)
-selective reabsorption by epithelial cells of tubules
-water reabsorbed by osmosis + reabsorbed sodium
-solutes reabsorbed through active (requires energy) + passive mechanisms (pressure gradient)
-maintains homeostasis

TUBULAR SECRETION
tubular secretion removes excess or unneeded substances from blood → tubular filtrate (occurs in PCT and collecting duct; H+ and NH3 secreted through PCT; H+ and K+ secreted in collecting duct through urine)
-secretion of H+ regulates pH
-secretion of other ions involved in electrolyte homeostasis
-eliminates metabolic by-products + some drugs detected by urinalysis

REABSORPTION ROUTES
1.) apical membrane of epithelial cell, in contact w/ tubule filtrate
*part that faces urine in lumen

2.) basolateral membrane of epithelial cell, in contact w/ interstitial fluid near peritubular capillary
*part that faces bloodstream

3.) paracellular reabsorption
-passive through adjacent tubule cells that act as “leaky” intercellular junctions

4.) transcellular reabsorption
-occurs across cell membranes of tubule cells
-can be passive or active transport w/ membrane proteins (usually active)

TRANSPORT MECHANISMS
1.) selective
-membrane proteins allow movement in one direction only
-apical or basolateral location determines net effect

2.) primary active transport, through Na+/K+ ATPases (uses ATP to transport Na+ in blood and K+ out of blood)

3.) secondary active transport
-does not use ATP, but uses energy of movement
-can be coupled
-can be symporters or antiporters
(symporters: Na+ is able to exit through pore with other substance exiting, antiporters: Na+ is able to exit through pore with other substance entering opposing direction)

4.) water reabsorbed by osmosis
-passive transport (occurs Na+ enters urine)
-solution reabsorption to tubules promote water reabsorption (with help of ADH)

5.) obligatory water reabsorption
-PCT + descending limb of nephron loop always permeable to water

6.) facultative water reabsorption
-collecting duct has variable permeability to water
-regulated by ADH

REABSORPTION AND SECRETION IN PCT
1.) most reabsorption processes involve Na+
2.) Na+ symporters in apical membrane PCT cells reabsorb
-gluclose, amino acids, lactic acid, water-soluble vitamins, and other nutrients

3.) Na+/H+ antiporters
-carry Na+ → into cell; H+ out → to lumen into tubular fluid
-important for homeostasis of blood pH

4.) carbonic anhydrase (CA)
-catalyzes formation of carbonic acid (H2CO3) in cytosol, to produce H+ for secretion from tubule cell
-H+ reacts w/ bicarbonate (HCO3-) in filtrate
-catalyzes dissociation of carbonic acid in filtrate, to produce CO2 for diffusion → into tubule cell
-mechanism of bicarbonate reabsorption from filtrate
-important as blood buffer

5.) solute reabsorption promotes water reabsorbed by facultative osmosis
-creates osmotic gradient
-paracellular and transcellular routes
-restores osmotic homeostasis

6.) water reabsorption promotes passive reabsorption of important ions + minerals
-creates electrochemical gradients
-paracellular and transcellular routes
-reabsorbed ions inclue Cl-, K+, Ca2+, Mg2+, urea

7.) ammonia (NH3) and urea secreted by PCT → tubular fluid

REABSORPTION IN NEPHRON LOOP
1.) ascending limb has Na+/K+/2Cl- symporters, and is relatively impermeable to water
-reabsorption of Na+ and 2Cl-
-K+ leakage channel → returns to filtrate
-passive eletrochemical gradient created for reabsorption of other cations (Na+, K+, Ca2+, Mg2+)

2.) descending limb permeable to water, for 15% water reabsorption

3.) substances diffuse → into vasa recta capillaries around loop

REABSORPTION IN DCT
1.) Na+/Cl- symporters continue Na+ and Cl- reabsorption
2.) 10-15% of filtered water is reabsorbed
3.) Ca2+ reabsorption controlled by parathyroid hormone

REABSORPTION AND SECRETION IN LATE DCT AND CT
1.) principal cells
-contain leakage channels and Na+/K+ pumps
-reabsorb Na+
-variable secretion of K+, which adjusts for dietary intake
-ADH receptors control variable water reabsorption
-aldosterone receptors control variable Na+ and Cl- reabsorption

2.) intercalated cells
-reabsorb K+ and HCO3-; secrete H+
-important for homeostasis of blood pH

TUBULAR REABSORPTION AND SECRETION HORMONAL REGULATION OVERVIEW
1.) renin-angiotensin-aldosterone system, which regulates electrolyte reabsorption + secretion
(Na+ reabsorbed from urine to blood → water reabsorbed)
2.) antidiuretic hormone (ADH), which regulates water reabsorption into blood and less water in urine
3.) atrialnatriuretic peptide (ANP), which inhibits electrolyte + water absorption
4.) parathyroid hormone, which stimulates DCT cells to reabsorb more Ca2+


RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM
1.) decrease in blood pressure → stimulates release of renin (from juxtaglomerular cells)
2.) renin converts angiotensinogen → into angiotensin I
3.) ACE activates angiotensin I → to angiotensin II
-decreases glomerular filtration rate
-enhances PCT reabsorption of Na+, Cl-, water
-stimulates release of aldosterone → to reabsorb more Na+, Cl-, water (by principal cells)

net result: blood volume + blood pressure increases

ANTIDIURETIC HORMONE/VASOPRESSIN
1.) ADH release, from posterior pituitary, stimulated by…
-decrease of water concentration in blood (through osmoreceptors)
-decrease in blood volume (through baroreceptors)

2.) ADH regulates facultative water reabsorption
-inserts aquaporins → into apical membranes of principal cells
-basolateral membranes always permeable, so water moves → into capillaries

net result: production of concentration urine → to maintain fluid homeostasis

ATRIAL NATRIURETIC PEPTIDE
-released from heart, in response to large increase in blood volume
-inhibits Na+ and water reabsorption in PCT
-suppresses secretion of aldosterone + ADH

net result: increased urine output, to decrease blood volume and blood pressure

PARATHYROID HORMONE
-released from parathyroid gland, in response to drop in blood Ca2+ levels
-stimulates cells in DCT, to reabsorb more Ca2+
-inhibits phosphate reabsorption in PCT

net result: increased blood calcium (w/o increased phosphate)

FORMATION OF DILUTE URINE
-PCT lumen fluid remains isotonic to blood plasma, by obligatory water reabsorption following solutes

-descending limb of nephron loop reabsorbs more water passively → into renal pyramid interstitial fluid in lumen more concentrated

-ascending limb of nephron loop impermeable to water; so Na+, K+, Cl- ion absorption by symporters makes the fluid in lumen more dilute than blood plasma

-early DCT not very permeable to water; so as more solute reabsorbed, fluid in lumen becomes more dilute

-late DCT and collecting duct impermeable to water when ADH normally low; so continued solute absorption makes fluid draining → into minor calyx up to 4x more dilute than blood plasma

FORMATION OF CONCENTRATED URINE
-ADH allows concentration of tubular fluid, due to increasing osmotic gradient of solutes in interstitial fluid from renal cortex to medulla

-three major solutes contribute to this gradient: Na+, Cl-, and urea

-two major factors establish, maintain this gradient: different water permeability + solute reabsorption in different sections of loop, countercurrent fluid flow in loop and vasa recta

-symporters in ascending limb create buildup of Na+ and Cl- in renal medulla

-different permeability, as countercurrent flow in descending limb + ascending limb
(descending very permeable to water but impermeable to most solutes; ascending reabsorb Na+ and Cl= but impermeable to water)

net result: tubular fluid becomes more concentrated as it flows down → into medulla; but becomes more dilute as it flows back up → to cortex

FORMATION OF CONCENTRATION URINE PART 2
-under influence of ADH, principal cells in collecting duct become permeable to water

-as collecting duct fluid passes through increasing osmotic gradient, more water is reabsorbed

-urea recycling causes buildup in renal medulla, promoting more water reabsorption
(collecting duct permeable, so reabsorbed; descending + thin ascending loop permeable, so secreting → into tubule lumen again)

-vasa recta countercurrent provides oxygen + nutrients → to renal medulla (w/o diminishing osmotic gradients)

net result in renal medulla:
1.) nephron loop (especially juxtamedullary) establishes osmotic gradient
2.) urea recycling enhances osmotic gradient
3.) vasa recta maintains the osmotic gradient
4.) ADH changes collecting duct permeability, to regulate rate of water loss in urine

FLUID BALANCE
fluid balance is when required amounts of water + solutes + electrolytes are present, and correctly distributed throughout the body
1.) intracellular - cytosol/fluid within cells (eg. fluid inside red blood cells)
2.) extracellular - interstitial tissue fluid between cells, blood plasma, lymph, and other fluids of body cavities

kidneys maintain fluid homeostasis, by excreting dilute or concentrated urine

fluid gain/loss → changes blood volume and blood pressure

REGULATION OF WATER GAIN
water gain is sources of water from ingesting liquids + moist foods + metabolic synthesis of water

regulation of water gain includes:
1.) dehydration, which is when water loss > gain
2.) increased blood osmotic pressure, which occurs when osmoreceptors stimulate hypothalamus thirst center
3.) decreased blood volume, which occurs when renin-angiotensin II stimulates hypothalamus thirst center

net effect: increased fluid intake → restores normal fluid volume

REGULATION OF WATER AND SOLUTE LOSS
water loss occurs through variable urine concentration, perspiration, feces, water vapor, or other fluid loss
-two major extracellular electrolytes eliminated in urine, if excess: Na+ and Cl-
-urinary salt (NaCl) loss mainly determines body fluid volume
-in osmosis, water follows solutes
-blood osmotic pressure is closely related to Na+ concentration

hormonal regulation of solute loss, when blood volume is increased
-release of ANP- → increase in Na+ loss AND Cl- loss, and water follows
-decreased renin-angiotensin II → increase in GFR and decrease in Na+, Cl-, water reabsorption
-decreased aldosterone → decrease in Na+ and Cl- reabsorption, in collecting duct

ADH regulates water loss, when blood osmotic pressure is increased
-increased water absorption in collecting ducts

net effect: small volume of concentrated urine

MOVEMENT OF WATER INTO AND OUT OF CELLS
1.) changes in interstitial fluid osmotic pressure affects cells, moving water into or out of cells

2.) increased interstitial osmotic pressure can cause cells to shrink; usually corrected by ADH and decreased urinary water loss

3.) decreased interstitial osmotic pressure can cause cells to swell; water intoxication can occur if kidneys unable to excrete water quickly enough

ACID-BASE BALANCE
body fluid pH homeostasis is critical to proper function of proteins and cells
-normal blood pH: 7.35-7.45
-metabolism produces excess H+

mechanisms for H+ removal from body fluids
-buffer systems
-urine excretion of H+
-cellular excahnge of K+ and H+
-exhalation of CO2

PHOSPHATE BUFFER SYSTEM
-buffers: act quickly to temporarily bind or release H+ ions; convert strong acids/bases to weak acids/bases
-monohydrogen phosphate (HPO42-): weak base to buffer excess H+
-dihydrogen phosphate (H2PO4-): weak acid to release H+
-in kidney collecting duct, HPO42- also buffers acids in urine for excretion

KIDNEY EXCRETION OF H+
1.) Na+/H+ antiporters some secretion in PCT

2.) intercalated cells of late DCT/collecting duct
-apical proton pumps (H+ ATPases) able to secrete H+ → into tubule fluid against concentration gradient
-basolateral CI-/HCO3- antiporters able to increase HCO3- concentration in peritubular capillaries

3.) secreted H+ in tubule fluid buffered for excretion in urine
-HCO3-, H2PO4-, NH3 (ammonia)

FUNCTIONS OF K+
1.) most abundant cation in intracellular fluid; helps maintain intracellular fluid volume
2.) key role in resting membrane potential, and action potential of neurons + muscle fibers
3.) helps maintain body fluid pH, by exchange w/ H+ ion
4.) extracellular concentration of K+ maintained by aldosterone stimulating principal cells, in collecting duct to secrete more K+

URINE TRANSPORT: URETERS
transport urine from renal pelvis → to urinary bladder
-retroperitoneal (like kidneys)
-urine flow by peristalsis, hydrostatic pressure + gravity
-physiological valve prevents backflow, by compression of opening as bladder fills w/ urine

three layers of ureter wall
1.) inner mucosa of transitional epithelia: stretch and mucus protection
2.) muscularis of longitudinal + circular smooth muscle
3.) superficial adventitia of areolar connective tissue

URINE STORAGE: URINARY BLADDER
urinary bladder is a hollow, distensible muscular organ in pelvic cavity
-location: posterior to pubic symphysis, inferior to peritoneum

trigone
-posterior has two uretheral openings
-anterior has internal urethral orifice

three layers of bladder wall
1.) inner mucosa of transitional epithelia: rugae permits expansion
2.) detrusor muscle: contains three layers of smooth muscle
3.) superficial adventitia or peritoneum

sphincters control emptying of bladder
1.) internal urethral sphincter contains smooth muscle, is involuntary
2.) external urethral sphincter contains skeletal muscle, is voluntary

MICTURITION REFLEX
micturition reflex is the discharge of urine from urinary bladder → to urethra

stretch receptor stimulate sacral spinal reflex arc
-stimulates parasympathetic impulses (includes contraction of detrusor muscle, relaxation of internal urethral sphincter muscle)
-inhibits somatic motor neuron impulses (includes relaxation of external urethral sphincter muscle)

micturition reflex can initiate or delay voluntarily, through learned control by cerebral cortex

URETHRA
urethra leads from bladder → exterior of body, for discharge of urine from the body
-in females, urethra is short w/ exterior opening between clitoris and vaginal opening
-in males, urethra also discharges semen w/ exterior opening through the penis
(subdivided into three regions: prostatic urethra, membranous urethra, spongy urethra)