Urinary System 2024 PDF

 2 LEARNING OBJECTIVES Identify the components of the urinary system and describe the vital functions performed by this system.  Three Functions of the Urinary System 1. Excretion Removal of organic wastes from body fluids...

 2 LEARNING OBJECTIVES Identify the components of the urinary system and describe the vital functions performed by this system.  Three Functions of the Urinary System 1. Excretion Removal of organic wastes from body fluids 2. Elimination Discharge of waste products 3. Homeostatic regulation Of blood plasma volume and solute concentration © 2012 Pearson Education, Inc.  Homeostatic Functions of the Urinary System Regulates blood volume and blood pressure By adjusting volume of water lost in urine Releasing erythropoietin and renin Regulates plasma ion concentrations Sodium, potassium, and chloride ions (by controlling quantities lost in urine) Calcium ion levels (through synthesis of calcitriol) Helps stabilize blood pH By controlling loss of hydrogen ions and bicarbonate ions in urine Conserves valuable nutrients By preventing excretion while excreting organic waste products Assists liver - In detoxifying poisons © 2012 Pearson Education, Inc. An Introduction to the Urinary System Kidney Produces urine Ureter Transports urine toward the urinary bladder Urinary bladder Temporarily stores urine prior to elimination Urethra Conducts urine to exterior; in males, transports semen as well Anterior view © 2012 Pearson Education, Inc.  6 LEARNING OBJECTIVES Describe the structure of the kidneys. Identify the major blood vessels associated with each kidney and trace the path of blood flow through the kidney.  The Kidneys Are located on either side of vertebral column Left kidney lies superior to right kidney Superior surface capped by adrenal gland Position is maintained by: Overlying peritoneum Contact with adjacent visceral organs Supporting connective tissues © 2012 Pearson Education, Inc. The Position of the Kidneys Diaphragm Left kidney Renal artery and vein Adrenal gland 11th and 12th ribs Right kidney Lumbar (L1) vertebra Ureter Inferior vena cava Iliac crest Abdominal aorta Urinary bladder Urethra A posterior © 2012 Pearson Education, Inc. view of the trunk The Gross Anatomy of the Urinary System Esophagus (cut) Diaphragm Left adrenal gland Inferior vena cava Celiac trunk Right adrenal Left kidney gland Left renal artery Right kidney Left renal vein Hilum Superior mesenteric artery Quadratus Left ureter lumborum Abdominal aorta muscle Iliacus muscle Left common iliac Psoas major artery muscle Gonadal artery Peritoneum (cut) and vein Rectum (cut) Urinary bladder Anterior view © 2012 Pearson Education, Inc.  Typical Adult Kidney Is about 10 cm long, 5.5 cm wide, and 3 cm thick Weighs about 150 g Hilum Point of entry for renal artery and renal nerves Point of exit for renal vein and ureter Renal Cortex Superficial portion of kidney in contact with renal capsule Reddish brown and granular Renal Pyramids 6 to 18 distinct conical or triangular structures in renal medulla Base found in the cortex Tip (renal papilla) projects into renal sinus © 2012 Pearson Education, Inc. The Structure of the Kidney Renal cortex Renal medulla Renal pyramid Inner layer of fibrous capsule Renal sinus Connection to minor calyx Adipose tissue Minor calyx in renal sinus Major calyx Renal pelvis Hilum Renal lobe Renal papilla Renal columns Ureter Fibrous capsule A diagrammatic view of a frontal section through the left kidney © 2012 Pearson Education, Inc. The Structure of the Kidney Renal cortex Renal medulla Renal pyramids Renal sinus Renal pelvis Hilum Major calyx Minor calyx Ureter Renal papilla Renal columns Renal lobe Fibrous capsule A frontal section of the left kidney © 2012 Pearson Education, Inc.  Nephrons Microscopic, tubular structures in cortex of each renal lobe Where urine production begins Blood Supply to the Kidneys Kidneys receive 20%–25% of total cardiac output 1200 mL of blood flows through kidneys each minute Kidney receives blood through renal artery © 2012 Pearson Education, Inc.  Segmental Arteries Receive blood from renal artery Divide into interlobar arteries Which radiate outward through renal columns between renal pyramids Supply blood to arcuate arteries Which arch along boundary between cortex and medulla of kidney Afferent Arterioles Branch from each cortical radiate artery (also called interlobular artery) Deliver blood to capillaries supplying individual nephrons © 2012 Pearson Education, Inc. The Blood Supply to the Kidneys Cortical radiate veins Cortical radiate arteries Interlobar Cortex arteries Segmental artery Adrenal Medulla artery Renal artery Renal vein Interlobar Arcuate veins veins Arcuate arteries A sectional view, showing major © 2012 Pearson Education, Inc. arteries and veins The Blood Supply to the Kidneys Glomerulus Cortical radiate vein Afferent arterioles Cortical radiate artery Arcuate artery Cortical Arcuate vein nephron Renal Juxtamedullary pyramid nephron Interlobar vein Interlobar artery Minor calyx © 2012 Pearson Education, Inc. Circulation in a single renal lobe The Blood Supply to the Kidneys Renal vein Renal artery Segmental arteries Interlobar veins Interlobar arteries Arcuate veins Arcuate arteries Cortical radiate veins Cortical radiate arteries Venules Afferent arterioles NEPHRONS Peritubular Glomerulus capillaries Efferent arteriole © 2012 Pearson Education, Inc. A flowchart of renal circulation  18 LEARNING OBJECTIVES Describe the structure of the nephron, including the microanatomy of the renal corpuscle.  The Nephron Consists of renal tubule and renal corpuscle Renal tubule Long tubular passageway Begins at renal corpuscle Spherical structure consisting of: Glomerular capsule (Bowman’s capsule) Cup-shaped chamber Capillary network (glomerulus) © 2012 Pearson Education, Inc.  Glomerulus Consists of 50 intertwining capillaries Blood delivered via afferent arteriole Blood leaves in efferent arteriole Flows into peritubular capillaries Which drain into small venules And return blood to venous system © 2012 Pearson Education, Inc.  Three Functions of the Renal Tubule 1. Reabsorb useful organic nutrients that enter filtrate 2. Reabsorb more than 90% of water in filtrate 3. Secrete waste products that failed to enter renal corpuscle through filtration at glomerulus © 2012 Pearson Education, Inc.  Segments of the Renal Tubule Located in cortex Proximal convoluted tubule (PCT) Distal convoluted tubule (DCT) Separated by nephron loop (loop of Henle) U-shaped tube Extends partially into medulla © 2012 Pearson Education, Inc. The Functional Anatomy of a Representative Nephron & the Collecting System NEPHRON Proximal convoluted tubule Distal convoluted tubule Reabsorption of water, Secretion of ions, acids, ions, and all drugs, toxins organic nutrients Variable reabsorption of water, sodium ions, and calcium ions (under hormonal control) Renal tubule Capsular space Glomerulus Efferent arteriole Afferent arteriole Glomerular capsule Descending Ascending limb of limb of loop begins loop ends Renal corpuscle Production of filtrate Thin Thick descending ascending limb limb Descending Ascending limb limb KEY Filtrate Water reabsorption Nephron loop Variable water reabsorption Further reabsorption of water Solute reabsorption or secretion (descending limb) and both sodium and chloride Variable solute reabsorption ions (ascending limb) or secretion © 2012 Pearson Education, Inc.  Organization of the Nephron Travelling along tubule, filtrate (tubular fluid) gradually changes composition Changes vary with activities in each segment of nephron © 2012 Pearson Education, Inc.  Each Nephron Empties into the collecting system A series of tubes that carries tubular fluid away from nephron Collecting ducts Receive fluid from many nephrons Each collecting duct: Begins in cortex Descends into medulla Carries fluid to papillary duct that drains into a minor calyx © 2012 Pearson Education, Inc. The Functional Anatomy of a Representative Nephron & the Collecting System COLLECTING SYSTEM KEY Filtrate Water reabsorption Variable water reabsorption Solute reabsorption or secretion Variable solute reabsorption or secretion Collecting duct Collecting duct Variable reabsorption of water and reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ions Papillary duct Delivery of urine to minor calyx Minor calyx © 2012 Pearson Education, Inc.  Cortical Nephrons 85% of all nephrons Located mostly within superficial cortex of kidney Nephron loop (Loop of Henle) is relatively short Efferent arteriole delivers blood to a network of peritubular capillaries Juxtamedullary Nephrons 15% of nephrons Nephron loops extend deep into medulla Peritubular capillaries connect to vasa recta © 2012 Pearson Education, Inc. The Locations and Structures of Cortical and Juxtamedullary Nephrons Cortical nephron Juxtamedullary Cortex nephron Medulla Collecting duct Papillary duct Renal papilla Minor calyx The general appearance and location of © 2012 Pearson Education, Inc. nephrons in the kidneys The Locations and Structures of Cortical and Juxtamedullary Nephrons Efferent Peritubular arteriole capillaries Afferent arteriole Distal convoluted Renal tubule corpuscle Collecting duct Peritubular capillaries Nephron loop The circulation to a cortical nephron © 2012 Pearson Education, Inc. The Locations and Structures of Cortical and Juxtamedullary Nephrons Peritubular capillaries Proximal convoluted tubule (PCT) Glomerulus Distal convoluted tubule (DCT) Vasa recta Collecting duct Vasa recta Nephron loop The circulation to a © 2012 Pearson Education, Inc. juxtamedullary nephron  The Renal Corpuscle Each renal corpuscle is 150–250 µm in diameter Glomerular capsule Is connected to initial segment of renal tubule Forms outer wall of renal corpuscle Encapsulates glomerular capillaries Outer wall is lined by simple squamous capsular epithelium Continuous with visceral epithelium that covers glomerular capillaries Separated by capsular space © 2012 Pearson Education, Inc.  The Visceral Epithelium Consists of large cells (podocytes) With complex processes or “feet” (pedicels) that wrap around specialized dense layer of glomerular capillaries Filtration Slits Are narrow gaps between adjacent pedicels Materials passing out of blood at glomerulus Must be small enough to pass between filtration slits © 2012 Pearson Education, Inc. The Renal Corpuscle Glomerular capsule Capsular Glomerular Capsular Visceral space capillary epithelium epithelium (podocyte) Proximal Efferent convoluted arteriole tubule Distal convoluted tubule Macula densa Juxtaglomerular cells Juxtaglomerular complex Afferent arteriole Important structural features of a renal corpuscle © 2012 Pearson Education, Inc. The Renal Corpuscle Nucleus Podocyte Pores Mesangial cell Capillary endothelial cell Filtration Dense slits layer RBC Pedicels Capsular space Capsular epithelium This cross section through a segment of the glomerulus shows the components of the filtration membrane of the nephron. © 2012 Pearson Education, Inc.  The Glomerular Capillaries Are fenestrated capillaries Endothelium contains large-diameter pores Blood Flow Control Special supporting cells (mesangial cells) Between adjacent capillaries Control diameter and rate of capillary blood flow © 2012 Pearson Education, Inc.  The Filtration Membrane Consists of: Fenestrated endothelium Dense layer Filtration slits © 2012 Pearson Education, Inc.  37 LEARNING OBJECTIVES Describe the processes involved in the formation of urine.  The Goal of Urine Production Is to maintain homeostasis By regulating volume and composition of blood Including excretion of metabolic waste products © 2012 Pearson Education, Inc.  Three Organic Waste Products 1. Urea 2. Creatinine 3. Uric acid Organic Waste Products Are dissolved in bloodstream Are eliminated only while dissolved in urine Removal is accompanied by water loss © 2012 Pearson Education, Inc.  The Kidneys Usually produce concentrated urine 1200–1400 mOsm/L (four times plasma concentration) Kidney Functions To concentrate filtrate by glomerular filtration Failure leads to fatal dehydration Absorbs and retains valuable materials for use by other tissues Sugars and amino acids © 2012 Pearson Education, Inc.  Basic Processes of Urine Formation 1. Filtration 2. Reabsorption 3. Secretion © 2012 Pearson Education, Inc.  An Overview of Renal Function Water and solute reabsorption Primarily along proximal convoluted tubules Active secretion Primarily at proximal and distal convoluted tubules Long loops of juxtamedullary nephrons and collecting system Regulate final volume and solute concentration of urine © 2012 Pearson Education, Inc. An Overview of Urine Formation Proximal convoluted tubule Distal convoluted tubule Glomerulus Glomerular capsule Collecting duct KEY Nephron loop Filtration occurs exclusively in the renal corpuscle, across the filtration membrane. Water reabsorption occurs primarily along the PCT and the descending limb of the nephron loop, but also to a variable degree in the DCT and collecting system. Variable water reabsorption occurs in the DCT and collecting system. Solute reabsorption occurs along the PCT, the ascending limb of the nephron loop, the DCT, and the collecting system. Variable solute reabsorption or secretion occurs at Urine storage the PCT, the DCT, and the collecting system. and elimination © 2012 Pearson Education, Inc.  FILTRATION Occurs in renal corpuscle Hydrostatic pressure forces water through membrane pores Forces water and dissolved solutes out of glomerular capillaries into Bowman’s capscule (or capsular space) Small solute molecules pass through pores Larger solutes and suspended materials are retained Occurs across capillary walls Produces protein-free solution (filtrate) similar to blood plasm © 2012 Pearson Education, Inc.  The Process of Glomerular Filtration Involves passage across a filtration membrane Three components of membrane 1. Capillary endothelium 2. Dense layer 3. Filtration slits © 2012 Pearson Education, Inc.  Glomerular Capillaries Are fenestrated capillaries Have pores 60–100 nm diameter Prevent passage of blood cells Allow diffusion of solutes, including plasma proteins The Dense Layer Is more selective Allows diffusion of only: Small plasma proteins Nutrients Ions © 2012 Pearson Education, Inc.  The Filtration Slits Are the finest filters Have gaps only 6–9 nm wide Prevent passage of most small plasma proteins © 2012 Pearson Education, Inc. Glomerular Filtration Glomerulus Dense Efferent layer arteriole Capillary lumen Afferent arteriole Filtration Podocyte slit Pedicels Pore Capsular space Filtration membrane The glomerular filtration membrane © 2012 Pearson Education, Inc. REABSORPTION is the removal of water and solutes from the filtrate, and their movement across the tubular epithelium and into the peritubular fluid It involves: Diffusion Osmosis Channel-mediated diffusion Carrier-mediated transport © 2012 Pearson Education, Inc. SECRETION Secretion is the transport of solutes from the peritubular fluid, across the tubular epithelium, and into the tubular fluid. © 2012 Pearson Education, Inc.  Filtration Pressures Glomerular filtration is governed by the balance between: Hydrostatic pressure (fluid pressure) Colloid osmotic pressure (of materials in solution) on either side of capillary walls © 2012 Pearson Education, Inc.  Reabsorption and Secretion At the kidneys, it involves: Diffusion Osmosis Channel-mediated diffusion Carrier-mediated transport © 2012 Pearson Education, Inc.  53 LEARNING OBJECTIVES Describe how hydrostatic and colloid osmotic pressure influence glomerular filtration pressure Define glomerular filtration rate and discuss its regulation. Forces Governing Glomerular Filtration Hydrostatic Pressure Glomerular hydrostatic pressure is blood pressure in glomerular capillaries Tends to push water and solute molecules Out of plasma Into the filtrate Is significantly higher than capillary pressures in systemic circuit Due to arrangement of vessels at glomerulus © 2012 Pearson Education, Inc.  Glomerular Blood Vessels Blood leaving glomerular capillaries Flows into an efferent arteriole with a diameter smaller than afferent arteriole Efferent arteriole produces resistance Requires relatively high pressures to force blood into it © 2012 Pearson Education, Inc.  Capsular Hydrostatic Pressure (CsHP) Opposes glomerular hydrostatic pressure Pushes water and solutes Out of filtrate Into plasma Results from resistance to flow along nephron and conducting system Averages about 15 mm Hg © 2012 Pearson Education, Inc.  Net Hydrostatic Pressure (NHP) Is the difference between: Glomerular hydrostatic pressure and capsular hydrostatic pressure Colloid Osmotic Pressure Is the osmotic pressure resulting from the presence of suspended proteins Blood colloid osmotic pressure (BCOP) Tends to draw water out of filtrate and into plasma Opposes filtration and averages 25 mm Hg © 2012 Pearson Education, Inc.  Net Filtration Pressure (NFP) Is the average pressure forcing water and dissolved materials: Out of glomerular capillaries Into capsular spaces At the glomerulus is the difference between: Hydrostatic pressure and blood colloid osmotic pressure across glomerular capillaries © 2012 Pearson Education, Inc.  Glomerular Filtration Rate (GFR) Is the amount of filtrate kidneys produce each minute Averages 125 mL/min About 10% of fluid delivered to kidneys Leaves bloodstream Enters capsular spaces © 2012 Pearson Education, Inc.  Hormonal Regulation of the GFR By hormones of the: Renin–angiotensin system & Natriuretic peptides (ANP and BNP) The Renin–Angiotensin System Three stimuli cause the juxtaglomerular complex (JGC) to release renin 1. Decline in blood pressure at glomerulus due to decrease in blood volume, fall in systemic pressures, or blockage in renal artery or tributaries 2. Stimulation of juxtaglomerular cells by sympathetic innervation 3. Decline in osmotic concentration of tubular fluid at macula densa © 2012 Pearson Education, Inc.  The Renin–Angiotensin System: Angiotensin II Activation Constricts efferent arterioles of nephron Elevating glomerular pressures and filtration rates Stimulates reabsorption of sodium ions and water at PCT Stimulates secretion of aldosterone by adrenal cortex Stimulates thirst Triggers release of antidiuretic hormone (ADH) Stimulates reabsorption of water in distal portion of DCT and collecting system © 2012 Pearson Education, Inc.  The Renin–Angiotensin System: Angiotensin II Increases sympathetic motor tone Mobilizing the venous reserve Increasing cardiac output Stimulating peripheral vasoconstriction Causes brief, powerful vasoconstriction Of arterioles and precapillary sphincters Elevating arterial pressures throughout body The Renin–Angiotensin System Aldosterone Accelerates sodium reabsorption in DCT and cortical portion of collecting system © 2012 Pearson Education, Inc. The Response to a Reduction in the GFR Renin–Angiotensin System Integrated endocrine and neural mechanisms activated Renin in the bloodstream Endocrine triggers formation of response angiotensin I, which is then activated to angiotensin II Juxtaglomerular by angiotensin converting complex increases enzyme (ACE) in the production of renin. capillaries of the lungs. Angiotensin II triggers increased aldosterone Angiotensin II secretion by the triggers Angiotensin II constricts adrenal glands. peripheral arterioles and neural further constricts the responses. efferent arterioles. Aldosterone increases Na+ retention. HOMEOSTASIS RESTORED Increased Increased fluid stimulation of Increased Increased consumption Increased systemic thirst centers glomerular blood blood volume pressure pressure Increased fluid Increased ADH retention production HOMEOSTASIS Constriction of Normal venous reservoirs glomerular filtration rate Increased Increased cardiac output sympathetic motor tone Together, angiotensin II and sympathetic activation stimulate peripheral vasoconstriction. © 2012 Pearson Education, Inc.  Hormonal Regulation of the GFR Natriuretic Peptides Are released by the heart in response to stretching walls due to increased blood volume or pressure Atrial natriuretic peptide (ANP) is released by atria Brain natriuretic peptide (BNP) is released by ventricles Trigger dilation of afferent arterioles and constriction of efferent arterioles Elevate glomerular pressures and increase GFR © 2012 Pearson Education, Inc.  Autonomic Regulation of the GFR Mostly consists of sympathetic postganglionic fibers Sympathetic activation Constricts afferent arterioles Decreases GFR Slows filtrate production Changes in blood flow to kidneys due to sympathetic stimulation May be opposed by autoregulation at local level © 2012 Pearson Education, Inc.  66 LEARNING OBJECTIVES Discuss the hormonal influence on the volume and concentration of urine. Describe the characteristics of a normal urine sample.  Aldosterone Is a hormone produced by the adrenal cortex Controls ion pump and channels Stimulates synthesis and incorporation of Na+ pumps and channels In plasma membranes along DCT and collecting duct Reduces Na+ lost in urine © 2012 Pearson Education, Inc.  Natriuretic Peptides (ANP and BNP) Oppose secretion of aldosterone And its actions on DCT and collecting system Parathyroid Hormone and Calcitriol Circulating levels regulate calcium ion reabsorption at the DCT © 2012 Pearson Education, Inc.  ADH Hormone that causes special water channels (aquaporins) to appear in apical cell membranes Increases rate of osmotic water movement Higher levels of ADH increase: Number of water channels Water permeability of DCT and collecting system Without ADH Water is not reabsorbed All fluid reaching DCT is lost in urine Producing large amounts of dilute urine © 2012 Pearson Education, Inc.  The Composition of Normal Urine A urine sample depends on osmotic movement of water across walls of tubules and collecting ducts Is a clear, sterile solution Yellow color (pigment urobilin) Generated in kidneys from urobilinogens Urinalysis, the analysis of a urine sample, is an important diagnostic tool © 2012 Pearson Education, Inc. General Characteristics of Normal Urine © 2012 Pearson Education, Inc.  Seven Steps of Renal Function Step 1 Glomerulus Filtrate produced at renal corpuscle has the same composition as blood plasma (minus plasma proteins) Step 2 Proximal Convoluted Tubule (PCT) Active removal of ions and organic substrates Produces osmotic water flow out of tubular fluid Reduces volume of filtrate Keeps solutions inside and outside tubule isotonic Step 3 PCT and Descending Limb Water moves into peritubular fluids, leaving highly concentrated tubular fluid Reduction in volume occurs by obligatory water reabsorption © 2012 Pearson Education, Inc.  Step 4 Thick Ascending Limb Tubular cells actively transport Na+ and Cl– out of tubule Urea accounts for higher proportion of total osmotic concentration Step 5 DCT and Collecting Ducts Final adjustments in composition of tubular fluid Osmotic concentration is adjusted through active transport (reabsorption or secretion) Step 6 DCT and Collecting Ducts Final adjustments in volume and osmotic concentration of tubular fluid Exposure to ADH determines final urine concentration © 2012 Pearson Education, Inc.  Step 7 Vasa Recta Absorbs solutes and water reabsorbed by nephron loop and the ducts Maintains concentration gradient of medulla Urine Production Ends when fluid enters the renal pelvis © 2012 Pearson Education, Inc.  75 LEARNING OBJECTIVES Describe the structures and functions of the ureters, urinary bladder and urethra. Describe the micturition reflex. Discuss the voluntary and involuntary regulation of micturition.  Urine Transport, Storage, and Elimination Take place in the urinary tract Ureters Urinary bladder Urethra Structures Minor and major calyces, renal pelvis, ureters, urinary bladder, and proximal portion of urethra Are lined by transitional epithelium That undergoes cycles of distention and contraction © 2012 Pearson Education, Inc. A Pyelogram 11th and Minor Major 12th ribs calyx calyx Ureter Urinary Renal Kidney bladder pelvis © 2012 Pearson Education, Inc.  The Ureters Are a pair of muscular tubes Extend from kidneys to urinary bladder Begin at renal pelvis Are retroperitoneal, attached to posterior abdominal wall Penetrate posterior wall of the urinary bladder Pass through bladder wall at oblique angle Ureteral openings are slit-like rather than rounded Shape helps prevent backflow of urine when urinary bladder contracts © 2012 Pearson Education, Inc.  Peristaltic Contractions Begin at renal pelvis Sweep along ureter Force urine toward urinary bladder Every 30 seconds The Urinary Bladder Is a hollow, muscular organ Functions as temporary reservoir for urine storage Full bladder can contain 1 liter of urine © 2012 Pearson Education, Inc. Organs for the Conduction and Storage of Urine Median umbilical ligament Ureter Lateral umbilical ligament Detrusor muscle Rugae Ureteral Center of trigone openings Neck of Internal urethral urinary bladder sphincter Prostate gland External urethral Prostatic urethra sphincter (in urogenital diaphragm) Membranous urethra Urinary bladder in male © 2012 Pearson Education, Inc.  The Mucosa Lining the urinary bladder, has folds (rugae) that disappear as bladder fills The Trigone of the Urinary Bladder Is a triangular area bounded by: Openings of ureters Entrance to urethra Acts as a funnel Channels urine from bladder into urethra © 2012 Pearson Education, Inc.  The Urethra Extends from neck of urinary bladder To the exterior of the body The Female Urethra Is very short (3–5 cm; 1–2 in.) Extends from bladder to vestibule External urethral orifice is near anterior wall of vagina © 2012 Pearson Education, Inc.  The External Urethral Sphincter In both sexes Is a circular band of skeletal muscle Where urethra passes through urogenital diaphragm Acts as a valve Is under voluntary control Has resting muscle tone Voluntary relaxation permits micturition © 2012 Pearson Education, Inc.  The Micturition Reflex and Urination Begins when stretch receptors stimulate parasympathetic preganglionic motor neurons Volume >500 mL triggers micturition reflex As the bladder fills with urine: Stretch receptors in urinary bladder stimulate sensory fibers in pelvic nerve Stimulus travels from afferent fibers in pelvic nerves to sacral spinal cord Efferent fibers in pelvic nerves: Stimulate ganglionic neurons in wall of bladder © 2012 Pearson Education, Inc.  Postganglionic neuron stimulates detrusor muscle contraction Interneuron relays sensation to thalamus Projection fibers from thalamus deliver sensation to cerebral cortex Voluntary relaxation of external urethral sphincter causes relaxation of internal urethral sphincter © 2012 Pearson Education, Inc. The Micturition Reflex Parasympathetic preganglionic motor Afferent fibers in the fibers in pelvic nerves carry motor pelvic nerves carry the commands back to the urinary bladder. information to the sacral spinal cord. Postganglionic neurons in intramural The process begins with ganglia stimulate detrusor muscle Start Urinary contraction. This elevates hydrostatic distortion of stretch bladder receptors in the wall of pressure in the urinary bladder. the urinary bladder as urine volume increases. Voluntary relaxation of the external urethral sphincter causes relaxation of the internal urethral sphincter. Because the local pathway already has elevated pressures within the urinary bladder, relaxation of these sphincters leads to urination. Urination occurs © 2012 Pearson Education, Inc. The Micturition Reflex Brain Projection fibers relay information from the thalamus, delivering the sensation of urinary bladder fullness to the cerebral cortex. If convenient at the time, the individual then An interneuron then relays voluntarily relaxes the the sensation to the thalamus. external urethral sphincter. The afferent fibers stimulate two different neurons involved with: a local pathway, and a central pathway © 2012 Pearson Education, Inc. References Fundamentals of Human Anatomy and Physiology, by F.H. Martini. 6th Ed. Benjamin Cummings Publishing. Human Physiology, An Integrated Approach by D. Silverthorn. 5th Ed. Benjamin Cummings. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings