The Urinary System Part A - PDF

The Urinary System Part A 25 Chapter 25: Urinary System 1 Kidney Functions  Filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine  Regulate volume and chemical makeup of the blood ...

The Urinary System Part A 25 Chapter 25: Urinary System 1 Kidney Functions  Filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine  Regulate volume and chemical makeup of the blood  Maintain the proper balance between water and salts, and acids and bases Chapter 25: Urinary System 2 Other Renal Functions  Gluconeogenesis during prolonged fasting  Production of rennin to help regulate blood pressure and erythropoietin to stimulate RBC production  Activation of vitamin D Chapter 25: Urinary System 3 Other Urinary System Organs  Urinary bladder – provides a temporary storage reservoir for urine  Paired ureters – transport urine from the kidneys to the bladder  Urethra – transports urine from the bladder out of the body Chapter 25: Urinary System 4 Urinary System Organs Figure 5 25.1a Chapter 25: Urinary System Kidney Location and External Anatomy  The bean-shaped kidneys lie in a retroperitoneal position in the superior lumbar region and extend from the twelfth thoracic to the third lumbar vertebrae  The right kidney is lower than the left because it is crowded by the liver  The lateral surface is convex and the medial surface is concave, with a vertical cleft called the renal hilus leading to the renal sinus  Ureters, renal blood vessels, lymphatics, and nerves enter and exit at the hilus Chapter 25: Urinary System 6 Layers of Tissue Supporting the Kidney  Renal capsule – fibrous capsule that prevents kidney infection  Adipose capsule – fatty mass that cushions the kidney and helps attach it to the body wall  Renal fascia – outer layer of dense fibrous connective tissue that anchors the kidney Chapter 25: Urinary System 7 Kidney Location and External Anatomy Chapter 25: Urinary System 8 Figure 25.2a Internal Anatomy  A frontal section shows three distinct regions  Cortex – the light colored, granular superficial region  Medulla – exhibits cone-shaped medullary (renal) pyramids  Pyramids are made up of parallel bundles of urine-collecting tubules  Renal columns are inward extensions of cortical tissue that separate the pyramids  The medullary pyramid and its surrounding capsule constitute a lobe Chapter 25: Urinary System 9 Internal Anatomy  Renal pelvis – flat, funnel-shaped tube lateral to the hilus within the renal sinus Chapter 25: Urinary System 10 Internal Anatomy  Major calyces – large branches of the renal pelvis  Collect urine draining from papillae  Empty urine into the pelvis  Urine flows through the pelvis and ureters to the bladder Chapter 25: Urinary System 11 Internal Anatomy Figure 12 25.3b Chapter 25: Urinary System Blood and Nerve Supply  Approximately one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute  Arterial flow into and venous flow out of the kidneys follow similar paths  The nerve supply is via the renal plexus Chapter 25: Urinary System 13 Figure 25.3c The Nephron  Nephrons are the structural and functional units that form urine, consisting of:  Glomerulus – a tuft of capillaries associated with a renal tubule  Glomerular (Bowman’s) capsule – blind, cup- shaped end of a renal tubule that completely surrounds the glomerulus Chapter 25: Urinary System 14 The Nephron  Renal corpuscle – the glomerulus and its Bowman’s capsule  Glomerular endothelium – fenestrated epithelium that allows solute-rich, virtually protein-free filtrate to pass from the blood into the glomerular capsule Chapter 25: Urinary System 15 The Nephron Figure 16 25.4b Chapter 25: Urinary System Anatomy of the Glomerular Capsule  The external parietal layer is a structural layer  The visceral layer consists of modified, branching epithelial podocytes  Extensions of the octopus-like podocytes terminate in foot processes  Filtration slits – openings between the foot processes that allow filtrate to pass into the capsular space Chapter 25: Urinary System 17 Renal Tubule  Proximal convoluted tubule (PCT) – composed of cuboidal cells with numerous microvilli and mitochondria  Reabsorbs water and solutes from filtrate and secretes substances into it Chapter 25: Urinary System 18 Renal Tubule  Loop of Henle – a hairpin-shaped loop of the renal tubule  Proximal part is similar to the proximal convoluted tubule  Proximal part is followed by the thin segment (simple squamous cells) and the thick segment (cuboidal to columnar cells)  Distal convoluted tubule (DCT) – cuboidal cells without microvilli that function more in secretion than reabsorption Chapter 25: Urinary System 19 Renal Tubule Figure 20 25.4b Chapter 25: Urinary System Connecting Tubules  The distal portion of the distal convoluted tubule nearer to the collecting ducts Chapter 25: Urinary System 21 Connecting Tubules  Two important cell types are found here  Intercalated cells  Cuboidal cells with microvilli  Function in maintaining the acid-base balance of the body  Principal cells  Cuboidal cells without microvilli  Help maintain the body’s water and salt balance Chapter 25: Urinary System 22 Nephrons  Cortical nephrons – 85% of nephrons; located in the cortex  Juxtamedullary nephrons:  Are located at the cortex-medulla junction  Have loops of Henle that deeply invade the medulla  Have extensive thin segments  Are involved in the production of concentrated urine Chapter 25: Urinary System 23 Nephrons Chapter 25: Urinary System 24 Figure 25.5b Capillary Beds of the Nephron  Every nephron has two capillary beds  Glomerulus  Peritubular capillaries  Each glomerulus is:  Fed by an afferent arteriole  Drained by an efferent arteriole Chapter 25: Urinary System 25 Capillary Beds of the Nephron  Blood pressure in the glomerulus is high because:  Arterioles are high-resistance vessels  Afferent arterioles have larger diameters than efferent arterioles  Fluids and solutes are forced out of the blood throughout the entire length of the glomerulus Chapter 25: Urinary System 26 Capillary Beds  Peritubular beds are low-pressure, porous capillaries adapted for absorption that:  Arise from efferent arterioles  Cling to adjacent renal tubules  Empty into the renal venous system  Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons Chapter 25: Urinary System 27 Capillary Beds Chapter 25: Urinary System 28 Figure 25.5a Vascular Resistance in Microcirculation  Afferent and efferent arterioles offer high resistance to blood flow  Blood pressure declines from 95mm Hg in renal arteries to 8 mm Hg in renal veins Chapter 25: Urinary System 29 Vascular Resistance in Microcirculation  Resistance in afferent arterioles:  Protects glomeruli from fluctuations in systemic blood pressure  Resistance in efferent arterioles:  Reinforces high glomerular pressure  Reduces hydrostatic pressure in peritubular capillaries Chapter 25: Urinary System 30 Juxtaglomerular Apparatus (JGA)  Where the distal tubule lies against the afferent (sometimes efferent) arteriole  Arteriole walls have juxtaglomerular (JG) cells  Enlarged, smooth muscle cells  Have secretory granules containing renin  Act as mechanoreceptors Chapter 25: Urinary System 31 Juxtaglomerular Apparatus (JGA)  Macula densa  Tall, closely packed distal tubule cells  Lie adjacent to JG cells  Function as chemoreceptors or osmoreceptors  Mesanglial cells:  Have phagocytic and contractile properties  Influence capillary filtration PLAY InterActive Physiology®: Urinary System: Anatomy Review Chapter 25: Urinary System 32 Juxtaglomerular Apparatus (JGA) Chapter 25: Urinary System 33 Figure 25.6 Filtration Membrane  Filter that lies between the blood and the interior of the glomerular capsule  It is composed of three layers  Fenestrated endothelium of the glomerular capillaries  Visceral membrane of the glomerular capsule (podocytes)  Basement membrane composed of fused basal laminae of the other layers Chapter 25: Urinary System 34 Filtration Membrane Figure 35 25.7a Chapter 25: Urinary System Filtration Membrane Figure 36 25.7c Chapter 25: Urinary System Mechanisms of Urine Formation  The kidneys filter the body’s entire plasma volume 60 times each day  The filtrate:  Contains all plasma components except protein  Loses water, nutrients, and essential ions to become urine  The urine contains metabolic wastes and unneeded substances Chapter 25: Urinary System 37 Mechanisms of Urine Formation  Urine formation and adjustment of blood composition involves three major processes  Glomerular filtration  Tubular reabsorption  Secretion Chapter 25: Urinary System 38 Figure 25.8 Glomerular Filtration  Principles of fluid dynamics that account for tissue fluid in all capillary beds apply to the glomerulus as well  The glomerulus is more efficient than other capillary beds because:  Its filtration membrane is significantly more permeable  Glomerular blood pressure is higher  It has a higher net filtration pressure  Plasma proteins are not filtered and are used to maintain oncotic pressure of the blood Chapter 25: Urinary System 39 Net Filtration Pressure (NFP)  The pressure responsible for filtrate formation  NFP equals the glomerular hydrostatic pressure (HPg) minus the oncotic pressure of glomerular blood (OPg) combined with the capsular hydrostatic pressure (HPc) NFP = HPg – (OPg + HPc) Chapter 25: Urinary System 40 Glomerular Filtration Rate (GFR)  The total amount of filtrate formed per minute by the kidneys  Factors governing filtration rate at the capillary bed are:  Total surface area available for filtration  Filtration membrane permeability  Net filtration pressure Chapter 25: Urinary System 41 Glomerular Filtration Rate (GFR)  GFR is directly proportional to the NFP  Changes in GFR normally result from changes in glomerular blood pressure Chapter 25: Urinary System 42 Glomerular Filtration Rate (GFR) Figure 43 25.9 Chapter 25: Urinary System Regulation of Glomerular Filtration  If the GFR is too high:  Needed substances cannot be reabsorbed quickly enough and are lost in the urine  If the GFR is too low:  Everything is reabsorbed, including wastes that are normally disposed of Chapter 25: Urinary System 44 Regulation of Glomerular Filtration  Three mechanisms control the GFR  Renal autoregulation (intrinsic system)  Neural controls  Hormonal mechanism (the renin-angiotensin system) Chapter 25: Urinary System 45 Intrinsic Controls  Under normal conditions, renal autoregulation maintains a nearly constant glomerular filtration rate  Autoregulation entails two types of control  Myogenic – responds to changes in pressure in the renal blood vessels  Flow-dependent tubuloglomerular feedback – senses changes in the juxtaglomerular apparatus Chapter 25: Urinary System 46 Extrinsic Controls  When the sympathetic nervous system is at rest:  Renal blood vessels are maximally dilated  Autoregulation mechanisms prevail Chapter 25: Urinary System 47 Extrinsic Controls  Under stress:  Norepinephrine is released by the sympathetic nervous system  Epinephrine is released by the adrenal medulla  Afferent arterioles constrict and filtration is inhibited  The sympathetic nervous system also stimulates the renin-angiotensin mechanism InterActive Physiology®: PLAY Urinary System: Glomerular Filtration Chapter 25: Urinary System 48 Renin-Angiotensin Mechanism  Is triggered when the JG cells release renin  Renin acts on angiotensinogen to release angiotensin I  Angiotensin I is converted to angiotensin II  Angiotensin II:  Causes mean arterial pressure to rise  Stimulates the adrenal cortex to release aldosterone  As a result, both systemic and glomerular hydrostatic pressure rise Chapter 25: Urinary System 49 Renin Release  Renin release is triggered by:  Reduced stretch of the granular JG cells  Stimulation of the JG cells by activated macula densa cells  Direct stimulation of the JG cells via 1-adrenergic receptors by renal nerves  Angiotensin II Chapter 25: Urinary System 50 Renin Release Chapter 25: Urinary System 51 Figure 25.10 The Urinary System Part B 25 Chapter 25: Urinary System 52 Other Factors Affecting Glomerular Filtration  Prostaglandins (PGE2 and PGI2)  Vasodilators produced in response to sympathetic stimulation and angiotensin II  Are thought to prevent renal damage when peripheral resistance is increased  Nitric oxide – vasodilator produced by the vascular endothelium  Adenosine – vasoconstrictor of renal vasculature  Endothelin – a powerful vasoconstrictor secreted by tubule cells Chapter 25: Urinary System 53 Tubular Reabsorption  A transepithelial process whereby most tubule contents are returned to the blood  Transported substances move through three membranes  Luminal and basolateral membranes of tubule cells  Endothelium of peritubular capillaries  Only Ca2+, Mg2+, K+, and some Na+ are reabsorbed via paracellular pathways Chapter 25: Urinary System 54 Tubular Reabsorption  All organic nutrients are reabsorbed  Water and ion reabsorption is hormonally controlled  Reabsorption may be an active (requiring ATP) or passive process Chapter 25: Urinary System 55 Sodium Reabsorption: Primary Active Transport  Sodium reabsorption is almost always by active transport  Na+ enters the tubule cells at the luminal membrane  Is actively transported out of the tubules by a Na+-K+ ATPase pump Chapter 25: Urinary System 56 Sodium Reabsorption: Primary Active Transport  From there it moves to peritubular capillaries due to:  Low hydrostatic pressure  High osmotic pressure of the blood  Na+ reabsorption provides the energy and the means for reabsorbing most other solutes Chapter 25: Urinary System 57 Routes of Water and Solute Reabsorption Figure 58 25.11 Chapter 25: Urinary System Reabsorption by PCT Cells  Active pumping of Na+ drives reabsorption of:  Water by osmosis, aided by water-filled pores called aquaporins  Cations and fat-soluble substances by diffusion  Organic nutrients and selected cations by secondary active transport Chapter 25: Urinary System 59 Reabsorption by PCT Cells Figure 60 25.12 Chapter 25: Urinary System Nonreabsorbed Substances  A transport maximum (Tm):  Reflects the number of carriers in the renal tubules available  Exists for nearly every substance that is actively reabsorbed  When the carriers are saturated, excess of that substance is excreted Chapter 25: Urinary System 61 Nonreabsorbed Substances  Substances are not reabsorbed if they:  Lack carriers  Are not lipid soluble  Are too large to pass through membrane pores  Urea, creatinine, and uric acid are the most important nonreabsorbed substances Chapter 25: Urinary System 62 Absorptive Capabilities of Renal Tubules and Collecting Ducts  Substances reabsorbed in PCT include:  Sodium, all nutrients, cations, anions, and water  Urea and lipid-soluble solutes  Small proteins  Loop of Henle reabsorbs:  H2O, Na+, Cl, K+ in the descending limb  Ca2+, Mg2+, and Na+ in the ascending limb Chapter 25: Urinary System 63 Absorptive Capabilities of Renal Tubules and Collecting Ducts  DCT absorbs:  Ca2+, Na+, H+, K+, and water  HCO3 and Cl  Collecting duct absorbs:  Water and urea Chapter 25: Urinary System 64 Na+ Entry into Tubule Cells  Passive entry: Na+-K+ ATPase pump  In the PCT: facilitated diffusion using symport and antiport carriers  In the ascending loop of Henle: facilitated diffusion via Na+-K+-2Cl symport system  In the DCT: Na+-Cl– symporter  In collecting tubules: diffusion through membrane pores Chapter 25: Urinary System 65 Atrial Natriuretic Peptide Activity  ANP reduces blood Na+ which:  Decreases blood volume  Lowers blood pressure  ANP lowers blood Na+ by:  Acting directly on medullary ducts to inhibit Na+ reabsorption  Counteracting the effects of angiotensin II  Indirectly stimulating an increase in GFR reducing water reabsorption Chapter 25: Urinary System 66 Tubular Secretion  Essentially reabsorption in reverse, where substances move from peritubular capillaries or tubule cells into filtrate  Tubular secretion is important for:  Disposing of substances not already in the filtrate  Eliminating undesirable substances such as urea and uric acid  Ridding the body of excess potassium ions  Controlling blood pH Chapter 25: Urinary System 67 Regulation of Urine Concentration and Volume  Osmolality  The number of solute particles dissolved in 1L of water  Reflects the solution’s ability to cause osmosis  Body fluids are measured in milliosmols (mOsm)  The kidneys keep the solute load of body fluids constant at about 300 mOsm  This is accomplished by the countercurrent mechanism Chapter 25: Urinary System 68 Countercurrent Mechanism  Interaction between the flow of filtrate through the loop of Henle (countercurrent multiplier) and the flow of blood through the vasa recta blood vessels (countercurrent exchanger)  The solute concentration in the loop of Henle ranges from 300 mOsm to 1200 mOsm  Dissipation of the medullary osmotic gradient is prevented because the blood in the vasa recta equilibrates with the interstitial fluid Chapter 25: Urinary System 69 Osmotic Gradient in the Renal Medulla Figure 70 25.13 Chapter 25: Urinary System Loop of Henle: Countercurrent Multiplier  The descending loop of Henle:  Is relatively impermeable to solutes  Is permeable to water  The ascending loop of Henle:  Is permeable to solutes  Is impermeable to water  Collecting ducts in the deep medullary regions are permeable to urea Chapter 25: Urinary System 71 Loop of Henle: Countercurrent Exchanger  The vasa recta is a countercurrent exchanger that:  Maintains the osmotic gradient  Delivers blood to the cells in the area PLAY InterActive Physiology®: Urinary System: Early Filtrate Processing Chapter 25: Urinary System 72 Loop of Henle: Countercurrent Mechanism Chapter 25: Urinary System 73 Figure 25.14 Formation of Dilute Urine  Filtrate is diluted in the ascending loop of Henle  Dilute urine is created by allowing this filtrate to continue into the renal pelvis  This will happen as long as antidiuretic hormone (ADH) is not being secreted Chapter 25: Urinary System 74 Formation of Dilute Urine  Collecting ducts remain impermeable to water; no further water reabsorption occurs  Sodium and selected ions can be removed by active and passive mechanisms  Urine osmolality can be as low as 50 mOsm (one- sixth that of plasma) Chapter 25: Urinary System 75 Formation of Concentrated Urine  Antidiuretic hormone (ADH) inhibits diuresis  This equalizes the osmolality of the filtrate and the interstitial fluid  In the presence of ADH, 99% of the water in filtrate is reabsorbed Chapter 25: Urinary System 76 Formation of Concentrated Urine  ADH-dependent water reabsorption is called facultative water reabsorption  ADH is the signal to produce concentrated urine  The kidneys’ ability to respond depends upon the high medullary osmotic gradient InterActive Physiology®: PLAY Urinary System: Late Filtrate Processing Chapter 25: Urinary System 77 Formation of Dilute and Concentrated Urine Figure 78 25.15a, b Chapter 25: Urinary System Diuretics  Chemicals that enhance the urinary output include:  Any substance not reabsorbed  Substances that exceed the ability of the renal tubules to reabsorb it  Substances that inhibit Na+ reabsorption Chapter 25: Urinary System 79 Diuretics  Osmotic diuretics include:  High glucose levels – carries water out with the glucose  Alcohol – inhibits the release of ADH  Caffeine and most diuretic drugs – inhibit sodium ion reabsorption  Lasix and Diuril – inhibit Na+-associated symporters Chapter 25: Urinary System 80 Renal Clearance  The volume of plasma that is cleared of a particular substance in a given time  Renal clearance tests are used to:  Determine the GFR  Detect glomerular damage  Follow the progress of diagnosed renal disease Chapter 25: Urinary System 81 Renal Clearance RC = UV/P RC = renal clearance rate U = concentration (mg/ml) of the substance in urine V = flow rate of urine formation (ml/min) P = concentration of the same substance in plasma Chapter 25: Urinary System 82 Physical Characteristics of Urine  Color and transparency  Clear, pale to deep yellow (due to urochrome)  Concentrated urine has a deeper yellow color  Drugs, vitamin supplements, and diet can change the color of urine  Cloudy urine may indicate infection of the urinary tract Chapter 25: Urinary System 83 Physical Characteristics of Urine  Odor  Fresh urine is slightly aromatic  Standing urine develops an ammonia odor  Some drugs and vegetables (asparagus) alter the usual odor Chapter 25: Urinary System 84 Physical Characteristics of Urine  pH  Slightly acidic (pH 6) with a range of 4.5 to 8.0  Diet can alter pH  Specific gravity  Ranges from 1.001 to 1.035  Is dependent on solute concentration Chapter 25: Urinary System 85 Chemical Composition of Urine  Urine is 95% water and 5% solutes  Nitrogenous wastes include urea, uric acid, and creatinine  Other normal solutes include:  Sodium, potassium, phosphate, and sulfate ions  Calcium, magnesium, and bicarbonate ions  Abnormally high concentrations of any urinary constituents may indicate pathology Chapter 25: Urinary System 86 Ureters  Slender tubes that convey urine from the kidneys to the bladder  Ureters enter the base of the bladder through the posterior wall  This closes their distal ends as bladder pressure increases and prevents backflow of urine into the ureters Chapter 25: Urinary System 87 Ureters  Ureters have a trilayered wall  Transitional epithelial mucosa  Smooth muscle muscularis  Fibrous connective tissue adventitia  Ureters actively propel urine to the bladder via response to smooth muscle stretch Chapter 25: Urinary System 88 Urinary Bladder  Smooth, collapsible, muscular sac that temporarily stores urine  It lies retroperitoneally on the pelvic floor posterior to the pubic symphysis  Males – prostate gland surrounds the neck inferiorly  Females – anterior to the vagina and uterus  Trigone – triangular area outlined by the openings for the ureters and the urethra  Clinically important because infections tend to persist in this region Chapter 25: Urinary System 89 Urinary Bladder  The bladder wall has three layers  Transitional epithelial mucosa  A thick muscular layer  A fibrous adventitia  The bladder is distensible and collapses when empty  As urine accumulates, the bladder expands without significant rise in internal pressure Chapter 25: Urinary System 90 Urinary Bladder Figure 91 25.18a, b Chapter 25: Urinary System Urethra  Muscular tube that:  Drains urine from the bladder  Conveys it out of the body Chapter 25: Urinary System 92 Urethra  Sphincters keep the urethra closed when urine is not being passed  Internal urethral sphincter – involuntary sphincter at the bladder-urethra junction  External urethral sphincter – voluntary sphincter surrounding the urethra as it passes through the urogenital diaphragm  Levator ani muscle – voluntary urethral sphincter Chapter 25: Urinary System 93 Urethra  The female urethra is tightly bound to the anterior vaginal wall  Its external opening lies anterior to the vaginal opening and posterior to the clitoris  The male urethra has three named regions  Prostatic urethra – runs within the prostate gland  Membranous urethra – runs through the urogenital diaphragm  Spongy (penile) urethra – passes through the penis and opens via the external urethral orifice Chapter 25: Urinary System 94 Urethra Figure 95 25.18a. b Chapter 25: Urinary System Micturition (Voiding or Urination)  The act of emptying the bladder  Distension of bladder walls initiates spinal reflexes that:  Stimulate contraction of the external urethral sphincter  Inhibit the detrusor muscle and internal sphincter (temporarily)  Voiding reflexes:  Stimulate the detrusor muscle to contract  Inhibit the internal and external sphincters Chapter 25: Urinary System 96 Micturition (Voiding or Urination) Chapter 25: Urinary System 97 Figure 25.20a, b Developmental Aspects  Three sets of embryonic kidneys develop, with only the last set persisting  The pronephros never functions but its pronephric duct persists and connects to the cloaca  The mesonephros claims this duct and it becomes the mesonephric duct  The final metanephros develop by the fifth week and develop into adult kidneys Chapter 25: Urinary System 98 Developmental Aspects Figure 99 25.21a, b Chapter 25: Urinary System Developmental Aspects Figure100 25.21c, d Chapter 25: Urinary System Developmental Aspects  Metanephros develop as ureteric buds that incline mesoderm to form nephrons  Distal ends of ureteric tubes form the renal pelves, calyces, and collecting ducts  Proximal ends called ureteric ducts become the ureters  Metanephric kidneys are excreting urine by the third month  The cloaca eventually develops into the rectum and anal canal Chapter 25: Urinary System 101 Developmental Aspects  Infants have small bladders and the kidneys cannot concentrate urine, resulting in frequent micturition  Control of the voluntary urethral sphincter develops with the nervous system  E. coli bacteria account for 80% of all urinary tract infections  Sexually transmitted diseases can also inflame the urinary tract  Kidney function declines with age, with many elderly becoming incontinent Chapter 25: Urinary System 102