Chapter 28: Urinary System

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Chapter 28 Urinary System

• Dr. Kim Wilson

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Urinary System

• Kidneys
• Principal organs of the urinary system
• Accessory organs
• Ureters, urinary bladder, and urethra
• Function - Regulates the content of blood plasma
  to maintain dynamic constancy, or homeostasis,
  of the internal fluid environment within normal
  limits

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Anatomy of the Kidney

• Shape Roughly oval with a medial indentation
• Size Approximately 11 cm 7 cm 3 cm
• Location Left kidney often larger than the
  right right located a little lower
• Both kidneys located in a retroperitoneal
  position
• Lie on either side of the vertebral column
  between T12 and L3
• Superior poles of both kidneys extend above the
  level of the twelfth rib and the lower edge of
  the thoracic parietal pleura
• Renal fasciae anchor the kidneys to surrounding
  structures
• Renal fat pad heavy cushion of fat that
  surrounds each kidney
• Hilum concave notch on medial surface where
  vessels and tubes enter kidney

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Internal Structures of the Kidney

• Cortex and medulla
• Renal pyramids comprise much of the medullary
  tissue papilla is at the tip of each pyramid and
  releases urine through multiple ducts
• Renal columns where cortical tissue dips into
  the medulla between the pyramids
• Calyx cuplike structure at each renal papilla to
  collect urine minor calyces join to form major
  calyces, which in turn join to form the renal
  pelvis
• Renal pelvis narrows as it exits the kidney to
  become the ureter acts as a collection basin to
  drain urine from the kidney

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Internal Structures of the Kidney

• Blood vessels of the kidneys
• Kidneys are highly vascular
• Renal artery large branch of the abdominal
  aorta brings blood into each kidney
• Interlobular arteries between the pyramids of
  the medulla, the renal artery branches
• Interlobular arteries extend toward the cortex,
  arch over the bases of the pyramids, and form the
  arcuate arteries
• From the arcuate arteries, the interlobular
  arteries penetrate the cortex
• Afferent arterioles extend to the nephrons
  (microscopic functional units of kidney tissue)

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Ureter

• Tube running from each kidney to the urinary
  bladder
• Composed of three layers
• Mucous lining
• Muscular middle layer
• Fibrous outer layer

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Urinary Bladder

• Urinary bladder
• Structure collapsible bag located behind the
  pubic symphysis made mostly of smooth muscle
  tissue lining forms rugae can distend
  considerably
• Functions
• Reservoir for urine before it leaves the body
• Aided by the urethra, it expels urine from the
  body

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Urethra

• Structure small mucous membranelined tube
  extending from the trigone to the exterior of the
  body
• Females lies posterior to the pubic symphysis
  and anterior to the vagina approximately 3 cm
  long
• Males after leaving the bladder, passes through
  the prostate gland where it is joined by two
  ejaculatory ducts
• From the prostate, it extends to the base of the
  penis, then through the center of the penis,
  ending as the urinary meatus
• Approximately 20 cm long part of the urinary
  system as well as the reproductive system

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Urination (Micturition)

• Mechanism for voiding bladder
• As bladder volume increases, micturition
  contractions (of detrusor muscle) increase and
  the internal urethral sphincter relaxes
• External urethral sphincter muscle contracts at
  first, then at appropriate time relaxes to
  release urine

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Microscopic Anatomy - Nephrons

• Comprise the bulk of the kidney
• Each nephron is made of two regions
• Renal corpuscle made of the glomerulus tucked
  inside a Bowman capsule (cup-shaped mouth of the
  nephron)
• Located within the cortex of the kidney
• Renal tubule
• Connects to a shared collecting duct

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Microscopic Anatomy - Nephrons

• Glomerulus network of fine capillaries
  surrounded by Bowman capsule
• Fenestrations pores in capillary walls that
  permit filtration
• Mesangial cells located between glomerular
  capillaries various structural and functional
  support functions
• Basement membrane lies between the glomerulus and
  Bowman capsule
• Glomerular capsular membrane formed by
  glomerular endothelium, basement membrane, and
  the visceral layer of Bowman capsule function is
  filtration

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Microscopic Anatomy - Nephrons

• Renal tubule
• Proximal convoluted tubule first part of the
  renal tubule nearest to Bowman capsule follows a
  winding, convoluted course
• Also known as the proximal tubule
• Loop of Henle (nephron loop)
• Renal tubule segment just beyond the proximal
  tubule
• Consists of a thin descending limb, a sharp turn,
  and an ascending limb ascending limb made of
  thin ascending limb followed by thick ascending
  limb

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Microscopic Anatomy - Nephrons

• Distal convoluted tubule convoluted tubule
  beyond the Henle loop
• Also known as the distal tubule
• Juxtaglomerular apparatus located where the
  afferent arteriole brushes past the distal
  convoluted tubule
• Made of macula densa (wall of distal tubule) and
  juxtaglomerular) cells surrounding afferent
  arteriole
• Important to maintenance of blood flow
  homeostasis by reflexively secreting renin when
  blood pressure in the afferent arteriole drops
• Along with other distal tubules, it joins a
  common collecting duct

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Microscopic Anatomy - Nephrons

• Collecting duct
• Straight duct joined by the renal tubules of
  several nephrons
• Collecting ducts of one renal pyramid converge to
  form one tube that opens at a renal papilla into
  a minor calyx

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Blood Supply to Nephrons

• Blood supply of the nephron
• Afferent arteriole enters glomerular capillary
  network
• Efferent arteriole leaves glomerulus and extends
  to the peritubular blood supply
• Vasae rectae straight arterioles that run
  alongside Henle loop
• Peritubular capillaries surround renal tubule

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Types of Nephrons

• Juxtamedullary nephron a nephron with a renal
  corpuscle near the medulla and a Henle loop that
  dips far into the medulla
• Cortical nephron a nephron with a Henle loop
  that does not dip into the medulla but remains
  almost entirely within the cortex
• Constitute approximately 85 of the total
  nephrons

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Kidney Function

• Chief function of the kidneys are to process
  blood and form urine
• Basic functional unit of the kidney is the
  nephron
• Forms urine by three processes
• Filtration movement of water and protein-free
  solutes from plasma in the glomerulus into the
  capsular space of Bowman capsule
• Tubular reabsorption movement of molecules out
  of the tubule and into peritubular blood
• Tubular secretion movement of molecules out of
  peritubular blood and into the tubule for
  excretion

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Filtration

• First step in blood processing
• Where? Occurs in renal corpuscles
• From blood in the glomerular capillaries,
  approximately 180 L of water and solutes filter
  into Bowman capsule each day takes place through
  the glomerular capsular membrane
• How? Filtration occurs as a result of a pressure
  gradient
• Glomerular capillary filtration occurs rapidly
  because of the increased number of fenestrations
• Glomerular filtration rate (GFR) determined
  mainly by glomerular hydrostatic pressure and
  therefore directly related to systemic blood
  pressure

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Reabsorption

• Second step in urine formation
• How? Occurs as a result of passive and active
  transport mechanisms from all parts of the renal
  tubules
• Where? major portion of reabsorption occurs in
  the proximal convoluted tubules
• Most water and solutes are recovered by the
  blood, leaving only a small volume of tubule
  fluid left to move on to the Henle loop
• Mechanisms of tubular reabsorption
• Sodium actively transported out of tubule fluid
  and into blood
• Glucose and amino acids passively transported
  out of tubule fluid by sodium cotransport
  mechanisms transport maximum is the maximal
  capacity of reabsorption and depends on carrier
  availability
• Chloride, phosphate, and bicarbonate ions
  passively move into blood because of an imbalance
  in electrical charge
• Water movement of sodium and chloride into blood
  causes an osmotic imbalance, moving water
  passively into blood
• Urea approximately half of urea passively moves
  out of the tubule, with the remaining urea moving
  on to the Henle loop

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Reabsorption

• Reabsorption in the Henle loop
• Two countercurrent mechanisms
• Countercurrent multiplier mechanism in Henle loop
  concentrates sodium and chloride in the
  interstitial fluid of renal medulla (Figure
  28-23)
• Countercurrent exchange mechanism in vasae rectae
  maintains high solute concentration in medullary
  interstitial fluid (Figure 28-24)
• Water is reabsorbed from the tubule fluid, and
  urea is picked up from the interstitial fluid in
  the descending limb
• Sodium and chloride are reabsorbed from the
  filtrate in the ascending limb, where the
  reabsorption of salt makes the tubule fluid
  dilute and creates and maintains a high osmotic
  pressure of the medullas interstitial fluid

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Reabsorption

• Distal tubules and collecting ducts
• The distal convoluted tubule reabsorbs sodium by
  active transport but in smaller amounts than in
  the proximal convoluted tubule
• Antidiuretic hormone (ADH) is secreted by the
  posterior pituitary and targets the cells of
  distal tubules and collecting ducts to make them
  more permeable to water
• With reabsorption of water in the collecting
  duct, urea concentration of the tubule fluid
  increases, which causes urea to diffuse out of
  the collecting duct into the medullary
  interstitial fluid
• Urea participates in a countercurrent multiplier
  mechanism that, along with the countercurrent
  mechanisms of the Henle loop and vasae rectae,
  maintains the high osmotic pressure needed to
  form concentrated urine and avoid dehydration

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Tubular Secretion

• Def The movement of substances out of the blood
  and into tubular fluid
• Descending limb of the Henle loop secretes urea
  by diffusion
• Distal tubule and collecting ducts secrete
  potassium, hydrogen, and ammonium ions
• Aldosterone hormone that targets the cells of
  the distal tubule and collecting duct cells
  causes increased activity of the sodium-potassium
  pump
• Secretion of hydrogen ions increases with
  increased blood hydrogen ion concentration

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Regulation of Urine Volume

• ADH influences water reabsorption
• As water is reabsorbed, the total volume of urine
  is reduced by the amount of water removed by the
  tubules ADH reduces water loss
• Aldosterone, secreted by the adrenal cortex,
  increases distal tubule absorption of sodium,
  thereby raising the sodium concentration of blood
  and thus promoting reabsorption of water
• Atrial natriuretic hormone, secreted by atrial
  muscle fibers, promotes loss of sodium by urine
  opposes aldosterone, thus causing the kidneys to
  reabsorb less water and thereby produce more
  urine

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Composition of Urine

• Approximately 95 water with several substances
  dissolved in it
• Nitrogenous wastes result of protein metabolism
  include urea, uric acid, ammonia, and creatinine
• Electrolytes mainly the following ions sodium,
  potassium, ammonium, chloride, bicarbonate,
  phosphate, and sulfate amounts and kinds of
  minerals vary with diet and other factors
• Toxins during disease, bacterial poisons leave
  the body in urine
• Pigments (urochromes)
• Hormones high hormone levels may spill into the
  filtrate
• Abnormal constituents (e.g., blood, glucose,
  albumin, casts, calculi)