The Urinary System

1
The Urinary System

• Chapter 23

2
Introduction

• Kidneys - maintain purity and chemical constancy
  of blood and other extracellular body fluids
• - filters liters of fluid sending toxins,
  metabolic wastes, excess water, and excess ions
  out in urine
• - while returning needed substances from the
  filtrate to the blood
• Main waste products are 3 nitrogenous compounds
• 1) urea derived from breakdown of amino acids
• 2) uric acid results from the turnover of
  nucleic acids
• 3) creatinine formed by the breakdown of
  creatine phosphate, molecule in muscle that
  stores energy for manufacture of ATP

3

• Kidneys also regulate the volume and chemical
  makeup of the blood
• - maintains the proper balance of water and
  salts and of acids and bases
• Other organs of the urinary system include
• - the paired ureters (pertaining to urine),
  tubes that carry urine from the kidney to the
  bladder
• - urinary bladder, temporary storage sac for
  urine
• - urethra, a tube that carries urine from the
  bladder to the body exterior

4
Organs of the Urinary System

• Kidneys
• Ureters
• Urinary bladder
• Urethra

Fig 23.1a
5
Location and External Anatomy of Kidneys

• Bean-shaped kidneys lie retroperitoneal - lateral
  to T11,12 - L3 vertebrae
• - right kidney crowded by the liver lies
  slightly inferior to the left kidney
• - on superior part of each kidney is the
  suprarenal gland
• Average kidney 12cm tall, 6cm wide, 3cm thick
• - lateral surface, is convex medial surface is
  concave
• - with a vertical cleft, the renal hilum where
  vessels, ureters, and nerves enter and leave the
  kidneys

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• Several layers of supportive tissue surround each
  kidney
• Fibrous (renal) capsule surrounds the kidney
  surface
• - maintains its shape and forms a barrier that
  can inhibit the spread of infection from
  surrounding regions
• Perirenal and pararenal fat layers - cushion and
  help hold the kidneys in place
• - perirenal fat capsule lies external to the
  renal capsule
• - external to that is an envelope of renal
  fascia
• - pararenal fat lies external and mostly
  posterior to the renal fascia

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Relationship of the Kidneys to Vertebra and Ribs
Figure 23.1b
8
Kidneys within the Posterior Abdominal Wall
Figure 23.2a
9
Internal Gross Anatomy

• Frontal section through the kidney reveals 2
  distinct regions
• - superficial renal cortex, lighter in color
  with a granular appearance
• - deeper renal medulla, darker color consists of
  cone-shaped masses called renal pyramids
• - renal pyramids contain parallel bundles of
  tiny urine-collecting tubules
• - pyramids apex or papilla points internally
• - renal columns, inward extensions of the renal
  cortex, separate adjacent pyramids

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Internal Anatomy of the Kidneys
Figure 23.3b
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• Kidney lobes a single renal pyramid the
  cortical tissue that surrounds that pyramid
• - 5 to 11 lobes and pyramids in each kidney
• Renal sinus large space within the medial part
  opening to the exterior through the renal hilum
• - filled space, contains the renal vessels and
  nerves, some fat, and the urine-carrying tubes
• Renal pelvis flat, funnel-shaped tube
• - expanded superior part of the ureter
• - branching extensions form 2 or 3 major calices
  (sing. calyx cup), each divide to form several
  minor calices
• - calices collect urine draining from the
  papillae, empty it into the renal pelvis ?urine
  flows into the ureter ?bladder

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Gross Vasculature Nerve Supply

• About 1/4 of the hearts systemic output reaches
  the kidneys via the large renal arteries
• - divide into 5 segmental arteries that enter
  the hilum
• - within the renal sinus, each segmental artery
  divides into interlobar arteries
• - at the medulla-cortex junction, interlobar
  arteries branch into arcuate (shaped like a
  bow) arteries
• - radiating outward from arcuate arteries are
  the small cortical radiate arteries (supply the
  cortical tissue)
• - give rise to the glomerular arterioles, which
  feed into the peritubular capillaries

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• Veins trace the pathway of the arteries in
  reverse
• except there are no segmental veins
• Nerve supply renal plexus (offshoot of the
  celiac plexus)
• - a network of autonomic fibers and autonomic
  ganglia on the renal arteries
• - supplied by sympathetic fibers from the
  inferior thoracic splanchic and 1st lumbar
  splanchnic nerves, and other sources
• - fibers control the diameters of the renal
  arteries and influence the urine-forming
  functions of the uriniferous (urine-carrying)
  tubules

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Summary of Blood Vessels Supplying the Kidney
Figure 23.3c
15
Mechanisms of Urine Production

• Uriniferous tubule - main structural and
  functional unit of the kidney
• - more than a million within each kidney
• 3 interacting mechanisms filtration, resorption,
  secretion
• - in filtration, a filtrate (similar to blood
  plasma) leaves the kidney capillaries, it is
  processed into urine by resorption and secretion
• - during resorptionm nutrients, water, and
  essential ions are recovered and returned to the
  blood via tissue capillaries, remaining waste
  contribute to urine
• - secretion, removes additional undesirable
  molecules into the tubule

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Uriniferous Tubules

• 2 major parts
• 1) urine-forming nephron, where filtration,
  resorption, and secretion occur, and
• 2) a collecting duct, concentrates urine by
  removing water
• Uriniferous tubule is lined by simple epithelium,
  one cell thick, adapted for urine production

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Figure 23.4
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Nephrons

• Composed of the renal corpuscle and a tubular
  section
• - proximal convoluted tubule, loop of Henle,
  distal convoluted tubule
• Renal corpuscle (1st part of the nephron)
  located in the cortex, where filtration occurs
• - consist of a tuft of capillaries called the
  glomerulus (ball of yarn) surrounded by the
  glomerular capsule (Bowmans capsule)
• - glumerulus is supplied by an afferent
  arteriole and drained by an efferent arteriole

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• - endothelim is fenestrated, allowing large
  quantities of fluid and small molecules to pass
  from the capillary blood into the capsular space
• - about 20 of the fluid leaves the glomerulus
  and enters the capsular space 80 remains in the
  blood within the capillary
• - parietal layer of the capsule contributes only
  to the structure
• - visceral layer consists of branching
  epithelial cells called podocytes (foot cells),
  end in pedicels, foot processes that
  interdigitate
• - filtrate passes into the capsular space
  through thin clefts between the podocytes called
  filtration slits or slit pores

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Uriniferous Tubule
Figure 23.5a
21
Filtration Membrane

• Filtration barrier - lies between the blood in
  the glomerulus and the capsular space
• Consists of 3 layers
• 1) fenestrated endothelium of the capillary
• 2) filtration slits between the pedicels
• - each covered by a thin slit diaphragm
• 3) intervening basement membrane
• - consists of the fused basal laminae of the
  endothelium and the podocyte epithelium
• - capillary fenestration restrict passage of the
  largest molecules (blood cells)
• - basement membrane slit diaphragm allow small
  proteins and molecules (water, ions, glucose,
  amino acids, urea)

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Renal Corpuscle and the Filtration Membrane
Fig 23.6a
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Renal Corpuscle and the Filtration Membrane
Fig 23.6c
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Tubular Section of the Nephron

• After formation in the renal capsule the filtrate
  
• proceeds into the long tubular section
• Proximal convoluted tubule (renal cortex), is
  most active in resorption and secretion
• - walls are cuboidal epithelial cells with long
  microvilli on the exposed luminal surface
• - contain many mitochondria and a highly
  infolded basolateral membrane with
• - many ion-pumping enzymes responsible for
  resorbing molecules from the filtrate

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• U-shaped loop of Henle (the nephron) consists
  of a descending limb and ascending limb
• Descending limb, continuous with the proximal
  tubule (has a similar structure)
• - rest of the descending limb, the thin segment,
  is the narrowest part of the nephron with walls
  of permeable simple squamous epithelium,
  continues into the
• - ascending limb, joining the thick segment or
  thick ascending limb, cell structure resembles
  the distal convoluted tubule

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• Distal convoluted tubule (in the renal cortex)
  selective secretion and resorption of ions
• - walls of simple cuboidal epithelium
• - less active in resportion, cells do not have
  abundant microvilli
• - but do have many mitochondria and infoldings
  of the basolateral membrane (typical of all
  ion-pumping cells in the body)

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• 2 classed of nephrons
• 1) Cortical nephrons (85) almost entirely
  within the cortex
• - loops of Henle dip only a short distance into
  the medulla
• 2)Juxtamedullary (near the medulla) nephrons
  (15) renal corpuscles lie near the
  cortex-medulla junction
• - loops of Henle deeply invade the medulla
• - thin segments are much longer
• - long loops of Henle, with nearby collecting
  ducts, contribute to production of concentrated
  urine

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Fig 23.8

• Collecting tubules - receive urine from distal
  convoluted tubules

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Collecting Ducts

• Receive urine from several nephrons runs
  straight through the cortex into the deep medulla
• - adjacent collecting ducts join to form larger
  papillary ducts that empty into the minor calices
• - most important role to conserve body fluids is
  shared with the distal tubules
• - pituitary gland secretes ADH, increases
  permeability of the collecting ducts and distal
  tubules to water
• - water is resorbed from the filtrate into the
  surrounding BVs, decreasing the total volume of
  urine produced
• Note Alcohol inhibits the release of ADH,
  reduced water
• resorption from the renal tubules results in
  copious amounts
• of dilute urine

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Microscopic Blood Vessels

• Nephrons are associated with 2 types of capillary
  beds (a portal system) glomerulus and the
  peritubular capillaries
• Juxtamedullary nephrons also associate with the
  capillary-like vasa recta

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Micrograph through the Renal Medulla
Figure 23.7
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Glomeruli

• Capillaries produce the filtrate that moves
  through the uriniferous tubule to become urine
• Both fed and drained by an afferent arteriole and
  an efferent arteriole (respectively)
• - high-resistance vessels, the efferent
  arteriole is narrower than the afferent arteriole
• BP is high for a capillary bed and easily
  forces the filtrate out of the blood and into the
  glomerular capsule
• Kidneys generate 1 L of filtrate every 8 minutes
  only 1 ends up as urine
• - 99 resorbed by the uriniferous tubule, and
  returned to the blood in the peritubular
  capillary beds

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Peritubular Capillaries

• Or intertubular capillaries - arise from the
  efferent arterioles draining the cortical
  glomeruli
• - lie in the interstitial CT of the renal
  cortex, areolar CT surrounds the uriniferous
  tubules
• - capillaries cling closely to the convoluted
  tubules and empty into nearby venules of the
  renal venous system
• - are adapted for absorption low-pressure
  porous capillaries readily absorb solutes and
  water
• - all molecules secreted by the nephrons into
  the urine are from the blood of peritubular
  capillaries

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Vasa Recta (straight vessels)

• Located in the deepest part of the renal cortex
• efferent arterioles from the juxtamedullary
  glomeruli continue into these thin-walled looping
  vessels
• - part of the kidneys urine-concentrating
  mechanism

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Classes of Nephron
Fig 23.9a
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Juxtaglomerular Apparatus

• Near the glomerulus functions in the
  regulation of blood pressure
• - area of specialized contact between the
  terminal end of the ascending limb and the
  afferent arteriole
• - within the apparatus, structures of both the
  tubule and the arteriole are modified
• - granular cells (juxtaglomerular cells),
  modified smooth muscle cells (mechanoreceptors)
  secrete renin in response to falling blood
  pressure in the afferent arteriole

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• Macula densa (dense spot) the terminal
  portion of the loop of Henle
• - tall, closely packed epithelial cells, act as
  chemo-receptors, monitor solute concentrations in
  the filtrate
• - level of solute concentration drops, cells
  signal the granular cells to secrete renin
• - renin initiates the renin-angiotensin
  mechanism that results in secretion of
  aldosterone from the adrenal cortex
• - aldosterone increases Na resorption, water
  follows along the osmotic gradient, causing blood
  volume and BP to rise
• - extraglomerular mesangial cells interact with
  cells of the macula densa and granular cells to
  regulate blood pressure

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Figure 23.10
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Ureters

• Slender tubes 25cm (10in) long carry urine
  from the kidneys to the urinary bladder
• - begins superiorly at L2 as a continuation of
  the renal pelvis
• - descends retroperitoneal through the abdomen,
  enters the true pelvis into the posterolateral
  corner of the bladder
• - runs medially within the posterior bladder
  wall before opening into the bladders interior
• - oblique entry into the bladder prevents
  backflow of urine
• Innervated by both sympathetic and
  parasympathetic nerve fibers
• - although neural control of peristalsis is
  insignificant compared to local stretch response
  of smooth muscle

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• Histology of ureter 3 basic layers
• Mucosa lining of transitional epithelium that
  stretches when the ureters fill with urine
• - and a lamina propria composed of fibroelastic
  CT with patches of lymphoid tissue
• Muscularis consists of 2 layers
• - inner longitudinal layer
• - outer circular layer
• Adventitia in the inferior 1/3 of the ureter is
  an external longitudinal layer of muscularis
• - typical CT

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Microscopic Structure of the Ureter
Figure 23.12
43
Urinary Bladder

• Collapsible muscular sac
• - stores and expels urine
• Full bladder
• spherical
• - expands into the abdominal cavity
• Empty bladder
• lies entirely within the pelvis

Figure 23.13
44
Urinary Bladder

• Urachus closed remnant of the allantois
• Prostate gland - in males
• - lies directly inferior to the bladder
• - surrounds the urethra

Figure 23.14
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• Bladder wall has 3 layers
• Mucosa distensible transitional epithelium and
  a lamina propria
• Thick muscular layer, the detrusor (to thrust
  out) muscle intermingled smooth muscle fibers
• - arranged in inner and outer longitudinal
  layers and a middle circular layer
• - contraction squeezes urine from the bladder
• Fibrous adventitia (except on the superior
  surface which is covered by parietal peritoneum)
• Basic pyramidal shape contains little urine
• - walls are thick and mucosa thrown into folds,
  or rugae

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Histology of the Urinary Bladder
Fig 23.15a, b
47
Urinary Bladder and Urethra

• Trigone (triangle) openings for both the
  ureters and urethra
• - defines a triangular region on the posterior
  wall
• Urethra thin-walled tube
• - drains urine from the bladder, conveys it out
  of the body
• Internal urethral sphincter involuntary smooth
  muscle at the bladder-urethra junction
• External urethral sphincter surrounds the
  urethra
• - lies within the urogenital diaphragm muscle
• - voluntary skeletal muscle inhibits urination
  until the proper time, relaxes when one urinates

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Male Urinary Bladder and Urethra

• Long urethra of the male
• has 3 regions
• Prostatic passes through the prostate gland
• Membranous through the urogenital diaphram
• Spongy (penile) passes through the length of
  the penis

Fig 23.16a
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Male Urinary Bladder and Urethra

• In females length of 3-4 cm

Fig 23.16b
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Urethra

• Transitional epithelium at the proximal end (near
  the bladder)
• Stratified and pseudostratified columnar mid
  urethra (in males)
• Stratified squamous epithelium at the distal
  end (near the urethral opening)

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Micturition
Figure 23.17
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Disorders of the Urinary System

• UTI urinary tract infections, more common in
  females
• - burning sensation during micturition
• Renal calculi kidney stones
• Bladder cancer 3 of cancers, more common in
  men
• Kidney cancer
• - arises fro epithelial cells of uriniferous
  tubules

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Urinary System Throughout Life

• Embryo develops 3 pairs of kidneys pronephros,
  mesonephros, metanephros
• - only metanephros persists to become the adult
  kidneys
• - metanephric kidney produces urine by fetal
  month 3
• - contributes to the volume of amniotic fluid

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Development of the Urinary Organs
Figure 23.18a,b
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Figure 23.18c, d
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Urinary System Throughout Life

• Kidney and bladder function declines with
• advancing age
• Nephrons decrease in size and number
• Tubules less efficient at secretion and
  reabsorption
• Filtration declines
• Recognition of desire to urinate is delayed
• Loss of muscle tone in the bladder