The Urinary System

1
Chapter 25

• The Urinary System

G.R. Pitts, J.R. Schiller, and James F.
Thompson, Ph.D.
2
General

• During metabolism cells produce wastes
• waste - any substance acquired or produced in
  excess with no function in body
• e.g. - CO2, H2O, heat
• All wastes must be eliminated, or at least
  maintained at low concentrations
• Additionally, protein breakdown leaves
  nitrogenous wastes
• Excess sodium (Na), chloride (Cl-), potassium
  (K), sulfate (SO42-), phosphate (PO42-), and
  hydrogen ion (H) must be regulated

3
General

• Several organs transport, neutralize, store and
  remove wastes
• Body fluid and body fluid buffers
• Blood and blood buffers
• Liver
• Lungs
• Sudoriferous (sweat) glands (minor)
• Hair and nails (minor)
• GI tract and liver
• Kidneys

4
General

• Urinary system
• Maintains fluid homeostasis including
• regulation of volume and composition by
  eliminating certain wastes while conserving
  needed materials
• regulation of blood pH
• regulation of hydrostatic pressure of blood and,
  indirectly, of other body fluids
• Contributions to metabolism
• helps synthesize calcitriol (active form of
  Vitamin D)
• secretes erythropoietin
• performs gluconeogenesis during fasting or
  starvation
• deaminates certain amino acids to eliminate
  ammonia

5
Kidneys

• Paired reddish organs, just above waist on
  posterior wall of abdomen
• partially protected by 11th, 12th ribs
• right kidney sits lower than the left kidney
• receive 20-25 of the resting cardiac output
• Consume 20-25 of the O2 used by the body at rest

6
Kidneys (cont.)

• Retroperitoneal, as are ureters and urinary
  bladder

7
Kidney - Internal Gross Anatomy
Know these terms for lecture and lab exams!
8
Kidney - Internal Micro Anatomy

• Nephron the functional unit of kidney
• Three physiological processes 1) filtration,
  2) reabsorption , and 3) secretion
• These three processes cooperate to achieve the
  various functions of the kidney
• Different sites ? different primary functions

9
The Functions of the Kidney

• Nephron forms urine from blood plasma
• 1) formation of a plasma filtrate
• 2) reabsorption of useful molecules from the
  filtrate to prevent their loss in urine
• 3) secretion of excess electrolytes and certain
  wastes (nitrogenous wastes, H) in concentrations
  greater than their concentration in plasma
• 4) regulation of water balance by concentrating
  or diluting the urine
• 5) minor endocrine function releasing hormone
  erythropoietin to stimulate RBC production
• 6) releasing renin for angiotensinogen activation

10
Kidney - Internal Micro Anatomy

• A million nephrons are located in the cortex
• The filtrate is carried by the collecting duct
  system through the medulla
• The urine is collected at the papillae into the
  minor and major calyxes

Nephron
Papilla
Minor Calyx
11
Nephron

• 2 major parts to the nephron

12
Nephron

• Renal corpuscle
• site of plasma filtration
• 2 components
• glomerulus
• tuft of capillary loops
• fed by afferent arteriole
• drained by efferent arteriole

• glomerular (Bowman's) capsule
• double walled cup lined by simple squamous
  epithelium
• outer wall (parietal layer) separated from inner
  wall (visceral layer podocytes) by capsular
  (Bowman's) space

• as blood flows through capillary tuft
  filtration occurs
• water and most dissolved molecules pass into
  capsular space
• large proteins and formed elements in the blood
  do not cross

13
Nephron

• Renal tubule - where filtered fluid passes from
  capsule
• proximal convoluted tubule (PCT)
• loop of Henle (nephron loop)
• distal convoluted tubule (DCT)
• short connecting tubules
• collecting ducts
• merge to papillary duct
• then to minor calyx
• 30 pap ducts/papillae

DCT
PCT
ducts
Loop
14
Nephron

• Cortical vs. juxtamedullary nephrons
• Location related to the length of loop of the
  nephron
• 15-20 of the nephrons have longer loops and
  increased involvement in the reabsorption of water

H2O
15
Renal Corpuscle Histology

• Each nephron portion has distinctive features
• Histology of the glomerular filtration membrane
• Three components to the filter
• From inside to out, the layers prevent movement
  of progressively smaller particles

16
Histology of Filtration Membrane

• Endothelium of glomerulus
• Single layer of capillary endothelium with
  fenestrations
• Prevents RBC passage WBCs use diapedesis to get
  out

17
Histology of Filtration Membrane

• Basement membrane of glomerulus
• Between endothelium and visceral layer of glom.
  capsule
• Prevents passage of large protein molecules

18
Histology of Filtration Membrane

• Filtration slits in podocytes
• Podocytes
• specialized epithelium of visceral layer
• footlike extensions with filtration slits between
  extensions
• Restricts passage of medium-sized proteins

19
Histology of Filtration Membrane
20
Tubule Histology

• PCT - cuboidal cells with apical microvilli
• Descending Loop, and beginning of Ascending Loop
• simple squamous epithelium
• water permeable
• Remainder of Ascending limb of the Loop
• cuboidal to low columnar epithelial cells
• impermeable to water
• permeable to solute (ions)
• DCT, collecting ducts
• cuboidal with specialized cells
• principal cells - sensitive to ADH (antidiuretic
  hormone)
• intercalated cells - secrete H

21
Renal Blood Supply

• Important vessels
• Renal arteries
• 20-25 of resting CO
• 1200 ml/min
• Segmental arteries
• Interlobar arteries - through columns
• Arcuate arteries
• Interlobular arteries

Refer to the kidney models in lab.
22
Renal Blood Supply
peritubular capillaries

• Important vessels
• Afferent arterioles - each renal corpuscle
  receives one
• Glomerular capillaries
• Efferent arterioles - drain blood from glomerulus

cortex -------- medulla

• Peritubular capillaries - around cortical
  nephrons
• Vasa recta - long networks from the efferent
  arteriole around the Loop (juxtamedullary
  nephrons)

Vasa recta
23
Renal Blood Supply

• Important vessels
• Interlobular veins
• Arcuate veins
• Interlobar veins
• Segmental veins
• Renal veins - exits hilus

Refer to the kidney models in lab.
24
Renin-Angiotensin System

• Juxtaglomerular apparatus (JGA)
• Distal tubule contacts afferent arteriole at
  renal corpuscle
• Juxtaglomerular (JG) cells
• modified smooth muscle cells in afferent
  arteriole wall detect changes in blood pressure
  (a stretch reflex)
• Secrete enzyme renin to trigger Renin-Angiotensin
  System if blood pressure falls

JG
afferent art.
efferent art.
Distal Convoluted Tubule
25
Renin-Angiotensin System

• Juxtaglomerular apparatus (JGA)
• Distal tubule contacts afferent arteriole at
  renal corpuscle
• Macula Densa (MD) cells
• special cells in the wall of the distal tubule in
  this area monitor the osmotic potential in the
  filtrate in the distal tubule
• stimulate JG cells to release renin if filtrate
  is too dilute, indicating insufficient filtration
  and/or low blood pressure/low blood volume
• Both JG and MD cells work together to regulate
  blood pressure and blood volume

JG
afferent art.
MD
efferent art.
Distal Convoluted Tubule
26
Renin-Angiotensin System

• Hepatocytes secrete inactive precursor
  Angiotensinogen into the bloodstream
• Juxtaglomerular (JG) cells secrete the enzyme
  renin to convert Angiotensinogen to Angiotensin I
  in the bloodstream
• Angiotensin I is transported to the lungs where
  Angiotensin Converting Enzyme (ACE) converts
  Angiotensin I to Angiotensin II
• Both Angiotensin I and Angiotensin II act as
  circulating hormones to increase blood pressure
  and blood volume AII is stronger

27
Renal Nerve Supply

• Nerves from renal plexus of Sympathetic Division
  of ANS innervate the kidney
• Vasomotor nerves accompany the renal arteries and
  their branches
• What is the role of sympathetic stimulation on
  renal blood flow?
• In Fight or Flight or muscular exertion
• decrease renal arterial flow
• decrease urine production
• maintain blood volume
• increase systemic blood pressure

28
Physiology of Urine Formation

• Glomerular filtration - first step in urine
  formation
• forcing of fluids and dissolved solutes through
  membrane by hydrostatic pressure
• same process as in systemic capillaries
• results in a filtrate
• 180 L/day, about 60 times plasma volume
• 178-179 L/day is reabsorbed (99)

1-2 L/day of urine is typical
29
Glomerular Filtration

• 3 structural features of the renal corpuscles
  enhance their filtering capacity
• Glomerular capillaries are relatively long which
  increases their surface area for filtration
• Filter (endothelium-capsular membrane) is thin
  and porous
• Fenestrated glomerular capillaries are 50 times
  more permeable than regular capillaries
• The filtration slits of the basement membrane
  only permit passage of small molecules
• Glomerular Capillary blood pressure is high the
  efferent arteriole diameter is less than the
  afferent arteriole diameter -- increasing
  filtration pressure in the renal corpuscle

30
Glomerular Filtration

• Net filtration pressure (NFP) depends on 3
  pressures
• glomerular blood hydrostatic pressure (GBHP)
• capsular hydrostatic pressure (CHP)
• blood colloid osmotic pressure (BCOP)

NFP GBHP CHP BCOP 10 55 - 15 - 30

1
2
3
31
Glomerular Filtration Rate (GFR)

• GFR
• Volume of filtrate that forms in all renal
  corpuscles in both kidneys/min
• Adults GFR ? 125 mL/min (180 L/day)
• Regulation of GFR
• When more blood flows into glomerulus, GFR ?
• GFR depends on systemic blood pressure, and the
  diameter of afferent efferent arterioles
• If glomerular capillary blood pressure falls much
  below 45 mm Hg, no filtration occurs ? anuria (no
  urine output)

32
Glomerular Filtration Rate (GFR)

• 3 principal regulators of GFR
• Renal autoregulation of GFR
• the kidneys are able to maintain a relatively
  constant internal blood pressure and GFR despite
  changes in systemic arterial pressure
• there is negative feedback from the
  JuxtaGlomerular Apparatus adjusting blood
  pressure and blood volume

33
Glomerular Filtration Rate (GFR)

• 3 principal regulators of GFR (cont.)
• Hormonal regulation of GFR
• Angiotensin I II
• activated by renin released from JG cells and
  further by ACE in the lungs
• 5 important functions
• direct vasoconstriction
• ? aldosterone secretion
• ? thirst generated at the hypothalamus
• ? ADH secretion
• ? Na reabsorption (H2O follows passively)
• Net Effect ? increased blood pressure and blood
  volume

34
Glomerular Filtration Rate (GFR)

• 3 principal regulators of GFR (cont.)
• Hormonal regulation of GFR
• Angiotensin I II
• Atrial Natriuretic Peptide (ANP)
• secreted by cells in atria of heart in response
  to stretch
• ? GFR, promotes excretion of H2O, Na, but
  retention of K
• suppresses output of ADH, aldosterone, and renin
• Net Effect ? decreased blood pressure and blood
  volume
• Aldosterone
• secreted by cells in adrenal cortex in response
  to angiotensin I II (and ACTH)
• ? GFR, promotes retention of H2O, Na, but
  excretion of K
• antagonist to Atrial Natriuretic Peptide
• Net Effect ? increased blood pressure and blood
  volume

35
Glomerular Filtration Rate (GFR)

• 3 principal regulators of GFR (cont.)
• Neural regulation
• kidneys blood vessels supplied by
  vasoconstrictor fibers from Sympathetic Division
  of ANS which release Norepinephrine
• strong sympathetic stimulation causes JG cells to
  secrete renin and the adrenal medulla to secrete
  Epinephrine

36
GFR Control
modest
37
Tubular Reabsorption

• Movement of water and certain solutes back into
  bloodstream from the renal tubule
• Filter 180 L/day of fluid and solutes
• nutrients (Na, K, Glucose, etc.) are needed by
  body
• body will expend ATP to get them back into blood
• about 99 of the filtrate volume is reabsorbed
  from the tubule by active transport and osmosis
• Epithelial cells in PCT (microvilli) increase
  surface area for tubular reabsorption
• DCT and collecting ducts play a lesser role in
  nutrient/solute reabsorption

38
Reabsorption of Na in PCT

• PCT is site of most electrolyte reabsorption
• Mechanisms which aid Na transport
• Na/ K ATPase on basolateral side is
  fundamental
• Concentration of Na inside the tubular cells is
  low
• Interior of the cell negatively charged
• Double gradient for Na movement from filtrate to
  tubular cell
• Requires ATP energy

39
Reabsorption of Nutrients in PCT

• 100 of the filtered glucose and other sugars,
  AA's, lactic acid, and other useful metabolites
  are reabsorbed
• Na symporters power secondary active transport
  systems
• Why secondary? They rely on the Na/ K ATPase
  pump.

40
Reabsorption of Na in PCT

• Na is passively transported from the filtrate in
  tubule lumen into tubular cells to replace the
  Na being actively transported into the
  peritubular capillaries. Glucose moves with Na.

41
Reabsorption of H2O in PCT

• H2O follows Na passively by osmosis from the
  filtrate through the tubular cells into the
  peritubular capillaries

42
Reabsorption of Nutrients in PCT

• The movement of water back to the bloodstream
  concentrates the remaining solutes in the filtrate

H2O ? solutes ?
43
Reabsorption of Nutrients in PCT

• The new concentration gradients increase the
  diffusion of some of the other remaining solutes
  in the filtrate from lumen to the blood stream.

44
Transport Maximums (Tm)s

• each type of symporter has an upper limit
  (maximum) on how fast it can work
• any time a substance is in the filtrate in an
  amount greater than its transport maximum, some
  of it will be left behind in the urine
• only Na has no transport maximum because Na is
  being actively transported by the Na/ K ATPase
  pump at all times.

45
Renal Thresholds

• The Renal Threshold is the plasma concentration
  at which a substance begins to spill into the
  urine because its Tm has been surpassed.
• If the plasma filtrate concentration is too
  high, all of the substance cannot be reabsorbed.
• For example, glucose spills into the urine in
  untreated diabetics.
• Tm for glucose 375 mg/min
• If blood glucose gt 400 mg/100 mL, large
  quantities of glucose will appear in the urine

46
Reabsorption in the PCT

• By the end of the PCT the following reabsorption
  has occurred
• 100 of filtered nutrients (sugars, albumin,
  amino acids, vitamins, etc.)
• 80-90 of filtered HCO3-
• 65 of Na ions and water,
• 50 of Cl- and K ions

47
Reabsorption in Loop of Henle

• Cells in the thin descending limb are only
  permeable to water
• H2O reabsorption is not coupled to reabsorption
  of filtered solutes (osmosis) in this area as it
  had been in the PCT
• Note illustration at right is not thin
  descending limb of nephron loop

48
Reabsorption in Loop of Henle

• Cells in the thicker ascending Loop feature
  sodium-potassium-chloride symporters
• reabsorb 1 Na, 1 K, 2 Cl-
• depend on the low cytoplasmic Na concentration
  to function
• little or no H2O is reabsorbed from the thick
  ascending Loop
• Loop reabsorbs 30 of K, 20 of Na, 35 of
  Cl-, and 15 of H2O
• H2O reabsorption is not coupled to reabsorption
  of filtered solutes (osmosis)

49
Reabsorption in DCT and Collecting Ducts

• Filtrate reaching the DCT has already had 80 of
  the solutes and H2O reabsorbed
• Fluid now has the characteristics of urine
• DCT is the site of final adjustment of urine
  composition
• less work to do, so no need for microvilli
  brush border to increase surface area for
  tranporters
• Na/K/Cl- symporter is a major DCT transporter
• DCT reabsorbs another 10 of filtrate volume

50
Reabsorption in DCT and Collecting Duct

• Principal cells are present in the distal DCTs
  and collecting ducts
• 3 hormones act on principal cells to modify ion
  and fluid reabsorption
• 1 Anti-Diuretic Hormone (ADH) (from
  neurohypophysis)
• ? H2O reabsorption by increasing permeability to
  H2O in the DCT and collecting duct
• details discussed later on

51
Reabsorption in DCT and Collecting Duct

• 3 hormones act on principal cells . . .
• 2 Aldosterone (from adrenal cortex)
• ? Na reabsorption Cl- and H2O follow passively
  ? K reabsorption
• ? numbers of basolateral Na/K ATPases
• ? activity and numbers of Na-K transporters and
  K channels
• 3 Atrial Natriuretic Peptide (ANP) is the
  antagonist to Aldosterone
• ? K reabsorption ? Na reabsorption Cl- and
  H2O follow passively adding salt and water to
  urine

52
Reabsorption Summary
Loop and DCT are sites for additional electrolyte
reabsorption
PCT is the site for reabsorption of all
nutrients and most electrolytes
Collecting Ducts complete electrolyte reabsorption
53
Reabsorption in the Nephron

• Note reabsorption of electrolytes must maintain
  an electrostatic equilibrium. The Net Charge
  must remain in balance in each fluid compartment.
• For every cation (e.g., Na) which crosses a
  membrane in a particular direction, one of two
  things must also happen
• An anion (e.g., Cl-, HCO3-) must cross the
  membrane in the same direction, or
• A different cation (e.g., K) must cross the
  membrane in the opposite direction

54
Reabsorption in the Nephron

• Aldosterone and Atrial Natriuretic Peptide
  regulate the rate of tubular reabsorption of Na
  and Cl- and the concurrent secretion of K.
• Parathormone regulates the rate of tubular
  reabsorption of Ca and Mg and the concurrent
  secretion of HPO4-.

55
Fluid Reabsorption in the Nephron

• Use GFR (mLs/min) values to track reabsorption of
  filtrate

Start with a GFR of 125 mLs/min PCT reabsorbs
105 mLs/min and DCT reabsorbs 19 mLs/min leaving
1 mL/min as urinary output. This is obligatory
water reabsortion. 1440 mLs/day produced under
these standard conditions.
56
Tubular Secretion

• Removes substances from the blood, adds them to
  the filtrate
• includes H, K, NH4, HPO4-, creatinine, plant
  alkaloids (toxins), penicillin and other drugs
• Two primary functions
• Helps rid body of certain routinely generated
  waste substances and toxins
• Regulates blood pH by secretion of H (and to a
  lesser degree, reabsorption of HCO3-)

57
Secretion of K ions

• Principal cells in collecting ducts secrete
  variable amount of K in exchange for reabsorbed
  Na
• Most animal diets contain excess K but scarce
  Na
• Na/K ATPases are the ion pumps
• Controlled by Aldosterone and Atrial Natriuretic
  Peptide
• Aldosterone is released from the Adrenal Cortex
  in response to
• Angiotensin I II
• With excess K, Aldosterone secretion
  predominates Na
• (and Cl-) are reabsorbed while
  considerable K is secreted
• Atrial Natriuretic Peptide is released from the
  Atrial walls in the
• heart in response to stretching when
  blood volume or blood
• pressure increase
• With excess Na, Atrial Natriuretic Peptide
  secretion
• predominates K is reabsorbed while
  considerable Na (and
• Cl-) are secreted

58
Secretion of H ions

• Cells of the renal tubule can elevate blood pH in
  3 ways
• Secrete H ions into the filtrate
• Reabsorb filtered HCO3-
• Produce more HCO3-
• The key is the chemical relationship between H
  ions and HCO3- ions
• H2O CO2 ? H2CO3 ? H HCO3-
• This reaction occurs spontaneously and it is also
  catalyzed by the enzyme carbonic anhydrase.

59
Secretion of H ions

• In PCT
• 1 Na/H antiporter puts H ions into the
  filtrate
• H ions combine with HCO3- in lumen to form CO2
  and H2O

1
H
1
HCO3-
60
Secretion of H ions

• In PCT
• 2 CO2 from the filtrate or plasma enters the
  tubular cell where it combines with H2O to form
  H2CO3

H
HCO3-
2
2
61
Secretion of H ions

• In PCT
• 3 H is pumped into the lumen
• 4 H2CO3- follows pumped Na back to the
  bloodstream

HCO3-
H
4
3
HCO3-
62
Secretion of H ions

• Collecting ducts also secrete H ions
• H pumps are a primary active transport process
  powered by ATPs
• generate as much as a 1000 fold concentration
  gradient ? strongly acid urine
• new bicarbonate ions are reabsorbed by the
  basolateral HCO3-/Cl- antiporter
• adding new HCO3- buffer to the bloodstream

HCO3-
H
63
Secretion of NH3 and NH4

• Ammonia is a toxic waste absorbed from bacterial
  metabolism in the large intestine and ammonia is
  generated from the deamination of amino acids in
  the liver
• Liver converts ammonia to urea, a much less toxic
  nitrogenous waste
• PCT cells can also deaminate certain amino acids
  and secrete additional NH4 with a Na/NH4
  antiporter when blood pH becomes acidic

64
Summary of Nephron FunctionsGFR ? 125 mL/min
65
Summary of Nephron FunctionsPCT reabsorbs
nutrients, electrolytes, and water
66
Summary of Nephron Functions
Loop also reabsorbs some electrolytes and water
67
Summary of Nephron FunctionsDCT and Collecting
Ducts continue the absorption of water and
electrolyte, especially Na and HCO3- DCT and
CDs also secrete K and H and ammonia ions into
the filtrate
68
Summary of Nephron FunctionsThe final process
to discuss is regulation of water balance
making a dilute or concentrated urine.
69
Nephron Reaborbs 99 of H2O

• Water balance determines the fate of the last 1!

Start with a GFR of 125 mLs/min PCT reabsorbs
105 mLs/min and DCT reabsorbs 19 mLs/min leaving
1 mL/min as urinary output. 1440 mLs/day
produced under these standard conditions.
1 mL/Min is adjusted as needed by ADH. That
is facultative water reabsorption.
70
Adjusting Water Balance

• Distal tubular cells and cells in the collecting
  ducts expend ATP energy to create an osmotic
  gradient between the cortex and medulla of the
  kidney
• The key substances transported are urea and NaCl
• Countercurrent flow mechanisms maintain the
  osmotic gradient

71
Countercurrent Flow Mechanisms

• Compare to a system of co-current flow
• two pipes are semi-permeable
• the fluids flow in the same direction
• solutes will diffuse along concentration
  gradients
• solutes will all reach equilibrium values

72
Countercurrent Flow Mechanisms

• In a system of countercurrent flow
• two pipes are still semi-permeable
• but the fluids flow in opposite directions
• solutes again diffuse along concentration
  gradients
• the gradient always favors transfer
• solutes do not reach equilibrium values

73
Countercurrent Flow Mechanisms

• Countercurrent flow is seen in a variety of
  physiological systems
• How do penguins stand in freezing water in their
  bare feet?
• blood flows in opposite directions
• heat is transferred along the heat gradient
• most of the heat moves from arterial to venous
  blood and is not lost to the water

heat
74
Countercurrent Flow Mechanisms

• Countercurrent flow is seen in a variety of
  physiological systems
• How do fish gills oxygenate blood?
• blood flows in opposite directions
• O2 is transferred along the O2 gradient
• O2 continues to move from water to the blood and
  the gradient is always favorable

O2
blood
75
Nephrons Countercurrents

• Renal tubule has a more complicated system of
  counter-current flow
• PCT descending Loop vs. ascending Loop and DCT
• arterial vasa recta vs. venous vasa recta
• Renal tubule versus vasa recta
• This system permits the osmotic gradient to
  develop

DCT
PCT
ducts
Loop
76
Nephrons Countercurrents

• complex countercurrent flow between the
  juxtamedullary nephrons and their vasa recta
• 1 the entire flow in the renal tubule (loop) is
  countercurrent to the flow in the vasa recta
• 2 each U-shaped vessel also has countercurrent
  flow between its descending and ascending limbs

1
2
77
Nephrons Countercurrents

• in the medulla, urea and NaCl are actively
  transported from the vessels exiting the medulla
• this increases the concentration of urea and NaCl
  in the medulla
• although urea and NaCl can diffuse into the
  vessels entering the medulla, they do not carry
  the solutes away

78
Nephrons Countercurrents

• the countercurrent flow is in a loop
• the active transport pumps work at all times
• therefore, the solutes accumulate at the bottom
  of the loop
• the vasa recta carry the water back to the
  medulla and, thus, back to the body

79
Nephrons Countercurrents

• the combination of complex countercurrent flow
  and the active transport pumping of urea and NaCl
  maintain the osmotic gradient between the cortex
  and the medulla at all times

80
Adjusting Water Balance

• water conservation is dependent on ADH
• normal osmotic concentration in the body fluids,
  plasma and interstitial fluids, including the
  kidneys cortex, is 300 mOsm/L
• glomerular filtrate is isosmotic to plasma
• thick limb of the ascending Loop is impermeable
  to water but urea and Na/Cl- ions are actively
  transported out of the filtrate
• DCT and collecting ducts are impermeable to water
  unless ADH is present

81
Producing a Dilute Urine

• With adequate H2O, the posterior pituitary
  releases little ADH
• the glomerular filtrate equilibrates with
  medullary conditions while passing down through
  the loop
• meanwhile, tubular reabsorption of solutes
  continues

82
Producing a Dilute Urine

• As the filtrate enters the ascending limb of the
  Loop, and the DCT, and then the collecting ducts,
  no water can diffuse out of the filtrate
• Meanwhile, continuing tubular reabsorption of
  solutes in the DCT CDs creates a dilute
  hypo-osmotic (hypotonic) urine

83
Producing a Concentrated Urine

• with inadequate H2O, the posterior pituitary
  releases more ADH
• the glomerular filtrate equilibrates with
  medullary conditions while passing down through
  the loop
• meanwhile, tubular reabsorption of solutes
  continues

84
Producing a Concentrated Urine

• the ascending limb of the Loop remains
  impermeable to H2O
• however, as the filtrate enters the DCT, and then
  the collecting ducts, ADH causes the tubular
  cells to become permeable to H2O
• water can diffuse in or out of the filtrate

85
Producing a Concentrated Urine

• the filtrate becomes hypo-osmotic (hypotonic) in
  the DCT while H2O and solutes are returned to the
  bloodstream
• however, the filtrate equilibrates with medullary
  conditions while passing down through the
  collecting ducts

86
Producing a Concentrated Urine

• even though the filtrate is still losing urea and
  NaCl to active transport, the other solutes
  cannot leave
• the filtrate becomes hyper-osmotic (hypertonic)
  as it equilibrates with the osmotic gradient
  surrounding the collecting ducts
• water is drawn into the vasa recta and back to
  the bloodstream

87
Producing a Concentrated Urine

• The effect of ADH is to create a concentrated
  hyper-osmotic (hypertonic) urine

88
The Final Common Pathway

• Ureters
• extensions of the renal pelvis
• enter the bladder medially from the posterior
• Histology - 3 layers
• inner mucosa lined with transitional epithelium
• muscularis smooth muscle in circular and
  longitudinal layers
• retroperitoneal (serosa or adventitia)

• Physiology
• transport urine to the bladder
• peristalsis primarily, but hydrostatic pressure
  of gravity helps in humans

89
The Final Common Pathway

• Urinary bladder
• hollow muscular organ
• generally smaller in females due to presence of a
  uterus
• retroperitoneal in the pelvic cavity, posterior
  to the pelvic symphysis
• freely movable
• Structure - trigone

90
The Final Common Pathway

• Bladder histology
• inner mucosa lined with transitional epithelium
• muscularis smooth muscle in three layers
• Sphincters control entry from ureters and exit at
  the urethra
• circular smooth muscle fibers form internal
  urethral sphincter
• lower is the external urethral sphincter with
  skeletal muscle for voluntary control
• retroperitoneal (serosa or adventitia)

91
The Final Common Pathway
Urethra routed differently in males and females
see chapter 28
92
The Final Common Pathway

• Urethra
• small tube from floor of bladder to exterior of
  body
• females -- fairly straight path exits anterior to
  vagina
• males -- passes through the prostate gland and
  exits through the penis
• histology
• female three coats
• inner mucosa, intermediate thin layer of spongy
  tissue with plexus of veins
• outer muscular coat continuous with the bladder
• male two layers
• inner mucous membrane and a muscularis
• outer submucosa tissue with various accessory
  structures which connect to it
• both genders have a stratified squamous
  epithelial lining

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The Final Common Pathway

• Urethra
• Physiology - terminal portion of urinary tract,
  in males the urethra also serves as the duct
  through which semen is discharged from the body
• Urine
• Volume
• 1000-2000 ml/day
• influenced by blood pressure, blood osmotic
  pressure, temperature, mental state, general
  health, diet, diuretics, other drugs
• Chemical Composition - 95 water, 5 solutes

94
Micturition

• Voluntary and involuntary (ANS) nerve impulses
  control the process
• 700-800 mL capacity
• when volume gt 200-400 mL, stretch receptors fire
• processed in cortex
• micturition reflex
• initiates a conscious desire to expel urine
• parasympathetic commands coordinate the process
• contraction of detrusor (bladder), relaxation of
  internal sphincter

A
B
2
1
3
95
End Chapter 25