The document provides an overview of the urinary system and kidney functions. It discusses: - The main components of the urinary system including kidneys, ureters, urinary bladder, and urethra. - The key functions of the kidneys which include filtering blood, regulating fluid and electrolyte balance, controlling blood pressure, and producing hormones. - The anatomy of the kidneys including their location, layers of tissue, blood supply, and the functional unit of the kidney called the nephron. - The three stages of urine formation: glomerular filtration, tubular reabsorption and secretion, and the regulation of glomerular filtration rate.

1. 23-1
Chapter 23
Urinary
System

2. 23-2
The Urinary System
• Functions of urinary system
• Anatomy of kidney
• Urine formation
– glomerular filtration
– tubular reabsorption
– tubular secretion
• Urine and renal function tests
• Urine storage and elimination

3. 23-3
Urinary System
Consists of 6 organs:
2 Kidneys
2 Ureters
Urinary Bladder
Urethra
We will focus mostly on the kidneys.

4. 23-4
Kidney Functions
• Fundamental role: eliminate waste & homeostatically
regulate body fluid
1. Filters blood plasma
– Separate waste from useful chemicals;
– eliminates waste while returning useful substances to blood;
2. Regulates
– Blood pressure & volume by eliminating or conserving water as
necessary;
– osmolarity of body fluids by controlling elimination or water and
solutes;
3. Secretes
– Renin: enzyme activate hormonal mechanisms that control BP
and electrolyte balance;
– Erythropoietin: hormone that helps activate Erythropoiesis.

5. Kidney Functions
4. Work with lungs
- help regulate PCO2 and acid base balance;
5. Calcium homeostasis
- by partaking in the synthesizing of calcitrol (vit. D);
6. Detoxify free radicals and drugs
- by using perioxisomes;
7. Gluconeogenesis
- during starvation they deaminate AA, excrete the –NH3
and synthesize glucose from the rest of the AA. 23-5

6. 23-6
Nitrogenous Wastes
• Waste: any substance that is useless to the body or in
excess of body’s need.
• Metabolic Waste: more specific, a waste product
produced by the body;
– i.e. feces is waste but not metabolic waste (not made by body);
• In great quantities are lethal to cells if allowed to
accumulate;
• Thus these small nitrogen containing substances
are nitrogenous waste.

7. 23-7
Nitrogenous Wastes
• Urea (50%)
– proteins→amino acids →NH2 removed →forms
ammonia, liver converts to urea
• Uric acid
– nucleic acid catabolism
• Creatinine
– creatine phosphate catabolism
• Blood Urea Nitrogen (BUN)
– nitrogenous wastes in expressed in blood
– Normal levels are 10-20mg/dL
– azotemia: ↑BUN= renal insufficiency
Can progress to…
– uremia: toxic effects as wastes accumulate
leading to diarrhea, vomiting, dyspnea, coma &
death within a few weeks, need kidney
transplant to remove nitrogenous waste.

8. 23-8
Excretion
Separation of wastes from body fluids and
eliminating them;
In part to four systems:
1. respiratory: excretes small amount of CO2 ,
water and other gases;
2. integumentary: water, salts, lactic acid, urea via
sweat;
3. digestive: water, salts, CO2, lipids, bile pigments,
cholesterol and other metabolic wastes;
4. urinary: excretes a variety of metabolic wastes,
toxins, drugs, hormones, salts, H+
and water.

9. 23-9
Anatomy of Kidney
• Position, weight and size
– retroperitoneal, level of T12 to L3
– about 160 g each
– about size of a bar of soap (12x6x3 cm)
• Shape
– lateral surface - convex; medial – concave

10. 23-10
Kidney Location

11. Gross Anatomy
• Tubular gland that’s functional units are called
nephrons;
• Hilum – on medial aspect, area that receives the
renal nerves, arteries and veins;
• Kidney is protected by three layers of CT:
1.renal fascia: fibrous layer binds kidney and associated
organs to abdominal wall;
2.adipose capsule: fat layer, cushions kidney, holds in place;
3.renal capsule: fibrous layer, encloses kidney like plastic
wrap, protects it from trauma and infection;
23-11

12. 23-12
Anatomy of Kidney
Renal parenchyma = glandular tissue that produces urine,
appears as a “C” shape in a fontal section;
-Parenchyma is divided into two zones:
1. Renal Cortex – outer zone
2. Renal Medulla – inner zone

13. Gross Anatomy
Renal cortex extends medially forming the renal columns that
separate the medulla into renal pyramids, each pyramid ends
at a renal papilla that faces the renal sinus.
Renal papilla of each pyramid “opens” a urine collecting duct
called a minor calyx, two or more minor calyces converge to
form a major calyx, two to three major calyces converge to
form the renal pelvis.
The ureter is the tube that connects to the renal pelvis that
drains urine down to the bladder.
23-13

14. 23-14
Renal Circulation
Kidneys account for 0.4% of body weight but receive about 21%
of the cardiac output.
Renal A. → segmental A.→ interlobar A.→ arcuate A.→ interlobular A. →
afferent arterioles (1 nephron) → glomerulus → efferent arteriole →
peritubular capillaries→ interlobular V. → arcuate V.→ interlobar V.→
Renal V.

15. 23-15
Renal Circulation

16. 23-16

17. 23-17
Innervation of Kidney
• Nerves arise from the superior mesenteric ganglion
enter the hilum of kidney;
• They follow the renal artery and innervate the
afferent and efferent arterioles;
• Contain sympathetic fibers that regulate blood flow
into and out of each nephron; thus control rate of
filtration and urine formation;
• If BP falls, they stimulate the secretion of renin
(enzyme) that activates hormonal mechanism for
restoring BP.

18. 23-18
The Nephron
• Each Kidney contains about 1.2 million
FUNCTIONAL units of the kidney = nephron
• Consists of two parts:
1. Renal corpuscle: where blood plasma is filtered;
2. Long renal tubule: process this filtrate into urine.

19. 23-19
• Consists of:
– Glomerular (Bowman’s) capsule
– Parietal layer- simple squamous epithelium
– Visceral layer - podocytes
– Glomerulus
– Capsular space – contains glomerular filtrate
Vascular pole – afferent
enters and efferent
exits;
Urinary pole – the
parietal wall gives
way to a renal tubule
on the opposite side.
The Renal Corpusle

20. 23-20
Renal Tubule
Leads away from the glomerular capsule and ends at the tip of
the medullary pyramid.
• Consists of 4 major regions:
1. Proximal convoluted tubule (PCT)
2. Nephron loop (loop of Henle)
3. Distal convoluted tubule (DCT)
4. Collecting ducts (CD)

21. Proximal Convoluted Tubule (PCT)
• Arises from the glomerular capsule;
• Longest and most coiled of the four regions;
• Simple cuboidal epithelium with microvilli; which
help with the great deal of absorption that takes
place.
23-21

22. Nephron Loop
“U” shaped loop following the PCT;
Portions:
- descending limb – passes from cortex to medulla;
- ascending limb – 180o
turn and returns to cortex;
Nephron loop divided into:
- thick segment – simple cuboidal epi., cells are highly active
in transport of salts (high metabolic activity & mitochondria;
- thin segment – simple squamous epithelium, low metabolic
rate but very permeable to water.
23-22

23. Distal Convoluted Tubule (DCT)
When the nephron loop returns to the cortex it becomes coiled
again forming the DCT;
Cuboidal epithelium – void of microvilli = end of the nephron.
23-23

24. 23-24
Collecting Ducts
Several DCT of several nephrons drain into a
straight tube called CD, which passes through the
medulla;
CD merge to form a papillary duct which will drain
into the minor calyx.
Flow of glomerular filtrate:
– glomerular capsule → PCT → nephron loop
→ DCT → collecting duct → papillary duct →
minor calyx → major calyx → renal pelvis →
ureter → urinary bladder → urethra

25. 23-25
Urine Formation I : Glomerular Filtration
Kidneys convert blood
plasma to urine in three
stages.
As we trace fluid through
the nephron it will change
names:
1. Glomerular filtrate: the
fluid in the capsular space,
similar to blood plasma but
lacks protein
2. Tubular Fluid: fluid that has
things added and removed
by the tubular cells
3. Urine: once it enters the
collecting ducts.

26. 23-26
The Filtration Membrane
Special case of capillary fluid exchange.
“The process by which water and some solutes in the blood
plasma pass from the capillaries of the glomerulus into the
capsular space of the nephron.”
In order for this to happen the fluid must pass through 3 barriers
that make up the filtration barrier.
1. Fenestrated endothelium of the capillary
2. Basement membrane
3. Filtration slits

27. 23-27
Filtration Barrier
3 barriers:
1. Fenestrated endothelium of the capillary
• Honeycomb capillaries with large holes (70-90 nm);
• Highly permeable but small enough to exclude blood cells from
filtrate;
2. Basement membrane
• Proteoglycan gel produce a negative charge, smaller fenestrations
• Exclude any particle > 8nm;
• Some smaller molecules are held back by negative electical
charge, produced by the proteoglycan gel;
3. Filtration slits
• Produced by the podocyte cells, produce negative charged
filtration slits, allow particles < 3nm to pass.

28. Anything smaller than 3 nm can pass freely into the capsular
space, include:
- water
- electrolytes
- glucose
-fatty acids
- AA
- nitrogenous wastes
- vitamins
23-28
Filtration Barrier
Some substances with a
low molecular weight are
retained in the blood
because they are bound
to plasma proteins (can
not pass through): iron,
calcium, & thyroid
hormone.
Trauma, kidney infection, strenuous exercise can
produce proteinuria (albuminuria) and hematuria
(due to damage of the filtration barriers).

29. 23-29

30. Filtration Pressure
23-30
Hydrostatic Pressure – is the physical force exerted
against a surface, (capillary), by a liquid. BP is one
example of hydrostatic pressure.
Colloid osmotic pressure (COP) – the portion of
osmotic pressure due to protein. Blood has a COP
of about 28mm Hg due mainly to albumin.

31. Filtration Pressure
23-31
Glomerular Filtration involves:
- High blood hydrostatic pressure (BHP) (60 mmHg),
due to the larger afferent arteriole compared to the
efferent;
- Hydrostatic pressure in the capsular space is about
18mm Hg, resulting in high rate of filtration, and
constant accumulation of fluid in capsular space;
- COP is about the same as anywhere = 28 – 32mm
Balance = one high outward pressure, opposed by two
inward pressures = net filtration pressure (NFP).

32. 23-32
Filtration Pressure
60out – 18in – 32in = 10 mm Hgout

33. Filtration Pressure
23-33
Do you foresee a potential problem with the glomerular
filtration process & its pressure?
Due to a higher blood hydrostatic pressure in the
glomeruli, the glomeruli make the kidney’s
vulnerable to hypertension.
Hypertension ruptures glomeruli capillaries resulting in
scarring of the kidneys = nephrosclerosis.
Over time hypertension often leads to renal failure.

34. 23-34
Glomerular Filtration Rate (GFR)
• The amount of filtrate formed per minute by two
kidneys combined;
• For every 1 mm Hg of net filtration pressure, the
kidneys produce about 12.5 mL of filtrate/minute =
filtration coefficient (Kf).
• GFR = NFP x Kf ≈125 ml/min or 180 L/day, male
• GFR = NFP x Kf ≈105 ml/min or 150 L/day, female
– filtration coefficient (Kf) depends on permeability and
surface area of filtration barrier
• 99% of filtrate reabsorbed, 1 to 2 L urine excreted

35. 23-35
Regulation of Glomerular Filtration
GFR must be precisely controlled:
↑GFR, urine output rises → dehydration,
electrolyte depletion;
↓GFR → wastes reabsorbed (azotemia possible);
GFR controlled by adjusting glomerular blood pressure
This is achieved by 3 homeostatic mechanisms:
1. Renal autoregulation
2. sympathetic control
3. hormonal control (renin and angiotensin)

36. The ability of the nephrons to adjust their own blood
flow and GFR without external (nervous, hormonal)
control.
It enables them to maintain a stable GFR even when
BP rises. (100mm → 125mm = 2L/day → 45L/day)
Helps to ensure stable fluid and electrolyte balance.
2 Mechanisms:
1. Myogenic mechanism
2. Tubuloglomerular feedback
23-36
Renal Autoregulation

37. Stabilizes GFR on the premise that smooth muscle will
contract when stretched.
So as BP rises, it will stretch the afferent arteriole;
It will contract resulting in ↓ blood flow into glomerulus
preventing the change of blood flow;
Conversely, when BP drops the smooth muscle
relaxes so blood flows easier into glomerulus.
Either way blood flow and filtration remain fairly stable.
23-37
Myogenic Mechanism

38. 23-38
Juxtaglomerular Apparatus

39. 23-39
Renal Autoregulation of GFR
∀↑ BP → constrict afferent
arteriole, dilate efferent
• ↓ BP → dilate afferent
arteriole, constrict
efferent
• Stable for BP range of 80
to 170 mmHg (systolic)
• Cannot compensate for
extreme BP

40. 23-40
Renal Autoregulation of GFR
• Myogenic mechanism
↑ BP → stretches afferent arteriole → afferent
arteriole constricts → restores GFR
• Tubuloglomerular feedback
– Macula densa on DCT monitors tubular fluid
and signals juxtaglomerular cells (smooth muscle,
surrounds afferent arteriole) to constrict afferent
arteriole to ↓ GFR

41. 23-41
Negative Feedback Control of GFR

42. 23-42
Sympathetic Control of GFR
• Renal blood vessels are highly innervated by
sympathetic nerve fibers;
• Strenuous exercise or acute conditions (circulatory
shock) activate SNS & adrenal epinepherine to
stimulate the afferent arterioles to constrict;
• Redirecting blood flow to heart, brain and skeletal
muscles where it is urgently needed;
∀ ↓ GFR and urine production.

43. 23-43
Renin-Angiotensin Mechanism
As BP drops the SNS stimulates juxtaglomerular
cells to produce renin (enzyme); converting…
Angiotensinogen (plasma protein) to angiotensin I;
In the lungs angiotensin-converting enzyme (ACE)
converts angiotensin I to angiotensin II.
Angiotensin II has multiple effects; lets say they
collectively act to raise BP by reducing water loss,
encouraging water intake, and constricting blood
vessels.

44. Renin-Angiotensin Mechanism
23-44

45. 23-45
Urine Formation II: Tubular Reabsorption and
Secretion
Converting filtrate to urine
requires the addition
and removal of
chemicals by tubular
reabsorption and
secretion.
• Trace the course of the
fluid through the
nephron, from the PCT
to the DCT.

46. 23-46
Proximal Convoluted Tubules (PCT)
• Reabsorbs 65% of GF, while removing substances from the
blood and secretes them into the tubule for disposal;
• Great length, prominent microvilli (↑ absorptive surface area)
and abundant mitochondria (ATP) for active transport;
• Tubular reabsorption – is the process of reclaiming water and
solutes from the tubular fluid and returning it to the blood;
There are 2 routes of reabsorption:
1. Transcellular route – substances pass through the cytoplasm and out
the base of the epithelial cells
2. Paracelluar route- substances pass between the epithelial cells.
PCT absorbs a greater variety of chemicals than any other part of the nephron.

47. Those substances that take the paracelluar route of
absorption will end up in the ECF, and reabsorbed in
the peritubular capillaries.
Sodium : is the key to everything else because it
creates an osmotic and electrical gradient that
drives the reabsorption of water and other solutes.
Na+
, most abundant cation in the filtrate, reabsorbed
via trans- & paracellular routes.
23-47
PCT

48. Water : kidneys 180 L of filtrate → 1-2 L of urine;
2/3 of H2O is reabsorbed by the PCT;
Thanks to the reabsorption of Na+ and other solutes,
the tubular cells and tissue fluid become hypertonic
to the tubular fluid;
23-48
PCT

49. 23-49

50. 23-50
Tubular Secretion
Tubular secretion – process
in which the renal tubule
extracts chemicals from
the capillary blood and
secretes them into the
tubular fluid;
In the PCT & nephron loop,
tubular secretion has
2 purposes
1. Waste removal
2. Acid-base balance

51. 23-51
Tubular Secretion (TS)
1. Waste removal:
- Urea, uric acid, bile acids, ammonia, catacholamines,
prostaglandins and little creatine;
- TS also clears the blood of pollutant, morphine, penicillin,
aspirin, and other drugs;
- one reason prescription drugs are taken 3-4x/day is to
keep pace with the rate of clearance and maintain a
therapeutically effective drug [ ] in the blood.
2. Acid – base balance:
- TS of hydrogen and bicarbonate ions regulates pH of body
fluids.

52. 23-52
The Nephron Loop
• Primary function of nephron loop
– Generate a salinity gradient that allows the CD to
concentrate urine and conserve water.

53. Fluid arriving at the DCT is about 20% water and 7%
salts from glomerular filtrate;
If we passed this all in the urine it would amount to
36L/day, so we have to do some reabsorption in the
DCT and CD;
DCT and CD are regulated by hormones:
1. aldosterone
2. Atrial Natriuretic peptide
3. antidiuretic hormone
4. parathyroid hormone
23-53
DCT & Collecting Duct

54. 23-54
DCT and Collecting Duct
Two types of cells in the DCT and CD:
1. Principal cells – receptors for hormones; involved
in salt/water balance;
2. Intercalated cells – involved in acid/base balance,
high mitrochondria, reabsorb K+
and sectete H+

55. 23-55
DCT and Collecting Duct
• Aldosterone effects
↓ BP → renin release → angiotensin II
formation
– angiotensin II stimulates adrenal cortex
– adrenal cortex secretes aldosterone
• promotes Na+
reabsorption → promotes water
reabsorption → ↓ urine volume → maintains BP

56. 23-56
DCT and Collecting Duct
Antidiuretic Hormone (ADH):
- secreted by the posterior pituitary in response to
dehydration and rising blood osmolarity;
- it allows the CD to become more permeable to
water, so water enters the tubular fluid and
bloodstream instead of being lost in the urine.

57. 23-57
DCT and Collecting Duct
• Atrial natriuretic peptide (ANP):
– atria secrete ANP in response to ↑ BP
– has four actions that result in excretion of more salt and
water in urine = reducing blood volume and pressure:
1. dilates afferent arteriole, constricts efferent
arteriole - ↑ GFR
2. inhibits renin/angiotensin/aldosterone pathway
3. inhibits secretion and action of ADH
4. inhibits NaCl reabsorption

58. 23-58
DCT and Collecting Duct
• Effect of PTH
↑ calcium reabsorption in DCT - ↑ blood Ca2+
↑ phosphate excretion in PCT, ↓ new bone
formation
– stimulates kidney production of calcitriol

59. The kidneys serve not only to eliminate waste, but to
prevent water loss, thus supporting the body’s fluid
balance.
As water is returned to the tissue and bloodstream,
urine becomes more concentrated.
Remember the CD begins in the cortex, receiving fluid
from numerous nephrons and passes through the
medulla;
We will now see how the kidney’s function in
accomplishing this.
23-59
Urine Formation III: Water Conservation

60. When urine enters the upper portion of the collecting duct it is
isotonic, by the time it leaves the CD it has become highly
hypertonic (more concentrated);
Two conditions allow for the CD to produce hypertonic solution:
1. the osmolarity of the extracellular fluid is 4x as high in the
lower medulla as it is in the cortex;
2. the medullary portion of the CD is more permeable to
water than to NaCl.
As urine passes down the CD through the ↑ salty medulla,
water leaves by osmosis, thus urine becomes more
concentrated.
23-60
The Collection Duct

61. 23-61
The Collecting Duct

62. 23-62
Control of Water Loss
How [ ] your urine is depends upon the body’s state of
hydration:
Drink a large amount of H2O = hypotonic urine
(called water diuresis)
– CD reabsorb NaCl
– water remains in urine
Dehydration = hypertonic urine (more concentrated)
– High blood osmolarity in a dehydrated person → stim.
Pituitary gland → release ADH → ↑ aquaporin channels, ↑
CD’s water permeability.

63. 23-63
Countercurrent Multiplier
• Recaptures NaCl and returns it to renal medulla
• Descending limb
– reabsorbs water but not salt
– concentrates tubular fluid
• Ascending limb
– reabsorbs Na+
, K+
, and Cl-
– maintains high osmolarity of renal medulla
– impermeable to water
– tubular fluid becomes hypotonic
• Recycling of urea: collecting duct-medulla
– urea accounts for 40% of high osmolarity of medulla

64. 23-64
Countercurrent Multiplier of Nephron Loop
Diagram

65. 23-65
Countercurrent Exchange System
• Formed by vasa recta
– provide blood supply to medulla
– do not remove NaCl from medulla
• Descending capillaries
– water diffuses out of blood
– NaCl diffuses into blood
• Ascending capillaries
– water diffuses into blood
– NaCl diffuses out of blood

66. 23-66
Maintenance of Osmolarity in Renal
Medulla

67. 23-67
Summary of Tubular
Reabsorption and Secretion

68. 23-68

69. 23-69
Urine & Renal Function Tests
Urinalysis (UA) – examination of the physical and
chemical properties of urine;
This is used to evaluate renal functions.

70. 23-70
Composition and Properties of Urine
6 Basic compositions and properties are:
1. Appearance
2. Odor
3. Specific Gravity
4. Osmolarity
5. pH
6. Chemical composition

71. 23-71
Composition and Properties of Urine
• Appearance
– Urochrome provides the yellowish color of urine;
– Pyuria – pus in urine making it cloudy, suggest kidney
infection;
– Hematuria – blood in urine from UTI, trauma, kidney stone.

72. Odor – fresh urine has a distinct odor, as it stands bacteria
degrade urea to ammonia producing a pungent odor;
- asparagus and other foods can change the odor;
- Diabetes mellitus imparts a sweet, fruity odor of
acetone;
- rotten odor may indicate UTI.
Specific gravity
– density of urine ranges from 1.001 -1.028.
Osmolarity - ranges from 50 mOsm/L to 1,200 mOsm/L in
dehydrated person, compared to blood (300mOsm/L) urine
can be hyper- or hypotonic depending upon the situation.
23-72
Composition and Properties of Urine

73. pH - range: 4.5 - 8.2, usually 6.0 (mildly acidic);
Chemical composition: 95% water, 5% solutes
– Urea (most abundant), followed by NaCl, KCl, lesser
amounts of creatinine, uric acid;
– Abnormal to find glucose, free hemoglobin, albumin,
ketones, or bile pigaments in the urine = indicator of
disease.
23-73
Composition and Properties of Urine

74. 23-74

75. 23-75
Urine Volume
Normal volume - 1 to 2 L/day
Polyuria > 2L/day – fluid intake & some drugs can
temporarily ↑ output to as much as 20 L/day.
Oliguria < 500 mL/day
Anuria - 0 to 100 mL/day
Low output could result from kidney disease, dehydration,
circulatory shock, prostate enlargement, and other causes.
Output drops below 400mL/day, the body cannot maintain a
safe, low concentration of waste in the blood plasma =
azotemia.

76. 23-76
Diabetes
Is any metabolic disorder resulting in chronic polyuria.
4 Types:
1. DM Type 1
2. DM Type 2
3. Gestational Diabetes
4. Diabetes insipidus
Polyuria results in the high [ ] of glucose in the renal tubule.
Glucose opposes osmotic reabsorption of water and it
passes into the urine (osmotic diuresis); result dehydration.

77. DM & gestational diabetes – the high glucose [ ] in the
tubules results from hyperglycemia (high [ ] of
glucose in blood);
1%- 3% of pregnant women experience GD due to a
reduction in the mother’s insulin sensitivity =
hyperglycemia and glycosuria;
DI – results from ADH hyposecretion. Without ADH the
CD can not reabsorb much water = passes in urine.
DM & GD characterized by glycosuria = sweet.
DI lacks glucose in urine = not sweet. 23-77
Diabetes

78. 23-78
Diuretics
Chemicals that increase urine volume.
Effects
↑ urine output
↓ blood volume
Uses
– hypertension and congestive heart failure
2 Mechanisms of action
↑ GFR (ie caffeine dilates afferent arteriole)
↓ tubular reabsorption (ie alcohol inhibits ADH secretion)

79. 23-79
Renal Function Tests
Renal clearance: volume of plasma cleared of a waste
in 1 minute
Determine renal clearance (C) by assessing blood and
urine samples: C = UV/P
– U (waste concentration in urine)
– V (rate of urine output)
– P (waste concentration in plasma)
• Determine GFR: inulin is neither reabsorbed or
secreted so its GFR = renal clearance GFR = UV/P
• Clinical GFR estimated from creatinine excretion

80. Urine is continually produced but not continually
released from the body.
Certain structure for storage and our
neurological input allow for a timely release.
Such as:
Ureters
Urinary Bladder
Urethra
23-80
Urine Storage & Elimination

81. 23-81
Ureters
About 25 cm long
– from renal pelvis to bladder (retroperitoneal)
– Pass dorsally and enter bladder from below;
– 3 layers
• adventitia – CT that binds to surrounding tissue
• muscularis - 2 layers of smooth muscle
– urine enters, it stretches and contracts in peristaltic
wave (milks the urine from the renal pelvis to the
bladder);
• mucosa - transitional epithelium
– lumen very narrow, easily obstructed or injured by
kidney stones.

82. 23-82
Kidney Stones
• aka renal calculus (painful in some cases)
– Hard granule of calcium, phosphate, uric acid,
protein.
• Causes
– Hypercalcemia, dehydration, pH imbalance, freq
UTI
• Treatment
– Stone-dissolving drugs, surgery, lithotripsy

83. 23-83

84. 23-84
Urinary Bladder
• Located in pelvic cavity, posterior to pubic
symphysis
• 3 layers
– parietal peritoneum, superiorly; fibrous adventitia rest
– muscularis: detrusor muscle, 3 layers of smooth muscle
– mucosa: transitional epithelium (rugae)
• trigone: openings of ureters and urethra, triangular
(smooth floor)
• rugae: relaxed bladder wrinkled, highly distensible
• capacity: moderately full - 500 ml, max. 700- 800 ml

85. Conveys urine out of
the body.
Female
- 3 to 4 cm
- External urethral
orifice –opening lies
between vaginal
orifice and clitoris;
23-85
Urethra

86. Male:
3 regions:
1. prostatic urethra – urinary
bladder through prostate gland;
2. membranous urethra –
passes through muscular floor
of the pelvis;
3. spongy (penile) urethra-
through penis to external
urethral orifice.
23-86
Urethra

87. In both sexes the detrusor muscle is thickened near
the urethra producing an internal urethral sphincter;
IUS compresses the urethra and retains urine in the
bladder.
Composed of smooth muscle. Which means what?
When the urethra exits the pelvic floor it becomes
surrounded by skeletal muscle to form the External
urethral sphincter = voluntary control over urination.
23-87
Urethra

88. 23-88
Urinary Bladder and Urethra

89. 23-89
Voiding Urine
• Between urinations, the detrusor muscle must be
relaxed and the urethral sphincters remain closed.
• Due to:
– Sympathetic pathways from upper lumbar reg.
• These fibers relax the detrusor, excite the internal
sphincter.
External sphincter contains skeletal muscle
innervation is from:
– Somatic motor fibers from upper sacral region

90. 23-90
Voiding Urine – Micturition (involuntary)
• 200 ml urine in bladder, stretch receptors in bladder wall,
sending signals to sacral spinal cord
• Signals ascend to two area:
– Terminate at the inhibitory synapse of sympathetic neurons (this turns
them off thus allowing urination)
– Micturition center in the pons (integrates info from the full bladder with
other center of the brain like the amygdala and cerebellum. So here is
where urination can be prompted by fear or inhibited by the
knowledge that it is inappropriate circumstances to urinate.
• Signals return from MC via reticulospinal tract to the detrusor
muscle (via parasympathetic neurons). These are excitatory
and stimulate the bladder to contract.
– The contraction further excites the stretch receptors and starts a
positive feedback loop.
– Also relaxes internal urethral sphincter, (urine is voided unless
inhibited by the brain.)

91. Final obstacle is the external urethral sphincter:
- descending signals from the cerebral cortex travel
via corticospinal tracts to the sacral spinal cord, and
inhibits the somatic motor neruons;
- voluntary component of micturition;
If we must suppress the urge to urinate results in the
stretch receptors fatiguing and stop firing.
If the bladder is not full and we want to urinate (trip or
lecture) we use the valsalva maneuver to compress
the bladder and excite the stretch receptors.
23-91
Voiding Urine – Micturition (involuntary)

92. 23-92
Neural Control of Micturition

93. 23-93

94. 23-94
Hemodialysis
Responsible for INSIGHT 23.5 on page 925