The urinary system filters wastes from the blood and removes them from the body as urine. The kidneys filter blood to form urine and regulate fluid and electrolyte balance. Each kidney contains millions of nephrons, the functional units of filtration. Urine is transported from the kidneys to the bladder via ureters and is stored in the bladder until elimination through the urethra. Hormones help regulate urine concentration and output to maintain fluid homeostasis.

1. 23-1
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
• What is missing from the following
illustration?

4. 23-4
Urinary System
Two kidneys
Two ureters Urethra

5. 23-5
Kidney Location

6. 23-6
Kidney Functions
• Filters blood plasma
– returns useful substances to blood
– eliminates waste
• Regulates
– osmolarity of body fluids, blood volume, BP
– acid base balance
• Secretes
– renin and erythropoietin
• Detoxifies free radicals and drugs
• Gluconeogenesis

7. 23-7
Nitrogenous Wastes
• Urea
– by product of protein metabolism
– proteins→amino acids →NH2 removed
→forms ammonia, liver converts to
urea
• Uric acid
– nucleic acid catabolism
• Creatinine
– creatine phosphate catabolism
• Renal failure
– azotemia: ↑BUN (blood urea nitrogen),
nitrogenous wastes in blood
– uremia: toxic effects as wastes
accumulate
• Hemodialysis: removes nitrogenous
wastes

8. 23-8
Excretion
• Separation of wastes from body fluids and
eliminating them; by four systems
– respiratory: CO2
– integumentary: water, salts, lactic acid, urea
– digestive: water, salts, CO2, lipids, bile
pigments, cholesterol
– urinary: many 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
• CT coverings (3 layers protect kidney)
– renal fascia: binds kidney to abdominal wall
– adipose capsule: cushions and supports kidney;
holds it in place
– renal capsule: encloses kidney like cellophane wrap;
protects from trauma and infection

10. 23-10
Anatomy of Kidney
• Renal cortex: outer 1 cm
• Renal medulla: renal columns, pyramids - papilla
• Lobe of kidney: pyramid and it’s overlying cortex

11. 23-11
Wedge-Shaped Lobe of Kidney
• Glomerular filtrate collects in capsular space, flows into renal tubule

12. 23-12
Renal Corpusle

13. 23-13
The NephronRenal Corpusle
Proximal convoluted
tubule (PCT)
– longest, most coiled,
simple cuboidal with
brush border
• Nephron loop - U
shaped; descending
and ascending limbs
– thick segment
(simple cuboidal)
initial part of
descending limb and
part or all of
ascending limb,
active transport of
salts
– thin segment (simple
squamous) very
water permeable

14. 23-14
The Nephron - Continued
• Distal
convoluted
tubule (DCT)
– cuboidal,
microvilli
– minimal
• Collecting duct
– several DCT’s
join

15. 23-15
Fluid Flow
• Flow of glomerular filtrate: from filtrate to
exit from the body
– glomerular capsule → PCT → nephron loop →
– DCT → collecting duct → papillary duct →
– minor calyx → major calyx → renal pelvis →
– ureter → urinary bladder → urethra

16. 23-16
Nephrons
• True proportions of nephron
loops to convoluted tubules
shown
• Cortical nephrons (85%)
– short nephron loops
– efferent arterioles branch off
peritubular capillaries
• Juxtamedullary nephrons
(15%) – most responsible for
salt gradient
– very long nephron loops,
__________________________
Whether short or long nephron
loops they maintain salt
gradient, helps conserve water

17. 23-17
Nephron Diagram
• Peritubular capillaries shown only on right of nephron

18. 23-18
Path of Blood Through Kidney
• Renal artery
→ interlobar arteries (up renal columns, between lobes)
→ arcuate arteries (over pyramids)
→ interlobular arteries (up into cortex)
→ afferent arterioles
→ glomerulus (cluster of capillaries)
→ efferent arterioles (near medulla → vasa recta)
→ peritubular capillaries
→ interlobular veins → arcuate veins → interlobar veins
• Renal vein

19. 23-19
Blood Supply Diagram

20. 23-20
Urine Formation Preview

21. 23-21
Filtration Membrane Diagram

22. 23-22
Filtration Membrane
• Fenestrated endothelium
– 70-90nm pores exclude blood
cells
• Basement membrane
– proteoglycan gel, negative
charge excludes molecules >
8nm
– blood plasma 7% protein,
glomerular filtrate 0.03%
• Filtration slits
– podocyte arms have pedicels
with negatively charged filtration
slits, allow particles < 3nm to
pass

23. 23-23
Filtration = Diffusion dependent on -Pressure

24. 23-24
Glomerular Filtration Rate (GFR)
• Filtrate formed per minute
• 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

25. 23-25
Effects of GFR Abnormalities
∀↑GFR, urine output rises → dehydration,
electrolyte depletion
∀↓GFR → wastes reabsorbed (azotemia
possible)
• GFR controlled by adjusting glomerular
blood pressure
– autoregulation
– sympathetic control
– hormonal mechanism: renin and angiotensin

26. 23-26
Juxtaglomerular Apparatus
- vasomotion
- monitor salinity

27. 23-27
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

28. 23-28
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

29. 23-29
Negative Feedback Control of GFR

30. 23-30
Sympathetic Control of GFR
• Strenuous exercise or acute conditions
(circulatory shock) stimulate afferent
arterioles to constrict
∀↓ GFR and urine production, redirecting
blood flow to heart, brain and skeletal
muscles
• BP drop causes JG cells to secrete Renin
which acts on angiotensinogen >
angiotensin I

31. 23-31
Renin-Angiotensin-Aldosterone

32. 23-32
Effects of Angiotensin II

33. 23-33
Effects of Angiotensin II - continued
• Stimulates Adrenal Cortex to secrete
aldosterone > Na+
and water retention by
DCT
• Stimulates ADH secretion > water
retention
• Stimulates Hypothalmus thirst center

34. 23-34
Tubular Reabsorption and Secretion

35. 23-35
Peritubular Capillaries
• Blood has unusually high COP (colloid osmotic
pressure) here, and BHP (blood hydrostatic pressure) is
only 8 mm Hg (or lower when constricted by
angiotensin II); this favors reabsorption
• Water absorbed by osmosis and carries
other solutes with it (solvent drag)

36. 23-36
Proximal Convoluted Tubules (PCT)
• Reabsorbs 65% of GF
• Great length, prominent microvilli and abundant
mitochondria for active transport
• Reabsorbs greater variety of chemicals than
other parts of nephron
– transcellular route - through epithelial cells of PCT
– paracellular route - between epithelial cells of PCT
• Transport maximum: when transport proteins of cell
membrane are saturated; blood glucose > 220 mg/dL
some remains in urine (glycosuria); glucose Tm = 320
mg/min

37. 23-37
Mechanisms of Reabsorption in the Proximal Convoluted Tubule

38. 23-38
Tubular Secretion of PCT
and Nephron Loop
• Waste removal
– urea, uric acid, bile salts, ammonia,
catecholamines, many drugs
• Acid-base balance
– secretion of hydrogen and bicarbonate ions
regulates pH of body fluids
• Primary function of nephron loop
– water conservation -2/3 reabsorbed by PCT
– generates salinity gradient, allows CD to conc.
urine
– also involved in electrolyte reabsorption

39. 23-39
DCT and Collecting Duct
• Principal cells – receptors for hormones;
involved in salt/water balance
• Intercalated cells – involved in acid/base
balance
• Function
– fluid reabsorption here is variable, regulated
by hormonal action

40. 23-40
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

41. 23-41
DCT and Collecting Duct
• Effect of ADH
– dehydration stimulates hypothalamus
– hypothalamus stimulates posterior pituitary
– posterior pituitary releases ADH
– ADH ↑ water reabsorption
↓ urine volume

42. 23-42
DCT and Collecting Duct
• Atrial natriuretic peptide (ANP)
– atria secrete ANP in response to ↑ BP
– has four actions:
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
• Promotes Na+
and water excretion, ↑ urine
volume, ↓ blood volume and BP

43. 23-43
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

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Collecting Duct Concentrates
Urine
• Osmolarity 4x as concentrated
deep in medulla
• Medullary portion of CD is more
permeable to water than to NaCl

45. 23-45
Control of Water Loss
• Producing hypotonic urine
– NaCl reabsorbed by cortical CD
– water remains in urine
• Producing hypertonic urine
– dehydration → ↑ ADH → ↑ aquaporin
channels, ↑ CD’s water permeability
– more water is reabsorbed
– urine is more concentrated

46. 23-46
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

47. 23-47
Countercurrent Multiplier
of Nephron Loop Diagram

48. 23-48
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

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Maintenance of Osmolarity
in Renal Medulla

50. 23-50
Summary of Tubular
Reabsorption and Secretion

51. 23-51

52. 23-52
Composition and Properties of Urine
• Appearance
– almost colorless to deep amber; yellow color due to
urochrome, from breakdown of hemoglobin (RBC’s)
• Odor - as it stands bacteria degrade urea to ammonia
• Specific gravity
– density of urine ranges from 1.001 -1.028
• Osmolarity - (blood - 300 mOsm/L) ranges from
50 mOsm/L to 1,200 mOsm/L in dehydrated person
• pH - range: 4.5 - 8.2, usually 6.0
• Chemical composition: 95% water, 5% solutes
– urea, NaCl, KCl, creatinine, uric acid

53. 23-53

54. 23-54
Urine Volume
• Normal volume - 1 to 2 L/day
• Polyuria > 2L/day
• Oliguria < 500 mL/day
• Anuria - 0 to 100 mL/day

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Diabetes
• Chronic polyuria of metabolic origin
• With hyperglycemia and glycosuria
– diabetes mellitus I and II, insulin
hyposecretion/insensitivity
– gestational diabetes, 1 to 3% of pregnancies
• ADH hyposecretion
– diabetes insipidus; CD ↓ water reabsorption

56. 23-56
Diuretics
• Effects
↑ urine output
↓ blood volume
• Uses
– hypertension and congestive heart failure
• Mechanisms of action
↑ GFR
↓ tubular reabsorption

57. 23-57
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

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Urine Storage and Elimination
• Ureters (about 25 cm long)
– from renal pelvis passes dorsal to bladder and
enters it from below, with a small flap of
mucosa that acts as a valve into bladder
– 3 layers
• adventitia - CT
• muscularis - 2 layers of smooth muscle with
3rd
layer in lower ureter
– urine enters, it stretches and contracts in
peristaltic wave
• mucosa - transitional epithelium
– lumen very narrow, easily obstructed

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Urinary Bladder and Urethra -
Female

60. 23-60
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
• trigone: openings of ureters and urethra,
triangular
• rugae: relaxed bladder wrinkled, highly
distensible
• capacity: moderately full - 500 ml, max. - 800 ml

61. 23-61
Female Urethra
• 3 to 4 cm long
• External urethral orifice
– between vaginal orifice and
clitoris
• Internal urethral sphincter
– detrusor muscle thickened,
smooth muscle, involuntary
control
External urethral sphincter
skeletal muscle, voluntary
control

62. 23-62
Male Bladder and Urethra
• 18 cm long
• Internal urethral sphincter
• External urethral sphincter
3 regions
prostatic urethra
during orgasm receives semen
membranous urethra
passes through pelvic cavity
spongy urethra

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Voiding Urine - Micturition
• 200 ml urine in bladder, stretch receptors send
signal to sacral spinal cord
• Signals ascend to
– inhibitory synapses on sympathetic neurons
– micturition center (integrates info from amygdala, cortex)
• Signals descend to
– further inhibit sympathetic neurons
– stimulate parasympathetic neurons
• Result
– urinary bladder contraction
– relaxation of internal urethral sphincter
• External urethral sphincter - corticospinal tracts to
sacral spinal cord inhibit somatic neurons - relaxes

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Neural Control of Micturition

65. 23-65
Hemodialysis

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