1. Urinary System
Chapter 25

2. Urinary System: Anatomy
Kidneys
Ureters
Urinary Bladder
Urethra
Urethral opening
Figure 25.1a
Urinary System: Anatomy

3. Renal Anatomy Paired retroperitoneal bean-shaped organs
Located in the dorsal upper lumbar region
Encased in the renal fat pad

4. Renal Anatomy: Internal
Figure 25.3b
Superficial: renal cortex
Renal medulla: deep to the cortex
Contains medullary pyramids separated by renal columns
Medullary pyramids end in the papilla
Renal pelvis: merges with the ureter

5. Renal Anatomy: Functional
Nephron: the functional unit of the kidney
Figure 25.4b

6. Nephron: Anatomy Bowman’s capsule is continuous with the
Proximal convoluted tubule (PCT); cells cuboidal with microvilli
Loop of Henle
Descending limb; simple squamous
Ascending limb; simple cuboidal to columnar
Distal convoluted tubule; cuboidal
Collecting duct; cuboidal
Figure 25.4b

7. Nephron Cortical nephrons: located in the renal cortex (about 80%)
Juxtamedullary nephrons: at the cortical-medullary junction with the loop of Henle deep into the medulla
Figure 25.5a

8. Nephron: Anatomy Afferent arteriole Efferent arteriole
Glomerulus: fenestrated capillaries
Efferent arteriole
Peritubular capillaries (PCT and DCT) involved in reabsorption
Vasa recta (medullary loop of Henle) involved in forming concentrated urine
Figure 25.5a

9. Nephron: Anatomy Afferent arteriole bifurcates to form the -
Figure 25.7a
Afferent arteriole bifurcates to form the -
Glomerulus (a capillary bed encased in the glomerular capsule) which is drained by the -
Efferent arteriole
{The only arteriole g capillary g arteriole sequence in the body}

10. Nephron: Anatomy
Figure 25.7a
Glomerular (Bowman’s) capsule surrounds the glomerulus
Podocytes adhere to the glomerulus (visceral membrane)
Spaces between the foot processes of the podocytes form filtration slits
Capsular space surrounded by simple squamous epithelium (parietal membrane)

11. Nephron: Anatomy
Figure 25.7a
Figure 25.7c
Filtration membrane; lies between the capillary blood and the capsular space
Fenestrated endothelium of the capillaries
Visceral membrane of glomerulus (podocytes)
Fused basement membranes

12. Nephron: Anatomy Juxtaglomerular (JG) apparatus
Figure 25.6
Juxtaglomerular (JG) apparatus
DCT in close proximity to the afferent arteriole
JG cells around the afferent arteriole sense blood pressure and produce renin
Macula densa cells in DCT sense flow rate and chemical or osmotic changes in the filtrate

13. Mechanisms of Urine formation
Kidneys filter ~ 180 L of fluid/day
Urine production ~ 1.8 L/day

14. Mechanisms of Urine Formation
Urine formation involves three phases
Glomerular filtration
Tubular reabsorption
Tubular secretion
Figure 25.8

15. Glomerular Filtration
Figure 25.9
A passive process due to hydrostatic pressure across the filtration membrane
Filtration membrane: extremely permeable to water and solutes (molecules < 3nm pass freely)
Glomerular blood pressure is higher than other capillary beds (55mm Hg rather than 18mm Hg)
Filtrate formed is a protein free filtrate of plasma

16. Glomerular Filtration
Figure 25.9
Composed of 3 forces:
Glomerular hydrostatic pressure (HPg) = glomerular blood pressure
[Blood g capsule]
Colloid osmotic pressure (OPg) = osmotic flow toward blood;
[Capsule g blood]
Capsular hydrostatic pressure (HPc) = pressure exerted by capsular fluid
Net Filtration Pressure (NFP)

17. Glomerular Filtration
Net Filtration Pressure (NFP)
NFP = HPg – (OPg + HPc) = 55 – ( ) = 10mm Hg
Figure 25.9

18. Glomerular Filtration Rate (GFR)
GFR = volume of filtrate formed each minute
3 factors
Total surface area available for filtration
Filtration membrane permeability
NFP: pressure changes have dramatic impact on filtrate volume.
Normal GFR = ml/min

19. Glomerular Filtration Rate (GFR)
Regulation of GFR:
Intrinsic
Extrinsic
GFR needs to be held within a narrow range for appropriate reabsorption to occur

20. Regulation of GFR: Intrinsic
Renal autoregulation:
Myogenic mechanism: Afferent arteriole smooth muscle responds to stretch
h systemic BP g Afferent constriction
i systemic BP g Afferent dilation
Both help to maintain GFR

21. GFR: Intrinsic Tubuloglomerular feedback (macula densa)
Figure 25.6
Tubuloglomerular feedback (macula densa)
Macula densa cells of Juxtaglomerular (JG) Apparatus sense filtrate flow rate & osmotic signals [NaCl]
i Flow or i OSM promotes afferent arteriole dilation
h Flow or h OSM promotes afferent arteriole constriction

22. Regulation of GFR: Extrinsic
Extrinsic control: Neural & Hormonal
Neural = SNS
Stress shunts blood to vital organs:
Norepinephrine causes vasoconstriction of afferent arterioles & inhibits filtrate formation
Triggers renin / angiotensin system (hormonal)

23. Regulation of GFR: Extrinsic
Renin / angiotensin system
Other factors:
Prostaglandins
Nitric Oxide
Adenosine
Endothelin
Atrial Natriuretic Peptide

24. Extrinsic GFR regulation
Figure 25.6
Renin release:
Reduced stretch (i.e. i BP) stimulates JG cells to release renin
Stimulation of macula densa that cause vasodilation (decreased flow or decreased OSM) also causes JG cells to release renin
Direct stimulation of JG cells by b-adrenergic receptors (SNS)

25. Renin/Angiotensin System
Renin acts on Angiotensin to form Angiotensin I
Angiotensin I is converted to Angiotensin II
By Angiotensin Converting Enzyme (ACE) in lung capillary endothelium.

26. Renin/Angiotensin System
Angiotensin II: actions
Vasoconstrictor: h systemic BP
Enhances Na+ reabsorption in PCT (Proximal Convoluted Tube)
Stimulates the adrenal cortex to release aldosterone causing renal tubule reabsorption of Na+

27. Renin/Angiotensin System
Angiotensin II: actions
Vasoconstriction of efferent arteriole is greater than afferent resulting in h HPg which helps to maintain GFR near normal
Causes mesangial cells to reduce surface area of glomerular capillaries which i GFR
Stimulates thirst centers

28. Regulation of GFR: Extrinsic
Other factors:
Prostaglandins (PGE2, PGI2) vasodilators in response to SNS & angiotensin II:
oppose effects of norepinephrine and angiotensin II
Nitric Oxide: vasodilator from vascular endothelium
Adenosine: systemic vasodilator but acts to constrict renal vasculature
Endothelin: vasoconstrictor from vascular endothelium
Atrial Natriuretic Peptide (ANP)
Responds to h BP to inhibit Renin/Angiotensin system

29. Figure 25.10

30. Tubular Reabsorption
Every 45 min the entire blood plasma volume is filtered by the kidneys.
Most tubular contents are reclaimed (reabsorbed).

31. Routes of Water and Solute Reabsorption
Three membrane barriers
Luminal membrane
Basolateral membrane
Endothelial membrane
Figure 25.11

32. Routes of Water and Solute Reabsorption
Na+ Reabsorption:
Diffuses into tubular cell (co-transport)
Luminal membrane
Actively transported to interstitial fluid
Basolateral membrane
Diffusion through endothelium
Endothelial membrane
Na+ reabsorption provides energy & mechanisms for reabsorbing most other solutes
Figure 25.12

33. Reabsorption of water / ions / nutrients
Passive tubular reabsorption:
Na+ ions establish an electro-chemical gradient favoring anions (Cl- & HCO3-)
Na+ establishes an osmotic gradient allowing water (via aquaporins) to leave water permeable region (PCT & Loop)
As water leaves the tubules the remaining solutes become more concentrated & follow their diffusion gradient out of the filtrate (cations, fatty acids, urea)

34. Reabsorption of water / ions / nutrients
Secondary active transport:
Na+ moves down its diffusion gradient in co-transport with specific substances (glucose, amino acids, lactate, vitamins, most cations)
Transport maximum for various solutes is dependent upon the number of carriers for Na+ co-transport
Plasma proteins: endocytosed & hydrolyzed to amino acids

35. Non-Reabsorbed Substances
Non-reabsorption due to:
Lack of carriers
Lipid insolubility
Molecules too large to pass through membrane pores
Non-reabsorbed substances are usually nitrogenous wastes
Urea (50-60% reclaimed)
Creatinine: not reabsorbed

36. Proximal Convoluted Tubule (PCT); Reabsorption
PCT is the most active in reabsorption:
All glucose, lactate, & amino acids
Most Na+, H2O, HCO3- , CL- and K+
65% Na+ and H2O
90% HCO3-
50% CL-
55% K+
Figure 25.4b
Table 25.1
Summary of Tubular Reabsorption

37. Loop of Henle; Reabsorption
Descending limb:
H2O reabsorbed by osmosis
Ascending limb:
Na+, Cl-, K+ active transport
Ca2+, Mg2+ passive transport
Figure 25.4b

38. Distal Convoluted Tubule; Reabsorption
Na+ : Aldosterone mediated
Ca2+ : PTH mediated
Cl- : diffusion
H2O : osmosis
Figure 25.4b

39. Collecting Duct; Reabsorption
Most reabsorption in collecting duct due to hormonal influences
Na+ : Aldosterone vs ANP
HCO3-, H+, K+, Cl- :
passive transport & co-transport
H2O : ADH dependent
Urea: facilitated diffusion

40. Tubular Secretion Important functions:
Tubules also secrete substances into the filtrate.
H+, K+, NH4+, creatinine & organic acids
Important functions:
Disposes of substances not in original filtrate (certain drugs)
Replaces substances in filtrate that were reabsorbed (urea/uric acid)
Disposes of excess K+
Controls pH (Ch. 26)
Urine consists of filtered & secreted substances

41. Tubular Secretion Important functions:
Figure 25.4b
Important functions:
Disposes of substances not in the original filtrate (certain drugs)
Replaces elements in filtrate that were reabsorbed (urea/uric acid)
Disposes of excess K+
Controls pH (Ch. 26)
Urine consists of filtered & secreted substances

42. Regulation of Urine Concentration and Volume
Body fluids ~ 300 mOsm (milliosmoles)
Counter current multiplier:
Interaction between the filtrate in the juxtamedullary Loop of Henle & blood flow in vasa recta.

43. Regulation of Urine Concentration
Osmotic Gradient in the Renal Medulla
Capillary Beds
Loop of Henle: Countercurrent Mechanism
Figure 25.5a
Figure 25.13
Figure 25.14

44. Counter Current Multiplier
Descending loop:
freely permeable to H2O
impermeable to solutes
H2O leaves filtrate by osmosis
Filtrate becomes highly concentrated to 1200 mOsm at deepest portion of loop
Figure 25.14

45. Counter Current Multiplier
Ascending Loop
impermeable to H2O
permeable to NaCl
Most NaCl re-absorption occurs in ascending thick segment
Filtrate becomes more dilute as it ascends (100 mOsm)
Figure 25.14
Interstitial fluid develops a concentration gradient that is maintained by the movement of H2O and NaCl.

46. Counter Current Multiplier
Medullary Collecting Ducts are permeable to urea
Urea diffuses into interstitium
Urea makes a large contribution to the increased interstitial osmolality
Figure 25.14

47. Vasa Recta: Counter Current Exchanger
Slow blood flow
Freely permeable to H2O & NaCl
As blood descends it loses H2O & gains NaCl
As blood ascends into cortex it gains H2O & loses NaCl
Protects medullary gradient by preventing rapid removal of salt
Figure 25.14

48. Loop of Henle: Countercurrent Mechanism
Figure 25.14

49. Regulation of Urine Concentration and Volume
Dilute urine: reaches the end of Ascending Loop
Production of dilute urine requires nothing further
Absence of ADH

50. Regulation of Urine Concentration and Volume
Concentrated urine:
ADH dependent reabsorption of H2O from collecting ducts
ADH induces production of aquaporin complexes in collecting duct

51. Formation of Dilute and Concentrated Urine
Figure 25.15a, b

52. Regulation of Urine Concentration and Volume
Diuretics:
Osmotic (retains H2O in filtrate)
Inhibition of ADH
Inhibition of Na+ reabsorption

53. Urine Physical Characteristics: Clear/yellow
Slight aroma g develops ammonia on standing
pH: usually ~ 6.0 (range )
Specific gravity:
Chemical Composition:
95% H2O
5% Solutes: mostly urea, uric acid & creatinine

54. Ureters Tri-layered: Mucosa Muscularis Adventitia
Moves urine by peristalsis
Figure 25.18a, b

55. Urinary Bladder Tri-layered: Mucosa: transitional epithelium
Figure 25.18a, b
Tri-layered:
Mucosa: transitional epithelium
Muscularis: Detrusor muscle
Adventitia: except superior portion – which has a peritoneal covering
Capacity: ml

56. Urinary Bladder
Figure 25.18a, b

57. Urethra Thin walled muscular tube
Figure 25.18a, b
Thin walled muscular tube
Internal urethral sphincter: involuntary
External urethral sphincter: voluntary

58. Urethra Females: short urethra g external urethral orifice
Figure 25.18a, b
Females: short urethra g external urethral orifice
Males: prostatic urethra g membranous urethra g penile urethra g external urethral orifice

59. Urine Storage Storage reflex:
As bladder fills a sympathetic spinal cord reflex causes;
Contraction of internal & external urethral sphincters
Inhibition of contraction of detrusor muscle
Figure 25.20a

60. Micturition (Voiding or Urination)
Activation by visceral afferent fibers
Stimulates Pontine Micturation center which then:
Stimulates contraction of detrusor muscle
Inhibits contraction of internal sphincter
Inhibits sympathetic & somatic fibers allowing relaxation of external sphincter
Voluntary contraction of external sphincter can postpone voiding
Figure 25.20b