The document summarizes the structure and function of the nephron, the functional unit of the kidney. There are three main types of nephrons - cortical, juxtamedullary, and intermediate. The nephron consists of a glomerulus and renal tubule. The glomerulus is a tuft of capillaries surrounded by Bowman's capsule that filters the blood to form urine. The renal tubule reabsorbs useful solutes and secretes waste products to regulate water and electrolyte balance as the urine is formed and concentrated.

1. urinary SyStem
1

2. Types of Nephron
(1) Cortical (sub-capsular) nephrons are the most
common type of nephron. Their renal corpuscles are
found in the outer cortex and their short loops are formed
by distal straight tubules in the outer medulla.
(2) Juxtamedullary nephrons account for approximately
20% of all nephrons; their corpuscles are found adjacent
to the medulla. They are long looped nephrons whose
loop is formed by thin limbs. These nephrons are
responsible for producing the urine-concentrating
mechanism of the kidney.
(3) Intermediate nephrons have their renal corpuscles in
the mid-region of the cortex and their loops are
intermediate in length.
2

3. Glomerulus

Structure of a Nephron:


2 main structures
 Glomerulus – a knot of
capillaries
 Renal tubule (about 2
inches long).
The endothelium of the
glomerular capillaries is
fenestrated
3

4. Glomerular capsule

a double-walled
epithelial cup capillaries


External parietal layer:
(simple sequamous
epithelium)
Visceral layer
(Podocysts)
4

5. Visceral layer of Bowman’s capsule


The podocytes possess processes called
pedicles that wrap around the capillaries and
interdigitate to form filtration slits approximately
25nm wide.
The mesangial cells serve a phagocytic function
and keep the basement membrane clear of
debris. All 3 components combine to retains
cells and macromolecules (proteins > 70 kd) but
permit the passage of water and small solutes
(salts, glucose, amino acids, nitrogenous
wastes) into the urinary space as filtrate.
5

6. Filtrate membrane

Consist of 3 layers



Capillary endothelium
Basement membrane
(intervening basement
membrane)
Foot processes of
podocytes of glomerular
capsule.
6

7. 7

8. Urinary Filtration Membrane

Endothelial cell


Basal lamina




70-90 nm fenestra restrict
proteins > 70kd
heparan sulfate is
negatively charged
produced by endothelial
cells & podocytes
phagocytosed by mesangial
cells
Podocytes



pedicels 20-40 nm apart
diaphragm 6 nm thick with
3-5 nm slits
podocalyxin in glycocalyx is
negatively charged
8

9. 9

10. Renal tubule – site of selective reabsorption / secretion of solutes


The renal tubule serves to recover water
and other desirable solutes (sugar, ions,
small proteins) from the filtrate.
The proximal convoluted tubule receives
filtrate from the urinary space and is the
site of the selective re-absorption of most
solutes including all the glucose and
amino acids and most of the water and
salts.
10

11. 11

12. Renal tubule

Proteins are absorbed by pinocytosis,
broken down by lysosomal degradation
and released as amino acids to the
peritubular capillary network. This is also
the site of pH balancing and elimination
(active transport) of creatine.
12

13. 13

14. Histology of tubules




Structurally the proximal convoluted tubule is formed by
a simple cuboidal to low columnar epithelium.
The apical surface is covered with microvilli creating a
light microscopic brush border that increases the surface
area for ion absorption.
The cells are tightly bound to one another to seal off the
intercellular spaces from the lumen using junctional
processes apically and interdigitating plicae (folds)
laterally.
Basally, interdigitating processes contain numerous
mitochondria which create light microscopic basal
striations that are associated with ion transport.
14

15. Histology…





Proximal convoluted tubules are the most abundant
tubule in the cortex.
They have a eosinophilic cytoplasm with a basal
nucleus.
The brush border is rarely preserved and the indistinct
cells margins are due to basal and lateral border
interdigitations.
The proximal straight tubules are located within or near
the medulla, depending upon the type of nephron.
They are formed by lower cuboidal epithelium and their
microvilli and basal and lateral interdigitations are less
well developed. Histologically they are similar to
proximal convoluted tubules
15

16. Histology….




Descending thin tubules are located within the medulla
and are formed by low cuboidal to squamous epithelium.
The microvilli are poorly developed as are the basal and
lateral interdigitations creating a very leak cell that
serves as the site of passive transport of ions (inward)
and water (outward) between the lumen and interstitium.
The ascending thin tubules are also located within the
medulla and are similar in appearance to descending
thin tubules.
However, these tubes are impermeable to water and
permit passive transport of NaCl into the interstitium.
16

17. Histology….




Distal straight tubules are located within both the
medulla and cortex .
Histologically these appear as a simple cuboidal
epithelium with sparse microvilli and lacking
lateral interdigitations.
The nucleus is apical and basal interdigitations
with abundant mitochondria are present.
These tubes are impermeable to water and are
the site of ion transport from the lumen to the
interstitium that establishes the ion gradient of
medullary interstitium.
17

18. Histology….

Distal convoluted tubules are located
within the cortex. They are approximately
1/3rd as long as their proximal
counterparts. They contact the renal
corpuscle forming a macula densa which
is part of the juxtaglomerular apparatus
(see below). Histologically they are similar
to the distal straight tubules and also
function in ion exchange.
18

19. Collecting tubules….

The collecting system starts in the cortex as a
continuation of the distal convoluted tubules and
descend through the medulla.

As the ducts coalesce and increase in size, the
cells of the tubes change from somewhat
squamous to cuboidal to columnar and similarly
become increasing stratified.
They terminate at the tip of the renal pyramid as
the papillary ducts .

19

20. Collecting tubules….



Histologically they appear as tubes with
distinguishable cells margins, central nuclei and
poorly staining cytoplasm.
At the EM level many posses a single cilium and
sparse microvilli.
The collecting system functions to concentrate
urine through ADH-regulated and ADHindependent water channels
20

21. PCT


the walls formed by cuboidal
epithelial cells
Their Luminal surfaces have
dense microvilli.
21

22. Renal (Uriniferous) Tubule



Proximal convoluted tubule (PCT)
 longest, most coiled, simple
cuboidal with brush border
Nephron loop - U shaped;
descending + 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
Distal convoluted tubule (DCT)
 cuboidal, minimal microvilli
22

23. Kidney Cortex – PCT (P) & DCT (D)
P
D
D
P
23

24. Long section of Collect tubules (CT) and PCT (P)
CT
P
CT
24

25. 25

26. Proximal Convoluted Tubules (PCT)




Reabsorbs 65% of GF to peritubular capillaries
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 plasma
membrane are saturated; glucose > 220 mg/dL remains in
urine (glycosuria)
26

27. 27

28. Loop of Henle

Descending limb



Proximal part: similar
cells of the PCT
Thin segment: simple
sequamous
epithelium
Ascending limb

Cuboidal epithelium
or low columnar→
thick segment
28

29. Thin loop of Henle
29

30. Thick loop of Henle
30

31. DCT



The wall: cuboidal
epithelium (like PCT)
Thinner
Lack micrivilli
31

32. Collecting ducts

Have 2 types of cells

Intercalated cells :



cuboidal cells with
extensive microvilli
Maintain the acid –base
balance (blood pH)
Principal cells :



short microvilli
maintain water-sodium
(Na+) balance
Have receptors to
antidiuretic hormone &
aldosterone
32

33. DCT and Collecting Duct

Effect of aldosterone







↓ BP causes angiotensin II formation
angiotensin II stimulates adrenal cortex
adrenal cortex secretes aldosterone
aldosterone promotes Na+ reabsorption
Na+ reabsorption promotes water reabsorption
water reabsorption ↓ urine volume
BP drops less rapidly
33

34. DCT and Collecting Duct 2

Effect of atrial natriuretic factor (ANF)





↑ BP stimulates right atrium
atrium secretes ANF
ANF promotes Na+ and water excretion
BP drops
Effect of ADH





dehydration stimulates hypothalamus
hypothalamus stimulates posterior pituitary
posterior pituitary releases ADH
ADH ↑ water reabsorption
34
urine volume ↓

35. Collecting Duct Concentrates Urine


Osmolarity 4x as
high deep in
medulla
Medullary portion of
CD is permeable to
water but not to
NaCl
35

36. Cortical Collecting Tubule
36

37. Medullary Collecting Tubule
37

38. Control of Water Loss

Producing hypotonic urine



NaCl reabsorbed by cortical CD
water remains in urine
Producing hypertonic urine






GFR drops
tubular reabsorption ↑
less NaCl remains in CD
ADH ↑ CD’s water permeability
more water is reabsorbed
urine is more concentrated
38

39. 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
39
medulla

40. Countercurrent Multiplier
of Nephron Loop Diagram
40

41. Countercurrent Exchange System

Formed by vasa recta



Descending capillaries



provide blood supply to medulla
do not remove NaCl from medulla
water diffuses out of blood
NaCl diffuses into blood
Ascending capillaries


water diffuses into blood
NaCl diffuses out of blood
41

42. Maintenance of Osmolarity
in Renal Medulla
42

43. Summary of Tubular
Reabsorption and Secretion
43

44. Interstitium…..

The interstitium is the connective tissue
matrix of the kidney. [Yes. it is the stroma
of the kidney, but remember not all
stromas are connective tissue.] It is
sparse in the cortex and most abundant
the deep medulla. The extracellular matrix
consists of collagen fibers and
glycosamionglycans (GAGs) and the cells
are fibroblasts and macrophages.
44

45. 45

46. 46

47. 47

48. 48

49. 49

50. 50

51. 51

52. 52

53. Ureters

Ureters:The ureters are muscular tubes
connecting the renal pelvis to the urinary
bladder.


25 cm long
Enters on the floor of bladder
53

54. 54

55. 55

56. Ureter
56

57. Ureter – folded mucus membrane
Transitional Epithelium 57

58. Urinary Bladder


Urinary Bladder
 Muscular sac on floor of
pelvic cavity.
Located in pelvic cavity, posterior to
pubic symphysis


Average bladder volume is
500 ml
Max capacity is 700-800 ml
58

59. Urinary Bladder





Consists from:
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
The outer layer may contain Pacinian corpuscles
59

60. Urinary Bladder and Urethra - Female
60

61. Bladder
61

62. Urinary Bladder
62

63. 63

64. 64

65. Urethra

Urethra






Conveys urine out of body
Female urethra – 3 - 4 cm
Opens into external urethral oriface
Lies between vaginal oriface and clitoris
Male urethra – 18 cm
3 regions



Prostatic urethra – 2.5 cm
Membranous urethra – 0.5 cm
Penile urethra – 15 cm
65

66. Urethra
66

67. Blood supply…



Arterial: renal artery > inter-lobar arteries >
arcuate arteries > inter-lobular arteries > afferent
arterioles > renal corpuscle (capillaries!) >
efferent arteriole > peritubular capillary network
(for cortical nephrons) --or-- > vasa recta (for
juxtamedullary nephrons).
Venous:: Peritubular capillary network >
interlobular veins > arcuate veins > interlobar
veins > renal vein.
The vasa recta (L. straight vessels) are
comprised of arteriolae rectae and venulae
rectae and their associated capillaries.
67

68. Blood Supply Diagram
68

69. Nerve Supply



The smooth muscle of the glomerular
arterioles receives sympathetic
innervation.
Vasoconstriction of afferent arteriole
decreases filtration rate (lowers pressure)
whereas vasoconstriction of the efferent
arterioles increases filtration rate.
Neither innervation is neither for normal
function.
69

70. 
















urinary space of Bowman's
capsule --- 
> convoluted proximal tubule
--- 
> descending, thick, straight,
proximal tubule --- 
> descending, thin, straight,
proximal tubule -- 
> ascending, thin, straight
proximal tubule --- 
> ascending, thick, straight,
distal tubule ---- 
> convoluted, distal tubule --- 
> collecting tubule ---- 
> collecting ducts ---- 
> papillary ducts of Bellini
----- 
> minor calyx ------ 
> major calyx ---- 
> renal pelvis ----- 
> ureter ----- 
> bladder ----- 
> urethra ----- 
> outside
Urine flow
70