The urinary system functions to filter waste from the blood and regulate fluid levels. The kidneys contain nephrons, which are the functional filtering units. Each nephron contains a renal corpuscle with glomerulus for blood filtration, and a renal tubule for reabsorption and secretion. Filtrate passes through the glomerulus and along the tubule, where it is modified before collection in the ureters and storage in the bladder for excretion. The juxtaglomerular apparatus regulates blood pressure and fluid balance.

1. State University of Medicine and Pharmacy “Nicolae Testemitanu”
Urinary System
Department of Histology, Cytology and Embryology
Tatiana Globa

2. Urinary System Functions
 Clear the blood of nitrogenous and other waste metabolic
products (urea, uric acid, toxic stuff, drugs) by filtration and
excretion
 Regulation of
– blood volume
– concentration of blood solutes
– pH of extracellular fluid
 Endocrine function: synthesis of erythropoietin, renin,
prostaglandins
 Makes calcitrol (from Vit D3: stim Ca2+ absorption by
intestinal epithelium)
 Recovers by reabsorbtion small molecules (amino acids,
glucose, and peptides), ions (Na, Cl, Ca, PO), and water, in
order to maintain blood homeostasis.
 Assists liver in detoxification of poisons

3. Urinary System
Consists:
 Kidneys
 Ureters
 Urinary bladder
(storage)
 Urethra

4. Kidneys
 Kidney are paired, bean-shaped organs, enveloped by
a thin capsule of connective tissue
 Renal hilum is the concavity on the medial border of
the kidney where there are:
– Renal artery, vein; nerves, lymphatic vessels and
ureter
 Sizes:10 cm X 5.5 cm X 3 cm
 Each kidney is divided into an outer cortex and an
inner medulla
 Each kidney contains about 2 million nephrons –
morpho-functional units

5. Kidney consists of
 Cortex, which is divided into inner and outer regions.
– Renal corpuscles and convoluted tubules
 Medulla, which is formed by conical masses, the
medullary pyramids, with their bases located at the
cortico-medullary border.
– 10-18 renal pyramids
– Each renal pyramid opens into the renal papilla
– A medullary pyramids, together with the associated
covering cortical region, constitutes a renal lobe
 Minor calyx
 Major calyx
 Renal pelvis (connected to ureter)

7. Medulla
Medulla divided
into pyramids
Tip of pyramid
like top of
salt shaker

8. Medulla
pyramid
….
….
….
Minor calyx
urine

9. Parts of the Kidney
Within the kidney, utilize the diagram on the right to identify the
capsule, cortex, renal corpuscles, and medulla, which has no renal
corpuscles. The slide on the left is a representative section from this part
of the kidney.
cortex
medulla
Slide B93 Monkey Kidney H&E X20

10. Parts of the Kidney
On the left, locate an area in the cortex where tubules run parallel to one
another and are cut longitudinally. This is a pars radiata or medullary
ray. On either side is a pars convoluta, which contains renal corpuscles
and coiled tubules.
cortex
M
ed
ull
ar
yR
ay
Pars Convoluta
medulla
B93 Monkey Kidney H&E X20

11. Kidney: Cortex versus Medulla
With the same image, note the medullary rays are composed of
collecting tubules. On either side is a pars convoluta, which contains
renal corpuscles and coiled tubules.
Pars Rad
M cortex
ed
ul
la Pars Convoluta
ry
ata i
R
ay
Pars Convoluta
medulla

12. Kidney Blood Supply
 The main function of the kidney is to filter the blood
 The kidneys receive 20-25% of the total cardiac output per
minute and filter about 1.25 L of blood per minute. All the
blood of the body passes through the kidneys every 5 minutes
 About 90% of the cardiac output goes to the renal cortex; 10%
of the blood goes to the medulla
 Approximately 125 ml of filtrate are produced per minute, but
124 ml of this amount are reabsorbed
 About 180 L of fluid ultrafiltrate are produced in 24 hours and
transported through the uriniferous tubules. Of this amount,
178.5 L are recovered by the tubular cells and returned to the
blood circulation, whereas only 1.5 L are excreted as URINE

13. Kidney Blood Supply
 Renal artery
– Branches until afferent arterioles to nephrons
– GLOMERULI capillaries
– Efferent arterioles
– Secondary capillary network, surrounding the cortical segments of the
superficial uriniferous tubules
 Venules    renal vein

14. Nephron- morphofunctional unit
Consists of 2 components:
1. Renal corpuscle
– Bowman’s capsule and
glomerulus
1.Renal tubule
 Proximal thick segment Proximal straight tubule
– Proximal convoluted
tubule and proximal
straight tubule
 Thin segment
Distal straight tubule
– descending and ascending
limbs of loop of Henle
 Distal thick segment
– Distal convoluted tubule
and distal straight tubule

15. Glomerulus

17. Types of Nephrons
Depending on the distribution
nephrons can be:
 Cortical (85%)
– Is located in the outer region
of the cortex
– Its loop of Henle is short and
does not enter the medulla
– Most of reabsorption and
secretion
 Juxtamedullary (15%)
– Is located in the cortex
region adjacent to the
medulla
– Its loop of Henle is longer
and extends deep into the
medulla
– Create conditions for making
concentrated urine

18. Terms
 Blood filtration and formation of the primary
urine:
Cause: high pressure in glomeruli, glomeruli caps
more permeable than others in body
 Reabsorption Formation
of the
 Secretion secondary
urine
 Peritubular caps/vasa recta

19. Renal Corpuscle - site of filtration
Consists of :
1) Glomerulus
2) Bowman’s capsule
 Glomerulus
– tufts of fenestrated capillaries; fed by afferent arteriole and drains to
efferent arteriole
 Mesangial cells
– Support capillaries
– Phagosytose
– Contraction and regulation of blood flow
– Secretion of amorphous extracellular matrix
 Bowman’s capsule – double-walled (visceral and parietal) epithelial capsule
– Visceral layer: is attached to the capillary glomerulus, is lined by podocytes
– modified simple squamous epithelium;
– Parietal layer: is lined by simple squamous epithelium
– Bowman space (containing primary urine)
– Vascular pole: site of afferent (incoming) and efferent (outgoing) arterioles
supplying glomerulus
– Urinary pole: leads to proximal convoluted tubule; route of filtrate

21. Renal Corpuscle
Bowman’s
Capsule

22. Kidney: Renal Corpuscle
Note the schematic of the renal corpuscle (glomerulus) on the
right and how it is suspended in the urinary (Bowman’s) space.
The afferent and efferent arterioles enter and leave the
glomerulus at the vascular pole.
DCT
Slide B92 Human Kidney PAS X200

23. Renal filtration barrier
 Capillaryfenestrated endothelium
 Basement Membrane
– Much thicker than typical basement membrane
 Lamina rara externa – an electron-lucent zone
 Lamina densa – an electron dense intermediate zone
 Lamina rara interna – an electron-lucent zone
 Bowman’s Capsule Visceral Epithelium
– Podocytes – have long and branching cell processes that
completely encicle the surface of the glomerular capillary. The
endings of the cell processes, the pedicels, from the same
podocyte or adjacent podocytes, interdigitate to cover the basal
lamina and are separated by gaps, the filtration slits (are bridged
by a membranous material, the filtration slit diaphragm).

26. Capillary Endothelium
Basement Membrane
Podocytes Podocyte

27. Composition of the primary urine
 Water
 Ions (K, Ca, Mg, bicarbonate, phosphate,
sulfate ions)
 Glucose
 Small-weight proteins (less than 69,000
Daltons
 Amino acids
 Urea

28. Renal tubule – site of selective re-absorption /
secretion of solutes
Proximal convoluted tubule
Functions:
 receives filtrate from urinary space
 site of selective re-absorption of most solutes
– all glucose and amino acids
 60 - 80% of NaCl (active) and water (passive)
 proteins absorbed by pinocytosis followed by lysosomal
degradation and release of amino acids
 re-absorbed materials released to peritubular capillary
network
 site of pH balance
 site of creatinine secretion

29. Proximal Convoluted Tubule
STRUCTURE
 tubules formed by simple cuboidal / columnar epithelia
 apical surface covered with microvilli creating LM brush
border
– increase surface area for ion absorption
 cells
tightly bound to one another to seal off intercellular
space from lumen
– tight junctions and zonula adherens apically; interdigitating plicae (folds)
laterally
 interdigitating
basal processes contain numerous
mitochondria; creates LM basal striations; associated with
ion transport
Histological appearance
 most abundant tubule in cortex
 eosinophilic cytoplasm with basal nucleus (polarized)
– brush border rarely preserved producing occluded lumen
– Indistinct cell margins due to basal and lateral border interdigitations

31. Proximal Straight Tubule
 locatedwithin or near medulla, depending upon type
of nephron
 lower cuboidal epithelium
– microvilli and basal and lateral interdigitations
simplified

32. Loop of Henle: Thin Segment
Descending thin tubule
 located within medulla
 low cuboidal to squamous epithelium
 microvilli and basal and lateral interdigitations
poorly developed creating leaky cell
 site of passive transport of ions (inward) and
water (outward) between lumen and
interstitium
Ascending thin tubules
 located within medulla
 similar in appearance to descending thin tubules
 water impermeable; passive transport of
NaCl into interstitium

36. Distal Convoluted & Straight Tubules
Distal straight tubule
 located within medulla and cortex
 simple cuboidal epithelium with sparse microvilli and lacking
lateral interdigitations
– apical nucleus
– basal interdigitations present with abundant mitochondria
 function: water impermeable; site of ion transport from lumen
to interstitum which establishes ion gradient of medulla
Distal convoluted tubule
 located within cortex
 approximately 1/3 as long as proximal
 contacts renal corpuscle at macula densa to form
juxtaglomerular apparatus (below)
 morphology similar to straight portion
 function: ion exchange

37. Kidney: Convoluted Tubules
Within the pars convoluta, identify proximal convoluted tubules (PCT)
and distal convoluted tubules (DCT). The PCT is more than twice as
long as the DCT, so the majority of tubules are PCT.
DISTINGUISHING
CHARACTERISTICS PCT
DCT DCT
PCT
star-shaped lumen DCT
glycocalyx debris DCT
in lumen highly PCT
DCT
eosinophilic tall PCT
cuboidal cell
DCT
DCT PCT
more cells per lumen
PCT
clear lumen (no debris) PCT
no or minimal
PCT PCT
brush border
less eosinophilic
cells normal cuboidal PCT
Slide B92 Human
epithelium Kidney PAS X200

38. Kidney: PCT versus DCT
The diameter of the distal convoluted tubules (DCT) is much smaller
than the proximal convoluted tubules (PCT), although the luminal
diameter of the two tubules are approximately the same.
DISTINGUISHING
CHARACTERISTICS
PCT renal
renal
star-shaped lumen is corpuscle
corpuscle PCT
due to the autolysis
of the brush border.
Fewer nuclei appear
in cross-section and
cell boundaries are
indistinct.
Basal infoldings due DCT PCT
to mitochondria
DCT
no precipitate in lumen
more nuclei with
distinct cell boundaries Slide B90 Human
paler cytoplasm DCT Kidney H&E X400

39. Collecting Tubules & Ducts
 Start in cortex and descend
through medulla
 Is lined by a cuboidal
epithelium composed of
two cell types:
– Principal cells – resorb Na
and water and secrete K in a
Na, K ATPase pump-
depending manner
– Intercalated cells – have
abundant mitochondria and
secrete either H and HCO3.
they are important regulators
of acid-base balance

40. Kidney: Collecting Ducts
Photo of renal papilla projecting into renal calyx. The apex of the papilla
contains openings, the collecting ducts (of Bellini). These ducts deliver
urine from the renal pyramid to the minor calyx.
Collecting tubules, renal calyx
widen to form
collecting ducts Collecting collecting ducts
(columnar tubules (of Bellini).
epithelium). The
outer portion of the renal papilla
minor calyx is lined
with transitional
epithelium.
renal (minor) calyx

42. Kidney: Renal Papilla
Higher magnification photo of renal papilla projecting into renal calyx. The
openings seen within the papilla are the collecting ducts (of Bellini).
Note the renal
transitional calyx
epithelium lining
the outer surface
*
of the minor calyx. *
The renal papilla
has a simple
columnar * *
epithelium renal papilla
renal
(minor)
* calyx

43. Juxtaglomerular Apparatus - site of blood
pressure regulation via renin-angiotensin-aldosterone system
 Macula densa: specialized cells in distal convoluted
tubule adjacent to renal corpuscle
– these cells have receptors for Na. If it is necessary
they stimulate production of aldosterone
 Juxtaglomerular cells: modified smooth muscle cells of
afferent and efferent arterioles
– produce renin. Renin provides the transformation
of angiotensinogen into angiotensin I, which
transforms into angiotensin II (in lungs) that
elevates the blood pressure
 Juxtavascular cells: extraglomerular mesangial cells
– their function is not well known. Probably they are
involved in the renin and erythropoietin secretion
and blood pressure regulation

45. Juxtaglomerular Apparatus
Mechanism
 macula densa cells monitor NaCl levels in afferent arteriole
 renin secretion juxtaglomerular cells is stimulated by
paracrine activity from the macula densa
 renin is a protease that cleaves plasma angiotensinogen into
angiotensin I
 angiotensin I converted to angiotensin II in the lung (by
enzyme in capillaries)
 angiotensin II promotes vascular smooth muscle contraction
and release of aldosterone from the adrenal cortex
 aldosterone stimulates absorption of NaCl and water in the
distal convoluted tubule thus increasing blood volume
 net result is to increase blood pressure

46. Kidney: Vascular Pole
Search for an area within the renal corpuscle where a distal
convoluted tubule makes contact with the vascular pole of the
renal corpuscle. Note the macula densa and juxtaglomerular cells
DCT
DCT
Slide B94 Rabbit Kidney PAS X200 Wheater’s Fig.16.18b

47. Kidney: Vascular Pole
The macula densa of the distal convoluted tubule and the
juxtaglomerular (JG) cells constitute a juxtaglomerular apparatus
(JGA). The JG cells secrete renin and erythropoietin.
PCT
PCT

48. Ureters and Urinary Bladder

49. Ureter
 Drains urine from kidney to
urinary bladder
Structure
 mucosa – lined by transitional
epithelium over connect tissue
lamina propria
– transitional epithelium –
impermeable to water and salts;
distendable
– lamina propria - loose connective
tissue
 muscularis externa – smooth
muscle layer
– bi-laminar: inner longitudinal and
outer circular; produce peristalsis
 adventitia / serosa – connective
tissue coat with or without
mesothelial covering

50. Ureter

52. Urinary Bladder
 Hollow muscular organ: distensible reservoir
 Full: ~1 liter
 receives bilateral ureters and empties via midline urethra
 smooth muscle forms detrussor muscle; specialized distally as
internal urethral sphincter

53. Urinary Bladder
The gross regions of the urinary bladder are
 Fundus
 Body
 Neck
The histology of the urinary bladder is as follows:
 Mucosa - transitional epithelium and lamina
propria
 Submucosa - connective tissue with blood supply
 Muscularis externa - 3 layers of smooth muscle
 termed the detrusor muscle
 Serosa/Adventitia

54. MUCOSA
SUBMUCOSA
MUSCULARIS
EXTERNA

55. Urethra
urethra
urethra

56. Urethra
 Neck of bladder to exterior
 Female:
– Short: 1-1.5 in
 UTI (bacteria or fungus)
– External urethral orifice: very close to
vaginal orifice
 Male:
– Long: 7-8 in
– terminal duct for both urinary and genital
systems
– Prosthatic, membranous, penile
 Urogenital diaphragm: external urethral
sphincter (skeletal muscle)
– resting to urinate

57. Urinary System
Male Sphincters Female Sphincters
Internal urethral
External Urethral
sphincter
Sphincter