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Histology of the urinary tract
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Urinary system consists of kidneys, ureters, bladder and urethra
Functions
o Filter cellular waste from blood
o Selective reabsorption of water and solutes
o Excretion of waste and excess water as urine
Additional functions of the kidneys
o Regulate the fluid and electrolyte balance
o Produce renin, involved in BP maintenance
o Produce erythropoietin, stimulating RBC production
o Hydroxylation of vitamin D to an active form for regulation of Ca
balance

Kidneys
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Concave medial border (hilum) where nerves, blood, lymph and
ureters enter/exit
Convex lateral border
Covered by thin fibrous capsule
Renal pelvis is the expanded upper end of the ureter, and divides into
two or three major calyces, which subdivide into minor calyces
Area surrounding the calyces is the renal sinus, containing adipose
tissue
Kidney contains outer cortex and inner medulla
Medulla contains renal pyramids, separated by cortical extensions
known as renal columns
Renal lobe comprises a medullary pyramid plus cortical tissue around
base and sides
The functional unit of the kidney is the nephron
o Renal corpuscle – initial dilated portion in cortex
o Proximal convoluted tubule – in cortex
o Nephron loop (loop of Henle) – descend into medulla and
ascend back to cortex
o Distal convoluted tubule
o Collecting tubule
Collecting tubules converge into collecting ducts which carry urine to
the calyces then to the ureter
Cortical nephrons (exclusively cortical) and juxtamedullary nephrons

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Renal corpuscles and blood filtration
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Renal corpuscle contains the glomerulus surrounded by a double
walled epithelial capsule called the glomerular capsule
In-between the visceral and parietal layers of the glomerular capsule is
the urinary/capsular space – receives filtered fluid
Vascular pole of the renal corpuscle where the afferent arteriole enters
and efferent arteriole leaves
Urinary/tubular pole of the renal corpuscle is where the proximal
convoluted tubule begins
Parietal layer of the glomerular capsule is simple squamous epithelia,
with basal lamina and thin layer of reticular fibres
o Changes at the tubular pole to simple cuboidal
Visceral layer (podocytes) have a cell body with primary processes
o Primary processes give rise to numerous secondary (foot)
processes/pedicles
o The pedicles embrace a portion of a glomerular capillary
o Pedicles interdigitate, creating filtration slits
o A thin, semipermeable diaphragm bridges the filtration slits

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Slit diaphragms are a specialised type of cell junction composed of
large nephrin molecules (transmembrane protein) that form a porous
structure
Glomerular basement membrane located between the fenestrated
endothelial cells and covering podocytes
o Most substantial part of the filtration barrier separating the blood
from the capsular space
o Formed from fusion of the capillary and podocyte basal laminae
o Selective macromolecular barrier; physical barrier and barrier
against negatively charged molecules
Initial glomerular filtrate has a composition similar to plasma apart from
containing very little protein
Glomerular filtration rate is aided by muscular activity in the efferent
and afferent arterioles
o Neural and hormonal input regulates constriction
Renal corpuscles also contain mesangial cells, which resemble
pericytes in producing components of an enveloping external lamina
o Provide physical support and contraction
o Phagocytose protein aggregates e
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Ab-Ag complexes
o Synthesise and secrete cytokines and prostaglandins important
for immune defence and repair

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Clinical note
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Glomerular filter can be altered in diseases such as diabetes
mellitus and glomerulonephritis
Can become more permeable to proteins, resulting in proteinuria

Proximal convoluted tubule
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Long, torturous tube lined with cuboidal epithelium
60-65% of water is resorbed here, along with most vitamins, ions,
nutrients and plasma proteins
Water and solutes are transferred across the tubular wall and taken up
by the peritubular capillaries
Numerous mitochondria in the cells, giving acidophilic stain
Cells have numerous microvilli forming a brush border
Capillaries and other vascular structures are abundant in the
connective tissue
Active pinocytosis ongoing at the apical surface – uptake plasma
proteins that passes through the glomerular filter
Cells have numerous basal membrane invaginations and lateral
interdigitations with neighbouring cells
Na+/K+ATPase located on basolateral membrane, for active transport
of Na out of the cell; mitochondria located in the invaginations
Active resorption of glucose and amino acids and ~85% of NaCl and
other ions; water moves passively
If amount of glucose in the filtrate exceeds the absorbing capacity
(e
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diabetes), the urine may contain glucose
Cells of proximal convoluted tubule can also move substances from
the peritubular capillaries to the tubular lumen (tubular secretion)

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o Organic anions may be secreted in this way
Cells of the proximal convoluted tubule are also involved in vitamin D
hydroxylation

Nephron loop (of Henle)
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Proximal tubule continues as a straight tubule that becomes the
nephron loop; narrows at this point
Comprises a descending limb and an ascending limb
Composed of simple cuboidal epithelia in the cortex and squamous in
the medulla
Loop has a wide lumen with flat squamous epithelial cell lining
1/7 nephrons are juxtamedullary
o Important in production of concentrated, hypertonic urine
o Long loops that extend into the medulla
Nephron loop and surrounding tissue involved in making urine
hypertonic and conserving water
Cuboidal cells of the thick ascending limbs actively transport NaCl out
of the tubule against a concentration gradient into the interstitial tissue,
making it hyperosmotic
Squamous cells of the thin descending limbs are feely permeably to
water but not salts, while the thin ascending limbs are permeable to
NaCl but not water
Counter current flow of the filtrate in the two parallel limbs of the
nephron loops creates an osmolarity gradient in the interstitium
Counter current blood flow of the vasa recta helps maintain the
gradient
High interstitial osmolarity draws water passively from the collecting
ducts into the medullary pyramids, concentrating the urine
Permeability of the ducts is increased by ADH
Counter current multiplier effect – nephron loop and vasa recta
establishing conditions for urine concentration

Distal convoluted tubule
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Thick ascending limb of the nephron loop is straight as it enters the
cortex, then become torturous as the distal convoluted tubule
Simple cuboidal cells with no brush border
Basal membrane invaginations and associated mitochondria due to
ion transporting function
o Na+ absorption and K+ secretion; regulation of the sodium
pumps by aldosterone (adrenal glands)
o Also H+ and NH4+ secretion for acid-base balance

Juxtaglomerular apparatus (JGA)
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The juxtaglomerular apparatus (JGA) is where the initial straight part of
the distal tubule contacts the vascular pole of the renal corpuscle
o Cells here establish a negative feedback regulation on renal
blood flow and keep glomerular filtration rate constant
Where the cells of the distal tubule contact the arterioles is the macula
densa
o Cells become closely packed, columnar with apical nucleus,
basal Golgi and elaborate and varied ion and transport system
The tunica media of the afferent arteriole contains modified smooth
muscle cells known as juxtaglomerular cells
o Smooth muscle cells are secretory with rounded nuclear, RER,
Golgi and granules
Lacis cells are extra-glomerular mesangial cells, likely provide
supportive function
Function of the JGA is autoregulation of glomerular filtration rate (GFR)
and controlling BP

JGA regulation of BP
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Increased arterial pressure
o Increased arterial pressure increases glomerular capillary
pressure, increasing the GFR
o Higher GFR increases luminal concentrations of Na+ and Cl- in
nephrons
o Ion levels monitored by the macula densa; increased levels
cause release of ATP, adenosine and other vasoactive
compounds
o Afferent arteriole is stimulated to contract, lowering glomerular
pressure and decreasing the GFR
o Tubular ion concentrations fall, turning off the release of
vasoactive mediator release from the macula densa
Decreased arterial pressure
o Local baroreceptors in the afferent arteriole (possibly the JG
cells) detect decreased arterial pressure
o JG cells secrete renin (protease)
o Renin cleaves the plasma protein angiotensinogen into
angiotensin I
o Angiotensin converting enzyme (ACE) in lung capillaries and
other locations cleaves angiotensin I to angiotensin II

o Angiotensin II is a potent vasoconstrictor that directly increases
BP and stimulates adrenals to release aldosterone
o Aldosterone promotes Na+ and water resorption in the distal
convoluted tubule, which increases blood volume and thus BP
o Return of normal BP switches off renin secretion by JG cells

Clinical note
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Substantial haemorrhage will decrease blood volume, thus
decreases BP
Detected by the JG cells, which secrete renin
Ang II and aldosterone act to increase BP and blood volume
Other factors that lower BP by lowering blood volume (such as
Na+ depletion, dehydration ) will also activate the renin system

Collecting tubules and ducts
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Urine passes from the distal convoluted tubules to the collecting
tubules, forming the last part of the nephron, which join to form larger,
straight collecting ducts
The collecting ducts empty into the minor calyces
Collecting tubules and ducts are composed of principal cells, with few
organelles and scant microvilli
Intercalated cells found amongst the principal cells, responsible for
regulating the acid-base balance by secreting H+ and absorbing
HCO3Collecting ducts of the medulla are main component of the urine
concentrating system
Cells of the collecting duct are rich in aquaporins – selective pores for
water molecules, sequestered in vesicles
Pituitary hormone ADH/vasopressin stimulates vesicles containing
aquaporins to fuse with the cell membrane, resulting in increased
water permeability

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Reference
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Junqueira’s Basic Histology: Text and Atlas
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