Lec 13/14 - Urinary System

Question 1

unipyramidal kidney structure; what species, how it empties,

Answer 1

• one single lobe, stretched into a bean shaped organ
• carnivores, horses, rodents
• single papilla (renal crest) empties into renal pelvis (enlarged start of
  ureter)

Question 1

components of urinary system, how does the system ensure optimal blood properties

Answer 1

kidneys, ureters, urinary bladder, urethra

• ensures optimal blood properties by:
• regulation of balance between water & electrolytes; acids & bases
• excretion of bioactive substances
• regulation of arterial blood pressure
• secretion of erythropoietin
• conversion of pro-vitamin D3 to active form
• gluconeogenesis (along with liver)

Question 2

porcine kidney structure; species, appearance, draining

Answer 2

• multipyramidal (multilobar) kidney with no obvious external lobar structure
• each lobe equivalent to renal pyramid
• each lobe drains into calyx which drains into ureter

Question 3

large ruminant kidney structure; lobes, draining

Answer 3

• multipyramidal (multilobar) kidney with obvious external lobar structure
• each lobe equivalent to renal pyramid
• each lobe drains into minor calyx; several join to form major calyx;
  several major calyces join to form ureter

Question 4

kidney structure, what happens at each part

Answer 4

Cortex
* outer region; many corpuscles and cross section of tubules
* site of blood filtration & some transport

Medulla
* inner region with conical structures: renal pyramids
* site of urine concentration

Hilum
* concave medial border; entrance/exit of blood & lymphatic vessels, nerves and ureter

• tip of each pyramid is papilla; connects to calyx
• pyramids separated by extensions of cortex: renal columns
• lobes of kidneys also contain medullary rays (400-500/kidney)
• rays consist of straight tubules & collecting ducts of several sets of nephrons

Question 5

function of the nephron, epithelium

Answer 5

• three key functions: filter, secrete, absorb
• spans junction of cortex and medulla
• simple epithelium along its entire length, but type varies

Question 6

what does the renal corpuscle consist of, what happens here, where is it found

Answer 6

• ball-like structure where nephron begins **CORTEX
• consists of glomerulus (tuft of capillary loops) surrounded by a thin-walled hollow sphere (Bowman’s/glomerular capsule)
• site of blood filtration

Question 7

location of proximal tubule, nephron loop (and parts of this), distal tubule (what does it contain)

Answer 7

Proximal tubule: long, convoluted tube located in cortex
* enters medulla as upper part
of nephron loop

Nephron Loop (Loop of Henle)
* continuous with proximal tubule within medulla
* thin and thick descending and ascending portions.

Distal tubule: short convoluted tubule in cortex
* continuous with ascending thick region of nephron loop
* connection point with renal corpuscle contains the macula densa

Question 8

features of collecting tubules and collecting ducts

Answer 8

Connecting tubule
* short and straight final portion of nephron
* connecting tubules from several
nephrons merge to form a collecting duct
* collecting duct and all of the nephrons that empty into it are a functional lobule or medullary ray

Collecting Ducts
* converge in renal papilla & deliver urine from several nephrons to calyx/pelvis
* urine from minor calyces flows into major calyces, then pelvis then ureter

Question 9

kidney circulations; arteries involved and where they are/how they branch

Answer 9

• renal artery enters kidney at the hilum and divides into two or more segmental arteries that branch into interlobar arteries at the renal pelvis
• interlobar arteries sit on either side of kidney lobe spanning the medulla
• at level of corticomedullary junction, they become arcuate arteries that run in arch along this junction
• smaller arteries leaving the arcuate
  arteries at right angles to go deep into the cortex are interlobular arteries
• interlobular arteries branch into afferent arterioles that then branch further to produce glomerulus
  (capillaries)
• capillaries merge together to form efferent arterioles
• efferent arterioles give rise to 2 capillary beds:
  -peritubular plexus around convoluted tubules (cortex)
  -vasa recta around nephron loop
  (medlla)
• unique situation of glomerular capillaries between
  two arterioles; afferent arteriole larger in diameter
• increases hydrostatic pressure, favouring filtration

Question 10

details of functions of the nephron

Answer 10

• central functions of the kidney are performed by the nephron:
  1) Filtration: water & solutes in blood move from glomerular capillary to lumen of nephron

2) Tubular secretion: substances transported by tubule epithelial cells from surrounding interstitium and capillaries into nephron lumen

3) Tubular reabsorption: substances transported from tubular lumen across the epithelium into interstitium and surrounding capillaries

Question 11

urine - formation, modifications, urine vs filtrate

Answer 11

• formed by combination of filtration & tubular secretion; is essentially what remains after reabsorption
• enters calyces/pelvis & undergoes excretion without further modification
• urine differs from filtrate in that it is more concentrated (lower volume, less water), lower in NaCl, and higher in soluble waste

Question 12

renal corpuscle; what begins here, capsules

Answer 12

• nephron begins with the renal corpuscle
• about 200 μm in diameter containing the glomerulus surrounded by a double-walled epithelial capsule
• outer epithelial layer of simple squamous epithelium known as
  Bowman’s capsule or glomerular capsule (‘parietal’ layer)
• inner epithelial layer in contact with capillaries is composed of modified epithelial cells called podocytes (‘visceral’ layer)

Question 13

epithelium of parietal layer and visceral layer

what does glomerular filtrate receive

filtrate enters what

Answer 13

Parietal layer: simple squamous epithelium; becomes simple cuboidal
at the junction with proximal tubule

Visceral layer: formed by podocytes which together with capillary
endothelial cells form filtration apparatus of nephron

• glomerular capsule receives fluid filtered through capillary wall and visceral layer of glomerulus into capsular space
• filtrate enters proximal convoluted tubule at urinary pole of renal corpuscle

Question 14

podocytes; what are they, processes, how filtrate travels

Answer 14

Podocytes
* modified epithelial cells that extend primary processes to curve around the length of glomerular capillary
* processes usually branch into 2° processes which extend many parallel, interdigitating small processes (pedicels) that cover capillary surface
* filtrate travels from blood through
fenestrated endothelium, then must traverse podocyte layer

Question 15

filtration barrier - what is between podocytes, waht other structures are present and how are they seen, what forms the filtration barrier

Answer 15

• between podocyte’s interdigitating pedicels are elongated spaces called filtration slits or slit pores
• zipper-like structures called slit diaphragms span adjacent pedicels & bridge the slit pores
• slit diaphragms only visible in electron microscopy
• slit diaphragm, along with glomerular basement membrane
  forms main filtration barrier of nephron

Question 16

glomerular filter - formed by what, what is abundant, what does it restrict

Answer 16

• thick glomerular basement membrane (GBM) is major site of filtration
• formed by fused basement
  membrane of the endothelial cell and the podocyte
• GAGs abundant in GBM and in slit diaphragms
• restrict passage of organic anions (due to –ve charge)
• endothelial fenestrations block passage of proteins ≥ 70 kDa

Question 17

filtration; how much is filtered into capsular space, plasma and protein amounts, reabsorption, what does presence of protein mean

Answer 17

• about 20 % of the plasma volume entering the glomerulus gets filtered into capsular space
• initial filtrate has a composition similar to normal plasma but with very little protein
• smaller plasma proteins (< 70 kDa) that pass the filter are reabsorbed by cells of renal tubules
• presence of protein in urine (proteinuria) often indicator of potential kidney disorders: glomerular filter is altered & becomes more permeable to proteins (e.g. diabetes mellitus, glomerulonephritis)

Question 18

mesangial cells - what do they form, waht type of cells are they, are they distinguishable, functions

Answer 18

• mesangial cells & surrounding matrix form mesangium of glomerulus
• pericyte-like cells that fill the spaces between capillaries on surfaces that lack podocytes
• cells are difficult to distinguish from podocytes in routine LM preparations

Functions:
- physical support of capillaries within glomerulus
- adjust vessel diameter in response to blood pressure changes
- phagocytosis of protein aggregates adhering to the glomerular filter
- secretion of cytokines, prostaglandins & other factors for immune defense and glomerular repair

Question 19

PCT: length, epithelium, staining and why, appearance

Answer 19

Proximal Convoluted Tubule
(PCT)
* simple squamous epithelium of capsule parietal layer becomes simple cuboidal epithelium of PCT
* PCT is longest part of nephron so most of the sections of tubules seen in the cortex are PCT
* cells have eosinophilic cytoplasm (many mitochondria) & central nuclei
* lumens appear ‘filled’, due to long microvilli & aggregates of small plasma proteins

Question 20

PCT functions and details

what is it important for

Answer 20

Reabsorption:
* 50% of water & electrolytes; 100% of organic nutrients (glucose, vitamins, amino acids) reabsorbed
* transfer across tubular wall followed by uptake by peritubular capillaries
* active transport: ion pumps, transporters, etc.
* passive movement of water, small solutes in spaces formed by leaky junctions between cells
* small proteins reabsorbed by receptor mediated endocytosis and
then degraded

Secretion:
* some organic anions and cations (bile salts, creatinin, antibiotics &
other drugs) actively transported from peritubular capillaries to PCT
epithelium
* higher rate of disposal compared to filtration
* secreted into filtrate by PCT cells

VERY IMPORTANT FOR DRUG CLEARANCE

Question 21

epithelium of thin vs thick limbs in loop of henle

functions of the loop of henle

Answer 21

• thin limbs: simple squamous epithelium
• thick limbs: simple cuboidal epithelium; apically located nuclei; many mitochondria
• thin limbs variably permeable to water and NaCl
• play primarily passive role in transcellular transport
• thick limb actively transports Na + and Cl - and is impermeable to water

Function:
* together with vasa recta (straight capillaries around loop) forms the countercurrent multiplier system
* creates & maintains gradient of hyperosmotic interstitial compartment as the system penetrates deeper into medulla
* works to adjust salt content in filtrate

Question 22

DCT - what is it associated with, length, epithelium, appearance

Answer 22

• associated with vascular pole of corpuscle where arterioles enter and leave
• shorter than PCT, so fewer profiles seen in sections of cortex
• simple cuboidal epithelium; paler, flatter and smaller cells than PCT
• lumen is clearer & more nuclei are visible in cross-section

Question 23

DCT function (2 main and details)

Answer 23

Ion Conservation
* Na + absorbed from filtrate & returned to blood in exchange for K+
* Na + /K + exchange modulated by aldosterone (adrenal gland)
* specialized patch of tall columnar cells in DCT where it touches arterioles of glomerulus: Macula Densa
* cells of macula densa sense Na + levels in filtrate
* can trigger vasodilation of afferent arteriole & release of renin from
JG cells: changes in glomerular filtration and Na + recovery

Water Conservation
* Antidiuretic hormone (ADH) released by pituitary gland when
blood volume is low
* induces insertion of special water channels (aquaporin channels) into luminal membranes of DCT cells (& collecting duct cells)
* water then enters cytoplasm of these cells, is transported out basal
surface, and returned to blood

Question 24

juxtaglomerular apparatus; what does it form, function, what does it consist of

Answer 24

• forms at the point of contact between DCT and vascular pole of
  nephron glomerulus
• functions to keep glomerular filtration constant
• consists of: Macula densa (DCT cells)
  Juxtaglomerular cells (modified smooth muscle cells of afferent
  arteriole); produce renin
  Extraglomerular mesangial cells (Lacis cells)

Question 25

collecting ducts; length, what it forms, epithelium, appearance

Answer 25

• short connecting tubule is last part of nephron
• several connecting tubules in cortical medullary rays empty into collecting ducts
• collecting ducts have large lumens, cuboidal to columnar pale cells
• very smooth apical surfaces and distinct cell-cell boundaries

Question 26

what cells make up the collecting ducts, where do CDs fuse and what do they look like, large CDs

Answer 26

• made up of two cell types (not easily
  distinguishable in LM) principal cells: respond to aldosterone & ADH; recover Na + and water intercalated cells: modify pH of blood; less abundant (dark cells)
• cuboidal cells in cortex
• CDs fuse together in medulla; become larger and cells more columnar
• final large collecting ducts (of Bellini) empty urine out of papilla

Question 27

urinary tract - when does urine form, flow, storage, histology of layers

Answer 27

• upon delivery to the calyx, filtrate is no longer modified and becomes urine
• urine flows passively into renal pelvis but moves by peristalsis along ureters
• temporarily stored at the urinary bladder & voided through the urethra

Histology:
Mucosa: urothelium (transitional cell epithelium) resting on basement membrane & lamina propria of loose CT (no muscularis mucosae in UT of most species; so they have lamina propria-submucosa)

Muscularis: mostly smooth muscle; generally spiral inner longitudinal layer and spiral outer circular layer

Adventitia (or Serosa) depending on location

• calyces, renal pelvis, ureter and bladder all have similar histology
• walls become increasingly thicker closer to bladder

Question 28

urothelium; cells layers and what they do, what increases as you move down the urinary tract

Answer 28

superficial layer: umbrella cells
* dome shaped; can stretch out to accommodate distension
* contain reinforced tight junctions on
lateral surfaces
* can be shed when bacterial infection

intermediate cell layer
* 3-5 layers of pear-shaped cells that
can slide along each other with
distension
* can differentiate into umbrella cells as needed

basal cell layer
* rest on basal lamina and act as stem
cells for urothelium

• increasing # cell layers as move down urinary tract

Question 29

ureter - layers present

Answer 29

• mucosa (epithelium and lamina propria/submucosa) thrown into
  folds around lumen
• muscularis thickest layer; smooth muscle bundles with extensive CT
• ureters are retroperitoneal therefore covered by an adventitia of CT

Question 30

urinary bladder; layers and their features

Answer 30

mucosa thrown into folds around
lumen
* extensive lamina propria but no
muscularis mucosae in some species
(lamina propria-submucosa)
muscularis (detrusor muscle)
* smooth muscle bundles in more
random arrangement
* extensive circular muscle (internal
urethral sphincter) at opening into
urethra
* most of bladder is retroperitoneal
therefore covered by an adventitia of
CT
* part of bladder protrudes into pelvic
cavity floor- covered by serosa with
simple squamous epithelium

Question 31

urethra - connects, epithelium, what type of muscle

Answer 31

• urethra connects urinary bladder to exterior; varies in length
• epithelium varies from typical urothelium (transitional cell
  epithelium) to stratified squamous/cuboidal or even pseudostratified columnar epithelium
• depends on species, sex & location
• external urethral sphincter (voluntary) is skeletal muscle