Urinary System

Question 1

Kidney

Functions

Answer 1

• Water and electrolyte balance
  
  • absorption of H₂O and Na⁺
• Excretion
  
  • urea
  • uric acid
  • creatinine
• Hormone production
  
  • EPO
  • renin
  • prostaglandins
• Hydroxylation of Vit D

Question 2

Kidney

Gross Structure

Answer 2

Capsule

• outer layer of fibroblasts and collagen fibers
• inner layer of myofibroblasts

Cortex

• Renal corpuscles
  
  • formed by the glomeruli and Bowman’s capsule
• Cortical labyrinths
  
  • region that surrounds the renal corpuscles
  • formed by the convoluted tubules​
    
    • Renal corpuscles
      Proximal convoluted tubules
      Distal convoluted tubules
      Arched collecting tubules
    • Looks the same regardless of the plane of section
• Medullary rays
  
  • longitudinal striations that emanate from the medulla
  • formed by the straight portions of uriniferous tubules
    
    • ​collecting ducts
    • straight proximal tubule
    • straight distal tubule

Medulla

• Often divided into an inner and outer zone
  
  • Tubules of cortical (superficial) nephrons extend into the outer zone
  • Tubules of juxtamedullary nephrons extend into the inner zone
• Medullary pyramids
  
  • Separated by medullary extensions of the cortex ⇒ cortical columns
• Straight portions of the uriniferous tubules
• Collecting ducts
• Vasa Recta

​

Renal sinus:

Fat-filled cavity near hilum

Contains minor and major calyces, most of renal pelvis, branches of renal vessels.

Question 3

Medullary Pyramids

Answer 3

• Characterized by numerous straight tubules, collecting ducts, and vasa recta converging towards the tip.
• Each pyramid divided into an outer medulla and inner medulla
  
  • Outer medulla subdivided into outer stripe and inner stripe
• Base of the pyramid is surrounded by cortical tissue ⇒ cortical arch
• Apex of the pyramid points towards the hilum
  
  • Called the renal papilla
  • Collecting ducts terminate here
  • Area cribrosa ⇒ perforated tip of the renal papilla
    
    • formed by the openings of the collecting (ducts of Bellini)
    • Drains into a minor calyx ⇒ major calyx ⇒ renal pelvis​
• Cortical columns flank each side of the pyramid
  
  • Are not considered part of the medulla

Question 4

Kidney

Lobe

Answer 4

Defined as all the tissue whose nephrons drain into the same minor calyx.

Includes both cortex & medulla:

1 medullary pyramid

Associated cortical arch

Half of each cortical column that surrounds it

Interlobar arteries and veins located in renal columns at the boundaries between lobes.

Question 5

Kidney Lobule

Answer 5

Defined as all the tissue whose nephrons drain into the same medullary ray.

Includes cortex but no medulla:

Medullary ray

Half of each cortical labyrinth on each side

Interlobular arteries and veins located at the boundaries between lobules.

Question 6

Uriniferous Tubule

Answer 6

Functional unit of the kidney.

Includes a nephron plus it’s associated collecting duct.

Seperate embryological origin.

Question 7

Nephron

Answer 7

~ 1.3 million / kidney

Consists of:

• Renal corpuscle
  
  • Glomerulus
  • Bowman’s capsule
• Proximal tubule
  
  • Proximal convoluted tubule
  • Proximal straight tubule (thick descending limb of loop of Henle)
• Thin limbs of Loop of Henle
• Distal tubule
  
  • Distal straight tubule (thick ascending limb of loop of Henle)
  • Distal convoluted tubule

Question 8

Cortical vs Juxtamedullary Nephrons

Answer 8

Cortical Nephrons

• ~ 85% of nephrons
• short LoH or absent
• contributes to medullary rays
• may extend into outer layer of medulla

Juxtamedullary Nephrons

• ~ 15% of nephrons
• long LoH extending deep into medulla

Question 9

Renal Corpuscle

Answer 9

Includes glomerulus and Bowman’s capsule

• Glomerulus
  
  • 10-20 fenestrated capillary loops
    
    • no diaphragms
  • afferent & efferent arterioles carry blood
    
    • located at the vascular pole of the renal corpuscle
• Bowman’s Capsule
  
  • visceral and parietal layers
    
    • urinary space in between
  • two poles
    
    • vascular pole
      
      • where efferent and afferent arterioles located
      • where visceral and parietal layers continuous
      • macula densa found here
    • urinary pole
      
      • where parietal layer is continuous with PCT
      • lumen of PCT continuous with urinary space
      • epithelium abruptly changes from simple squamous to simple cuboidal

Question 10

Visceral Layer

Bowman’s Capsule

Answer 10

• Invaginates and covers outer surface of glomerular capillaries
• Composed of specialized epithelial cells called podocytes
  
  • primary processes extend parallel to long axis of capillary
  • secondary processes extend perpendicular from primary processes
  • pedicels (foot processes) extend from secondary processes
    
    • filtration slits formed between interdigitating pedicels of neighboring podocytes
    • covered by a slit diaphragm
    • podocalyxin - polyanionic sialoprotein which forms the glycocalyx of the podocytes

Question 11

Parietal Layer

Bowman’s Capsule

Answer 11

• Formed from simple squamous epithelium
• The urinary space lies between visceral and parietal layers of Bowman’s capsule
  
  • Collects plasma ultrafiltrate from glomerular capillaries

Question 12

Intraglomerular Mesangial cells

Answer 12

• Scattered along the endothelial side of glomerular basement membrane
  
  • Located between capillary endothelium and basal lamina
• Functions:
  
  • support for capillaries
  • acts like macrophages and cleans basement membrane
  • contracts to help regulate blood flow through glomerular capillaries

Question 13

Glomerular

Filtration Barrier

Answer 13

3 main components:

1. Fenestrated endothelium of glomerular capillaries
   
   • 70-90 nm fenestrae
   • lack diaphragms
   • impedes passage of molecules > 70 K daltons
2. Fused basal lamina (glomerular basement membrane)
   
   • very thick
   • PAS+
   • jointly formed by podocytes and endothelial cells
   • consists of two laminae rarae surrounding lamina densa
     
     • Laminae rarae
       
       • laminin, fibronectin, and heparan sulfate
       • light staining
     • Laminae densa
       
       • mostly type IV collagen
       • dark staining
3. Podocytes
   
   • filtration slit membranes connecting interdigitating pedicles
     
     • made of a protein complex

Question 14

Proximal Tubule

Characteristics

Answer 14

• Reabsorbs ~ 80% of primary filtrate
• Begins at the urinary pole of renal corpuscle
• Drains Bowman’s space
• Absorbs:
  
  • Na, Cl, H2O
  • Amino acids, sugars, and polypeptides
• Secretes certain endogenous and exogenous organic solutes
• Proximal convoluted tubule (pars convoluta)
  
  • longer than other tubule types in cortical labyrinth
  • is the type most commonly seen there in sections
• Proximal straight tubule (pars recta)
  
  • contributes to medullary rays
  • extends into medulla
  • becomes continuous with thin limb of LoH

Question 15

Proximal Tubule

Structure

Answer 15

• Simple cuboidal epithelium
• Complex lateral membrane folds
  
  • lateral cell borders indistinct by LM
• Basal striations
  
  • infoldings of basal plasmalemma
  • mitochondria fill space between folds
• Aciophilic cytoplasm
• Prominent brush border with PAS+ glycocalyx
• Large cell size with nucleus spread further apart
• May have stellate shape to the lumen

Question 16

Thin Limbs

Loop of Henle

Answer 16

• Descending thin limb
  
  • continuous with pars recta of proximal tubule
• Ascending thin limb
  
  • continuous with pars recta of distal tubule
• Simple squamous epithelium
• Contributes to contercurrent multiplier system

Question 17

Distal Tubule

Characteristics

Answer 17

• Initial part is straight ⇒ pars recta
  
  • continuous with ascending thin limb of LoH
• Becomes convoluted in the cortex ⇒ pars convoluta
  
  • impermeable to water and urea
• Activated by aldosterone to absorb Na⁺
• Macula densa is part of the distal tubule
  
  • Contributes to juxtaglomerular apparatus

Question 18

Distal Tubule

Structure

Answer 18

• Low cuboidal epithelium
  
  • fewer profiles in section
  • less acidopihil
  • no brush border
    
    • sparse microvilli by EM
  • more clearly defined lumen than proximal tubules
• Lateral cell borders indistinct
• Cells smaller so more nuclei per cross section
• Nuclei located close to apical membrane

Question 19

Juxtaglomerular Apparatus

Answer 19

Includes:

1. Macula Densa
   
   • taller, narrower cells on side of distal straight tubule closest to the vascular pole
   • closely packed nuclei stains darker
   • basment membrane absent between macula densa and JG cells
   • respond to changes in BP, blood flow rate, fluid composition in distal tubule
2. Juxtaglomerular (JG) cells
   
   • modified smooth muscle cells in tunica media of afferent more frequently than efferent arterioles
   • cytoplasmic vacuoles containing renin
   • part of the RAAS system
3. Extraglomerular mesangial cells
   
   • occupy space between macula densa, afferent arteriole, and efferent arteriole
   • continuous with intraglomerular mesangial cells
   • likely transmit signals between macula densa and intraglomerular mesangial cells to regulate contractility of the latter

Question 20

Collecting Tubules and Ducts

Answer 20

• found in cortex and medulla
• distinguished from proximal and distal tubules due to distinct lateral boundaries
• have two distinct cell types:
  
  • Principal cells
  • Intercalated cells
    
    • Alpha and beta types

Question 21

Principal Cells

Collecting Ducts

Answer 21

• Light cells
• More common
• Single nonmotile cilia
  
  • senses fluid flow
  • defect in cilia causes polycystic kidney disease
• membranes contain aquaporin channels

Question 22

Intercalated Cells

Collecting Ducts

Answer 22

• darker staining
• interspersed amount prinicpal cells
• Found in the cortical CD and proximal part of the medullary CD
  
  • Absent near papilla
• Have microplicae on apical plasmalemma
• Two types help regulate acid/base balance:
  
  • 𝛼-intercalated cells actively secrete hydrogen ions
  • 𝛽-intercalated cells release bicarbonate via membrane exchanger

Question 23

Cortical Collecting Tubules

Answer 23

• arched collecting tubules
• leave cortical labyrinths and enter medullary rays
• both cells types present

Question 24

Medullary Collecting Ducts

Answer 24

• Run parallel with other tubule types and vasa recta
• Proximally they contain both cell types

Question 25

Ducts of Bellini

Answer 25

A.K.A. **Papillary collecting ducts** * formed from union of multiple medullary collecting ducts * **simple columnar epithelium** * _no intercalated cells_ * end in the **area cribrosa** of the renal papilla

Question 26

Renal Blood Supply

Answer 26

**renal arteries** ⇒ anterior and posterior divisions (hilum) ⇒ **segmental arteries** (in renal sinus) ⇒ **interlobar arteries** (bisect renal columns, run towards capsule) ⇒ **arcuate arteries** (runs parallel to corticomedullary boundary near base of pyramids) ⇒ **interlobular arteries** (bisect cortical labyrinths towards capsule) ⇒ **afferent arterioles** (arise from interlobular artery, lead to glomeruli) ⇒ **glomerular capillaries** ⇒ **efferent arterioles** (one from each glomerulus) ⇒ **peritubular capillaries** (irregular network anastomosing vessels mainly in cortex, empty into interlobular or arcuate vein) or **vasa recta** (long straight vessels in medulla, arise from efferent arterioles of juxtamedullary nephrons, empty into interlobular or arcuate vein) **Last 3 form an Arterial Portal System.**

Question 27

Cortical Interstitial Cells

Answer 27

* resemble **fibroblasts** * likely contribute to basement membrane of renal tubules * some are **macrophages**

Question 28

Medullary Interstitial Cells

Answer 28

Resemble **myofibroblasts** Help to express urine from the duct system.

Question 29

Renal Papillae

Answer 29

* Found at the **apex of medullary pyramids** * Each **fits into a minor calyx** * **Area cribrosa** is on the tip of the papilla * formed by the openings of the **collecting ducts of Bellini** * Covered by **transitional epithelium**

Question 30

Renal Calyces

Answer 30

* Each minor calyces receives a **renal papilla** * protrudes into lumen of minor calyx * epithelium covering papilla becomes continuous with epithelium lining calyx * Unit to form **major calycles** * Lined with **transitional epithelium** * Wall has **smooth muscle deep to lamina propria** * Propels urine to the renal pelvis

Question 31

Renal Pelvis

Answer 31

* Formed by the **union of all major calyces** * **Transitional epithelium** * **Thin layer of smooth muscle** deep to **lamina propria** * **Continuous with ureter** at hilum * **Adventitia** with fat that **blends with fat of renal sinus**

Question 32

Ureter

Answer 32

* **Mucosa** * transitional epithelium * lamina propria of dense fibroelastic CT * no muscularis mucosae * **Muscularis** * layers of smooth muscle produce peristaltic waves * proximal 2/3 of ureter * inner spinal layer = longitudinal * outer layer circular * distal 1/3 of ureter: addtional outer longitudinal layer * inner longitudinal * middle circular * outer longditudinal * **Adventitia** * Blends into surrounding CT

Question 33

Urinary Bladder

Answer 33

1. **Mucosa** * **transitional epithelium** * Thickened plasmalemmal plaques by EM in distended bladder * Fusiform vesicles in apical cytoplasm by EM in relaxed bladder * **lamina propria** * dense irregular Ct * collagen and elastic fibers * no glands except at urethral orifice 2. **Muscularis** * three interlaced layers of smooth muscle form a thick muscularis ⇒ **detrussor muscle** 3. **Adventitia or Serosa** * dense irregular CT * some elastic fibers * **Superior surface has a serosa** * **Remainder has an adventitia**

Question 34

Female Urethra

Answer 34

* mucosa lined by **transitional epithelium** * **Fibroelastic** lamina propria * small **mucous-secreting glands** empty into lumen * coat of **erectile tissue** * numerous wide, thin-walled vessels * where it passes through the **urogenital diaphragm** it is surrounded by **skeletal muscle ⇒ extrenal urethral sphincter**

Question 35

Male Urethra

Answer 35

* **Prostatic urethra** * lined by **transitional epithelium** * **Membranous urethra** * lined by **stratified columnar and pseudostratified columnar** epithelium * passes through skeletal muscle of the **urogenital diaphragm** * **Penile urethra** * lined by **stratified columnar / pseudostratified columnar** _proximally_ * lined by **stratified squamous** _distally_ * this portion surrounded by vascular erectile tissue of **corpus spongiosum** * **Lamina propria** * fibroelastic CT * contains mucous producing **glands of Littre**