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Anatomy and Histology of the Urinary System

Complete Seminar, Midterm & Final Exam Guide

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OVERVIEW OF THE URINARY SYSTEM

• Excretion of metabolic waste (urea, creatinine, uric acid)
• Regulation of water, electrolyte, and acid-base balance
• Regulation of blood pressure (renin secretion)
• Erythropoiesis regulation (erythropoietin)
• Activation of Vitamin D (calcitriol synthesis)
• Gluconeogenesis during prolonged fasting

Histology: A Text and Atlas, p. 1849; Junqueira's Basic Histology, p. 948

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ORGAN 1: THE KIDNEY

1. Definition

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2. Topography (Position and Relations)

• The kidneys lie in the retroperitoneal space, one on each side of the vertebral column, within the lumbar gutters - shallow indentations flanked by the quadratus lumborum (lateral) and psoas major (medial).
• They extend between vertebral levels T12-L3, with the right kidney slightly lower than the left (displaced by the liver).
• During inspiration and erect posture, the lower pole descends approximately 3 cm compared to expiration/supine.

• Right kidney: T12 - L3
• Left kidney: T11 - L2 (slightly higher)

• Diaphragm (superiorly)
• 12th rib (crosses the posterior surface)
• Quadratus lumborum, psoas major (medially)
• Subcostal, iliohypogastric, ilioinguinal nerves (cross posterior surface - important for pain radiation to the groin and genitalia)

1. Fibrous (renal) capsule
2. Perirenal fat (adipose capsule)
3. Renal fascia (Gerota's fascia) - encloses kidney + adrenal
4. Pararenal fat (posterior to fascia)

Color Atlas of Human Anatomy Vol. 2, p. 388; Junqueira's Basic Histology, p. 949

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3. External Morphology

• Size: 10-12 cm long, 5-6 cm wide, 4 cm thick
• Weight: 120-300 g (right usually slightly smaller than left)
• Shape: Bean-shaped

• Convex lateral border
• Concave medial border - bears the hilum

1. Renal vein (most anterior)
2. Renal artery
3. Renal pelvis/ureter (most posterior)

• Also transmits lymphatics and autonomic nerves

Color Atlas of Human Anatomy Vol. 2, p. 388

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4. Internal Structure (Gross Anatomy)

Renal Cortex

• Outermost zone, ~1 cm thick, reddish-brown
• Contains renal corpuscles (glomeruli + Bowman capsules), proximal and distal convoluted tubules
• Sends renal columns (of Bertin) inward between pyramids
• Contains medullary rays: longitudinal striations extending from the bases of the pyramids into the cortex - formed by collecting ducts and straight tubules

Renal Medulla

• Inner zone, lighter and striated in appearance
• Divided into 8-12 renal pyramids (conical, with bases toward cortex and apices = papillae toward hilum)
• Contains loops of Henle, straight collecting ducts
• Each pyramid has an outer zone (redder) and an inner zone (lighter)

Renal Lobe

Renal Lobule

Renal Pelvis and Calyces

• Minor calyces (8-10): cup-shaped, surround one (or occasionally 2-3) papilla
• Major calyces (2-3): formed by fusion of minor calyces
• Renal pelvis: formed by fusion of major calyces; volume 3-8 mL; either branching (dendritic) or ampullary type depending on caliceal branching pattern

Color Atlas of Human Anatomy Vol. 2, pp. 388-389; Histology: A Text and Atlas, p. 1940

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5. Blood Supply

• Renal artery (from abdominal aorta at L1-2) → 5 segmental arteries (end arteries, no anastomoses) → interlobar arteries (run in renal columns between pyramids) → arcuate arteries (arc along corticomedullary junction) → interlobular arteries (radiate into cortex) → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries (cortex) + vasa recta (medulla, accompany thin loops of Henle)

• Peritubular capillaries → interlobular veins → arcuate veins → interlobar veins → renal vein → inferior vena cava
• The left renal vein is longer and crosses anterior to the aorta; it receives the left gonadal vein and left suprarenal vein

Histology: A Text and Atlas, p. 1941

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6. Nerve Supply

• Sympathetic fibers from the renal plexus (derived from T10-L1 via the least splanchnic nerve and the renal branches of the celiac and aorticorenal ganglia)
• Parasympathetic from the vagus nerve
• Afferent (pain) fibers travel with sympathetic fibers → pain radiates to the flank, groin, and external genitalia (via subcostal, iliohypogastric, and ilioinguinal nerves)
• Nerves control renal blood flow via vasomotion of afferent/efferent arterioles

Color Atlas of Human Anatomy Vol. 2, p. 399

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7. Histology of the Kidney

Capsule

• Thin, tough fibrous capsule of dense connective tissue
• Easily stripped from a healthy kidney
• Contains collagen fibers; attaches to parenchyma via loose connective tissue

The Nephron (Structural and Functional Unit)

A. Renal Corpuscle (Malpighian Corpuscle)

• Located exclusively in the cortex
• Consists of:
  1. Glomerulus - a tuft of fenestrated capillaries
  2. Bowman's capsule - double-walled cup
     • Parietal layer: simple squamous epithelium
     • Visceral layer: podocytes (highly modified epithelial cells)
• Vascular pole: where afferent and efferent arterioles enter/exit
• Urinary pole: where proximal tubule begins
• The space between the two layers = capsular (Bowman's) space - collects the ultrafiltrate

1. Fenestrated capillary endothelium - blocks blood cells and platelets
2. Glomerular basement membrane (GBM) - 300-360 nm thick; type IV collagen, laminin, fibronectin, proteoglycans; restricts proteins >70 kDa and anions; fused basal laminae of endothelium + podocyte
3. Filtration slit diaphragms between pedicels (podocyte foot processes) - restrict small proteins

• Large cells with a central cell body giving off primary processes → secondary processes → pedicels (foot processes) that interdigitate on the GBM
• Between adjacent pedicels: filtration slits bridged by slit diaphragms (nephrin protein is key component)
• The pedicels give the GBM a "claw" or "octopus" appearance on EM

• Intraglomerular mesangial cells between capillary loops
• Functions: structural support, phagocytosis, contractile (regulate filtration surface), produce cytokines

Junqueira's Basic Histology, pp. 960-962; Histology: A Text and Atlas, p. 1661

B. Proximal Convoluted Tubule (PCT)

• Longest, most prominent segment in the cortex
• Lined by simple cuboidal epithelium with prominent brush border (microvilli) - greatly increases reabsorptive surface area
• Cells have abundant mitochondria (in basal striations), extensive lateral interdigitations, large eosinophilic cytoplasm
• Functions: reabsorbs ~65-70% of filtered Na+, water, glucose, amino acids, bicarbonate; secretes organic acids, some drugs

C. Loop of Henle (Nephron Loop)

1. Proximal straight tubule (thick descending limb): like PCT, simple cuboidal with brush border
2. Thin segment: simple squamous epithelium - permeable to water and urea; descending thin limb permeable to water, ascending thin limb permeable to NaCl
3. Distal straight tubule (thick ascending limb): simple cuboidal without brush border; actively pumps NaCl out (impermeable to water) - establishes the countercurrent multiplier

• Cortical nephrons (85%): short loops, glomeruli in outer cortex
• Juxtamedullary nephrons (15%): long loops reaching deep into medulla, associated with vasa recta - most important for urine concentration

D. Distal Convoluted Tubule (DCT)

• Shorter than PCT, in cortex
• Simple cuboidal, NO brush border, basal striations, smaller cells
• Aldosterone acts here to reabsorb Na+ in exchange for K+/H+

E. Juxtaglomerular Apparatus (JGA)

1. Macula densa: specialized cells of the DCT wall adjacent to the arterioles - monitors NaCl concentration in tubular fluid
2. Juxtaglomerular (granular) cells: modified smooth muscle cells in the wall of the afferent arteriole - synthesize and store renin (released when BP drops or NaCl decreases)
3. Extraglomerular mesangial cells (Lacis cells): between arterioles and macula densa; signaling role

F. Collecting Ducts

• Cortical collecting duct → medullary collecting duct → papillary duct (of Bellini) → opens at area cribrosa of papilla
• Lined by principal cells (light, respond to ADH and aldosterone) and intercalated cells (dark, regulate acid-base balance via H+ and HCO3- transport)
• ADH (vasopressin) inserts aquaporin-2 (AQP2) water channels into the apical membrane of principal cells → increased water reabsorption → concentrated urine

Histology: A Text and Atlas, pp. 1657-1658

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ORGAN 2: THE URETER

1. Definition

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2. Topography

• Length: 25-30 cm
• Course: Divided into abdominal part and pelvic part (+ intramural segment in the bladder wall)
• Runs retroperitoneally along the anterior surface of psoas major
• Enters the pelvic cavity by crossing anterior to the bifurcation of the common iliac artery at the pelvic inlet
• In the pelvis: runs along the lateral pelvic wall → then turns medially above the pelvic floor → enters the bladder base obliquely
• Crosses (from behind): ductus deferens in males / uterine artery in females ("water under the bridge" - artery crosses superior to ureter)

1. Pelviureteric junction (PUJ) - where renal pelvis meets ureter
2. Pelvic brim - where ureter crosses the common iliac bifurcation
3. Vesicoureteric junction (VUJ) - intramural segment as it enters the bladder wall

Gray's Anatomy for Students, p. 538; Color Atlas of Human Anatomy Vol. 2, p. 401

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3. External Morphology

• Slightly flattened, thick-walled tube
• Appears star-shaped in cross-section due to the collapsed lumen with longitudinal mucosal folds

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4. Nerve Supply

• Sympathetic: T10-L2 via renal, aortic, and hypogastric plexuses
• Parasympathetic: Pelvic splanchnic nerves (S2-S4) for the pelvic portion
• Pain: Referred along subcostal, iliohypogastric, ilioinguinal, genitofemoral nerves → flank pain radiating to groin and scrotum/labia (classical ureteric colic)

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5. Histology of the Ureter

a. Mucosa

• Transitional epithelium (urothelium): 5-7 cell layers; adapts to distension by changing cell layer height
• Lamina propria: loose connective tissue, relatively cellular, rich in lymphocytes and blood vessels

b. Muscularis

• Upper 2/3 (abdominal ureter): Inner longitudinal + outer circular smooth muscle
• Lower 1/3 (pelvic ureter): Inner longitudinal + middle circular + outer longitudinal smooth muscle
• Peristaltic contractions (1-5/min) propel urine to the bladder

c. Adventitia

• Loose connective tissue embedding the ureter in surrounding structures
• Contains blood vessels, lymphatics, nerves

Color Atlas of Human Anatomy Vol. 2, p. 401; Histology: A Text and Atlas, pp. 1445-1455

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ORGAN 3: THE URINARY BLADDER

1. Definition

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2. Topography

• Located in the lesser pelvis, anterior to all other pelvic viscera
• When empty: entirely within the pelvic cavity, behind the pubic symphysis
• When full: expands superiorly into the abdominal cavity, lifting the peritoneum off the anterior abdominal wall (clinically important for suprapubic catheterization without entering the peritoneum)

• Anterior: pubic symphysis, retropubic space (of Retzius)
• Superior: peritoneum, small bowel
• Posterior: seminal vesicles, vas deferens, rectovesical pouch, rectum
• Inferior (neck): prostate gland

• Posterior: uterus, vesicouterine pouch, vagina
• Inferior (neck): urogenital diaphragm

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3. External Morphology

• Apex: points anterosuperiorly toward the pubic symphysis; connected to the umbilicus by the median umbilical ligament (remnant of the urachus)
• Base (fundus): triangular, faces posteroinferiorly; the ureters enter the posterolateral angles
• Superior surface: covered by peritoneum
• Two inferolateral surfaces
• Neck: inferior angle where the bladder funnels into the urethra; fixed to pelvic floor by puboprostatic/pubovesical ligaments

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4. Internal Structure

Trigone

• Smooth triangular area on the internal posterior wall of the bladder base
• Bounded by the two ureteric orifices (posterolateral) and the internal urethral orifice (anteroinferior)
• The interureteric ridge (crest) connects the two ureteric orifices
• The mucosa here is smooth (unlike the rest which is folded when empty) - because it is firmly adherent to the underlying muscularis
• Clinically important: first site for bladder tumors and susceptible to infections

Detrusor Muscle

• Three interlaced layers of smooth muscle: inner longitudinal, middle circular, outer longitudinal
• At the neck, the circular fibers thicken to form the internal urethral sphincter (involuntary, sympathetic control - maintains continence at rest)

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5. Nerve Supply

• Parasympathetic (S2-S4, pelvic splanchnic nerves): Cause detrusor contraction and internal sphincter relaxation - voiding
• Sympathetic (T11-L2, hypogastric nerve): Inhibit detrusor, contract internal sphincter - filling/storage; also transmit pain
• Somatic (S2-S4, pudendal nerve): Voluntary control of the external urethral sphincter (striated muscle)
• Afferent: Stretch receptors signal bladder fullness (~300-400 mL triggers sensation); pain via sympathetic (T11-L2)

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6. Histology of the Urinary Bladder

Mucosa

• Transitional epithelium (urothelium): the most characteristic feature
  • When relaxed/empty: 6-8 cell layers thick; surface cells are large, dome-shaped "umbrella cells" or "facet cells" (25-250 μm), often binucleated
  • When distended/full: cells flatten to 2-3 layers; surface cells spread and flatten
  • The umbrella cells have unique fusiform vesicles (discoid vesicles) in the cytoplasm - act as membrane reserves that are inserted into the apical membrane upon stretching, allowing surface area to increase
  • The apical membrane of umbrella cells is highly asymmetric: the outer leaflet is thickened, rigid, forming "plaques" of uroplakins (transmembrane proteins) - makes the epithelium essentially impermeable to ions and water (key protective barrier against hypertonic urine)
• Lamina propria: loose connective tissue

Muscularis (Detrusor)

• Three interlaced layers of smooth muscle (inner longitudinal, middle circular, outer longitudinal) - difficult to distinguish at the histological level as they interdigitate

Serosa / Adventitia

• Superior surface: peritoneum (serosa)
• Inferolateral surfaces: adventitia (loose connective tissue)

Histology: A Text and Atlas, pp. 1455-1486; Junqueira's Basic Histology, pp. 3749-3762

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ORGAN 4: THE URETHRA

1. Definition

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2. Topography and Morphology

Female Urethra

• Length: 3-4 cm
• Runs from the bladder neck, embedded in the anterior wall of the vagina, to open at the external urethral orifice anterior to the vaginal opening in the vestibule
• Short length makes females more susceptible to ascending UTIs

Male Urethra

• Length: 15-20 cm; divided into four parts:

1. External urethral orifice (meatus) - narrowest point
2. Membranous urethra
3. Internal urethral orifice

1. Prostatic urethra
2. Bulbar urethra
3. Navicular fossa

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3. Nerve Supply (Urethra)

• Parasympathetic (S2-S4): Relaxation of internal sphincter; urge sensation
• Pudendal nerve (S2-S4): Somatic control of external sphincter and urethral sensation
• Sympathetic (T11-L2): Contracts internal sphincter during ejaculation (prevents retrograde ejaculation)

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4. Histology of the Urethra

Female Urethra

• Proximal (near bladder): transitional epithelium
• Middle: non-keratinized stratified squamous (or pseudostratified/stratified columnar)
• Distal (near meatus): non-keratinized stratified squamous
• Periurethral glands of Skene (homolog of male prostate)

Male Urethra

• Prostatic urethra: transitional epithelium (continuous with bladder)
• Membranous urethra: pseudostratified or stratified columnar epithelium
• Spongy urethra: pseudostratified columnar (stratified columnar) → stratified squamous at fossa navicularis
• Mucous glands of Littré open into the spongy urethra (small mucous glands)
• Lacunae of Morgagni: small pockets/outpocketings in the mucosa of the spongy urethra

Wall layers (all segments)

• Mucosa (epithelium + lamina propria)
• Muscularis (inner longitudinal + outer circular smooth muscle)
• Adventitia

Junqueira's Basic Histology, pp. 3762-3768; Histology: A Text and Atlas, pp. 1493-1526

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TRANSITIONAL EPITHELIUM (UROTHELIUM) - Summary

• Essentially impermeable to water and salts (uroplakin plaques on apical membrane)
• Adapts to distension by flattening (via insertion of fusiform vesicle membranes)
• X450 H&E: shows 4-5 layers in relaxed ureter; dome cells visible at surface

Histology: A Text and Atlas, pp. 1455-1468

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QUICK REFERENCE SUMMARY TABLE

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KEY EXAM POINTS TO REMEMBER

1. Hilum contents (A-V-U from anterior to posterior): Renal Artery, Renal Vein, Ureter - actually vein is most anterior, then artery, then ureter
2. "Water under the bridge": Uterine artery crosses SUPERIOR to the ureter in females
3. Three ureteric constrictions: PUJ, pelvic brim (iliac crossing), VUJ
4. Nephron types: Cortical (85%) vs. Juxtamedullary (15% - responsible for urinary concentration)
5. JGA components: Macula densa + JG (granular) cells + lacis cells → regulates renin + GFR
6. GFR: 125 mL/min; 180 L/day filtered; ~1.5 L excreted
7. Detrusor sympathetic: Stores urine (contracts sphincter, relaxes detrusor); Parasympathetic: Voids urine
8. Uroplakins = unique proteins forming plaques in umbrella cell apical membrane → impermeability
9. Right renal vein is short; left renal vein is long and receives left gonadal + left suprarenal vein
10. Segmental arteries are end arteries - no anastomoses → infarcts are wedge-shaped

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• Color Atlas of Human Anatomy Vol. 2: Internal Organs (Thieme), pp. 385-403
• Junqueira's Basic Histology: Text and Atlas, 17e, pp. 948-968
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 1847-1945
• Gray's Anatomy for Students, pp. 537-540This guide is compiled directly from four authoritative textbooks in your medical library. Here is a summary of what's covered:

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• Hilum order: vein → artery → ureter (anterior to posterior)
• 3 ureteric constrictions (stone lodgment sites)
• JGA = macula densa + JG cells + lacis cells (renin regulation)
• Sympathetic = storage; Parasympathetic = voiding
• Uroplakin plaques = urothelium impermeability mechanism
• Juxtamedullary nephrons (15%) = urine concentration
• GFR = 125 mL/min = 180 L/day