42-44 Urinary System

Urinary System. Embryonic Development. Kidneys and Renal Parenchyma. Macroscopic Characteristics.

Urinary System

• Composed of kidneys, ureters, urinary bladder, and urethra.

• Functions: filtration of blood, formation of urine, and excretion of waste products.

• Also involved in electrolyte balance, blood pressure regulation, and acid–base homeostasis.

Embryonic Development

• Urinary system develops from the intermediate mesoderm.

• Three kidney stages: pronephros (nonfunctional), mesonephros (transient), metanephros (permanent).

• Metanephros forms the definitive kidney around week 5 of development.

• Ureteric bud forms the collecting system; metanephric blastema forms the nephrons.

Kidneys and Renal Parenchyma

• Each kidney has renal cortex (outer) and renal medulla (inner).

• Nephrons are the functional units, made up of the glomerulus, tubules, and collecting ducts.

• Cortex contains glomeruli and convoluted tubules; medulla contains loops of Henle and collecting ducts.

Macroscopic Characteristics

• Bean-shaped organs, ~11 cm long, located retroperitoneally.

• Right kidney is usually lower due to the liver.

• Covered by fibrous capsule, surrounded by perirenal fat and renal fascia.

• Renal hilum: entry/exit point for renal artery, vein, and ureter.

Kidneys. Nephron - Microscopic, Ultramicroscopic and Functional Characteristics.

Kidneys

• Paired retroperitoneal organs that filter blood, produce urine, and regulate fluid/electrolyte balance.

• Comprised of renal cortex (outer) and renal medulla (inner), with pyramids and columns.

• Contain about 1 million nephrons each (functional units).

Nephron

Microscopic Characteristics

• Consists of renal corpuscle (glomerulus + Bowman's capsule) and renal tubule (proximal tubule, loop of Henle, distal tubule).

• Located mostly in the cortex; loops of Henle extend into the medulla.

Ultramicroscopic Characteristics

• Glomerular capillaries have fenestrated endothelium.

• Bowman's capsule lined by podocytes with foot processes creating filtration slits.

• Tubular epithelial cells show microvilli (especially proximal tubule) to increase absorption surface.

Functional Characteristics

• Filters blood plasma at glomerulus → forms primary urine.

• Reabsorbs useful substances (water, ions, glucose) mostly in proximal tubule.

• Concentrates urine via countercurrent mechanism in loop of Henle.

• Fine-tunes electrolyte and acid-base balance in distal tubule and collecting duct.

Kidneys. Filtration Barrier. Endocrine Apparatus. Blood and Nerve Supply.

Filtration Barrier

• Made up of three layers:

  1. Fenestrated endothelium of glomerular capillaries.

  2. Basement membrane (shared basal lamina).

  3. Podocyte slit diaphragm (between podocyte foot processes).

• Selectively filters based on size and charge; prevents proteins and cells from passing.

Endocrine Apparatus

• Juxtaglomerular apparatus (JGA): specialized cells near afferent arteriole.

• JGA secretes renin to regulate blood pressure and fluid balance.

• Macula densa cells monitor NaCl concentration to modulate renin release.

• Kidneys also produce erythropoietin (stimulates RBC production) and activate vitamin D.

Blood Supply

• Renal artery → segmental → interlobar → arcuate → interlobular arteries → afferent arteriole → glomerulus.

• Blood exits via efferent arteriole → peritubular capillaries and vasa recta → veins following arteries back to renal vein.

Nerve Supply

• Sympathetic nerves regulate renal blood flow, glomerular filtration, and renin secretion.

• Mostly from renal plexus, derived from celiac and aortic plexuses.

Excretory Renal Ducts - Minor and Major Calyces, Pelvis. Macroscopic, Microscopic, Ultramicroscopic and Functional Characteristics.

• Macroscopic: Minor calyces collect urine from renal papillae; several minor calyces join to form major calyces, which merge into the renal pelvis — a funnel-shaped structure leading to the ureter.

• Microscopic: Lined by transitional epithelium (urothelium), supported by lamina propria and smooth muscle.

• Ultramicroscopic: Urothelial cells have specialized tight junctions and umbrella cells to prevent urine leakage.

• Functional: Transport urine from nephrons to ureter, prevent backflow via smooth muscle contractions (peristalsis).

Ureter and Urinary Bladder. Macroscopic, Microscopic, Ultramicroscopic, and Functional Characteristics.

Ureter

• Macroscopic: Muscular tube (~25-30 cm), connects renal pelvis to urinary bladder.

• Microscopic: Three layers:

  1. Mucosa – transitional epithelium with lamina propria.

  2. Muscularis – inner longitudinal and outer circular smooth muscle layers (plus an additional outer longitudinal layer in lower ureter).

  3. Adventitia – connective tissue with blood vessels and nerves.

• Ultramicroscopic: Urothelium with umbrella cells and tight junctions to maintain barrier.

• Functional: Propels urine by peristaltic waves from kidney to bladder; prevents reflux during bladder contraction.

Urinary Bladder

• Macroscopic: Hollow, distensible muscular sac located in the pelvis.

• Microscopic:

  • Mucosa: transitional epithelium with rugae (folds).

  • Submucosa: connective tissue.

  • Muscularis (detrusor muscle): 3 layers of smooth muscle arranged in different directions for effective contraction.

  • Adventitia or serosa depending on location.

• Ultramicroscopic: Urothelial umbrella cells provide a tight barrier against toxic urine.

• Functional: Stores urine; contracts during micturition to expel urine via urethra.

Excretory Renal Ducts. Ureter and Urinary Bladder. Blood and Nerve Supply.

Blood Supply

• Renal ducts & ureter: Branches from renal, gonadal, common iliac, internal iliac, and vesical arteries.

• Urinary bladder: Mainly from superior and inferior vesical arteries.

• Venous drainage follows arteries into corresponding veins.

Nerve Supply

• Ureter: Sympathetic fibers from renal, aortic, and hypogastric plexuses; parasympathetic from pelvic splanchnic nerves.

• Bladder: Sympathetic fibers from hypogastric nerve (promote storage by relaxing detrusor and contracting internal sphincter); parasympathetic from pelvic splanchnic nerves (promote voiding by contracting detrusor).