The Urinary System: Structure and Function

The Urinary System

Overview and Importance

The urinary system is essential for maintaining the body's internal environment by eliminating wastes and toxins through urine. Understanding its function is crucial for diagnosing and managing renal diseases.

• Key Functions: Regulation of water and solute balance, excretion of metabolic wastes, and maintenance of acid-base homeostasis.

• Major Organs: Kidneys, ureters, urinary bladder, and urethra.

Functions of the Kidneys

Homeostatic Roles

• Regulation of total water volume and solute concentration in the body.

• Regulation of ion concentrations in extracellular fluid (ECF).

• Long-term acid-base balance maintenance.

• Excretion of metabolic wastes, toxins, and drugs.

• Production of hormones: erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).

• Activation of vitamin D and gluconeogenesis (if needed).

Gross Anatomy of the Kidneys

Location and External Anatomy

• Retroperitoneal position in the superior lumbar region (between T12 and L3 vertebrae).

• Right kidney is lower than the left due to the liver.

• Adrenal (suprarenal) gland sits atop each kidney.

• Renal hilum: Concave medial surface where ureters, blood vessels, lymphatics, and nerves enter/exit.

• Supportive tissue layers:

  • Renal fascia: Anchoring outer layer of dense connective tissue.

  • Perirenal fat capsule: Fatty cushion for protection.

  • Fibrous capsule: Transparent layer preventing infection spread.

Internal Gross Anatomy

• Renal cortex: Superficial, granular-appearing region.

• Renal medulla: Deep to cortex, composed of cone-shaped medullary (renal) pyramids.

  • Papilla: Tip of pyramid, points internally.

  • Renal columns: Inward extensions of cortical tissue separating pyramids.

  • Lobe: Medullary pyramid and surrounding cortical tissue; about eight per kidney.

• Renal pelvis: Funnel-shaped tube continuous with ureter.

  • Minor calyces: Cup-shaped areas collecting urine from papillae.

  • Major calyces: Collect urine from minor calyces and empty into renal pelvis.

• Urine flow: Renal pyramid → minor calyx → major calyx → renal pelvis → ureter.

Clinical Note: Homeostatic Imbalance

• Protection: Upper kidney parts protected by thoracic cage and perirenal fat.

• Trauma risk: Lower kidney parts susceptible to blunt trauma (e.g., falls, accidents).

• Renal artery: Vulnerable to injury from rapid deceleration (e.g., car crashes).

• Hematuria: Blood in urine may indicate trauma.

Blood and Nerve Supply

Renal Circulation

• Renal arteries deliver about 25% of cardiac output to kidneys each minute (~1200 ml/min).

• Arterial flow: Renal → segmental → interlobar → arcuate → cortical radiate arteries.

• Venous flow: Cortical radiate → arcuate → interlobar → renal veins.

• Nerve supply: Sympathetic fibers from the renal plexus.

Nephrons

Structure and Function

Nephrons are the structural and functional units of the kidney, responsible for urine formation. Each kidney contains over 1 million nephrons.

• Two main parts:

  • Renal corpuscle

  • Renal tubule

Renal Corpuscle

• Glomerulus: Tuft of fenestrated capillaries allowing efficient filtrate formation.

• Glomerular (Bowman's) capsule: Cup-shaped, hollow structure surrounding the glomerulus.

  • Parietal layer: Simple squamous epithelium.

  • Visceral layer: Branching epithelial podocytes with foot processes forming filtration slits for filtrate passage.

Renal Tubule and Collecting Duct

• About 3 cm long, composed of a single layer of epithelial cells with region-specific histology and function.

• Three major parts:

  1. Proximal convoluted tubule (PCT): Cuboidal cells with dense microvilli (brush border) and many mitochondria; functions in reabsorption and secretion; confined to cortex.

  2. Nephron loop (Loop of Henle): U-shaped with descending and ascending limbs.

     • Descending limb: Proximal part continuous with PCT; distal part is the thin limb (simple squamous epithelium).

     • Ascending limb: Thick in some nephrons (cuboidal/columnar cells), thin in others.

  3. Distal convoluted tubule (DCT): Cuboidal cells with few microvilli; more involved in secretion than reabsorption; confined to cortex.

• Collecting ducts: Receive filtrate from many nephrons, run through medullary pyramids, and fuse to deliver urine through papillae into minor calyces.

  • Principal cells: Sparse microvilli; maintain water and Na+ balance.

  • Intercalated cells: Abundant microvilli; maintain acid-base balance (types A and B).

Classes of Nephrons

• Cortical nephrons: 85% of nephrons; almost entirely in cortex.

• Juxtamedullary nephrons: Long loops deeply invade medulla; important for concentrated urine production.

Nephron Capillary Beds

• Each nephron is associated with two capillary beds:

  • Glomerulus: Specialized for filtration; fed and drained by arterioles (afferent and efferent).

  • Peritubular capillaries: Low-pressure, porous; adapted for absorption; arise from efferent arterioles and cling to renal tubules in cortex.

• Vasa recta: Long, thin-walled vessels parallel to juxtamedullary nephron loops; function in formation of concentrated urine.

Juxtaglomerular Complex (JGC)

• Each nephron has one JGC, involving the distal ascending limb of the nephron loop and the afferent arteriole.

• Regulates filtrate formation and blood pressure.

Key Terms

• Podocyte: Specialized epithelial cell in the glomerulus with foot processes forming filtration slits.

• Filtration slit: Narrow gap between podocyte foot processes, allowing filtrate to enter the capsular space.

• Renal corpuscle: Structure composed of the glomerulus and Bowman's capsule.

• Renal tubule: Tubular component of the nephron, including PCT, nephron loop, and DCT.

Example: Blood Flow Through the Kidney

Blood enters the kidney via the renal artery, passes through segmental, interlobar, arcuate, and cortical radiate arteries, then through the afferent arteriole into the glomerulus. After filtration, blood exits via the efferent arteriole, peritubular capillaries or vasa recta, and finally through the venous system (cortical radiate, arcuate, interlobar, and renal veins).

Additional info: This summary covers the structure and function of the urinary system, focusing on the kidneys and nephrons, as presented in the provided lecture slides. Further details on renal physiology, urine formation, and clinical correlations would be included in subsequent sections of the chapter.