The Urinary System

Overview and Functions

The urinary system is essential for maintaining the body's internal environment by regulating the volume and composition of blood, removing metabolic wastes, and balancing electrolytes and pH. It consists of paired kidneys, ureters, the urinary bladder, and the urethra.

• Excretion of metabolic wastes (e.g., urea, creatinine, uric acid)

• Regulation of blood volume and pressure via water and sodium balance

• Electrolyte and acid-base balance

• Production of hormones (e.g., erythropoietin, renin)

• Activation of vitamin D

Major Structures of the Urinary System

• Kidneys: Filter blood and form urine

• Ureters: Transport urine from kidneys to bladder

• Urinary bladder: Stores urine

• Urethra: Conducts urine out of the body

Kidney Anatomy

External Anatomy

• Location: Retroperitoneal, between T12 and L3 vertebrae

• Connective Tissue Layers:

  • Renal fascia: Anchors kidney to surrounding structures

  • Perirenal fat capsule: Cushions and protects kidney

  • Fibrous capsule: Prevents infection spread to kidney

• Renal ptosis: Dropping of kidney due to loss of fat capsule

• Hydronephrosis: Swelling of kidney due to urine backup

Internal Anatomy

• Renal cortex: Outer region, contains renal corpuscles and convoluted tubules

• Renal medulla: Inner region, contains renal pyramids

• Renal pyramids: Cone-shaped tissues containing nephron loops and collecting ducts

• Renal columns: Extensions of cortex between pyramids

• Lobes: Each consists of a pyramid and surrounding cortical tissue

• Renal pelvis: Funnel-shaped tube collecting urine from calyces

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

• Pyelitis: Infection of renal pelvis

• Pyelonephritis: Infection of entire kidney

Blood and Nerve Supply

• Renal arteries supply blood; renal veins drain blood

• Renal plexus: Network of nerves regulating blood flow and urine formation

Nephron Structure and Function

Nephron Overview

Nephrons are the functional units of the kidney, responsible for filtering blood and forming urine. Each kidney contains about 1 million nephrons.

• Location: Cortex and medulla

Renal Corpuscle

• Glomerulus: Tuft of fenestrated capillaries; site of filtration

  • Fenestrated endothelium: Allows passage of plasma but not blood cells

  • Filtrate: Fluid filtered from blood into capsule

• Glomerular (Bowman's) capsule:

  • Parietal layer: Simple squamous epithelium

  • Visceral layer: Podocytes with foot processes forming filtration slits

  • Capsular space: Collects filtrate

Renal Tubule

• Proximal convoluted tubule (PCT): Cuboidal cells with microvilli for reabsorption

• Nephron loop (Loop of Henle): Descending and ascending limbs; thin and thick segments

• Distal convoluted tubule (DCT): Cuboidal cells, fewer microvilli

• Collecting duct: Receives filtrate from multiple nephrons; principal and intercalated cells

Nephron Classes

• Cortical nephrons: 85%; short loops; mainly in cortex; function in filtration and reabsorption

• Juxtamedullary nephrons: Long loops; extend deep into medulla; concentrate urine

Nephron Capillary Beds

• Glomerulus: High-pressure capillary bed for filtration

  • Afferent arteriole: Brings blood in; larger diameter

  • Efferent arteriole: Drains blood; smaller diameter; maintains high pressure

• Peritubular capillaries: Low-pressure; surround tubules; reabsorb water and solutes

• Vasa recta: Long, straight capillaries in juxtamedullary nephrons; maintain medullary osmotic gradient

Juxtaglomerular Complex (JGC)

• Regulates filtration rate and systemic blood pressure

• Macula densa: Senses NaCl in DCT

• Granular (juxtaglomerular) cells: Secrete renin

• Extraglomerular mesangial cells: Communication between macula densa and granular cells

Urine Formation

Glomerular Filtration

• Filtration membrane:

  • Fenestrated endothelium: Permits passage of plasma components

  • Basement membrane: Blocks large proteins; contains glycoproteins

  • Podocyte foot processes: Filtration slits with diaphragms

• Glomerular mesangial cells: Remove trapped residues and regulate filtration

• Permeability: Water, glucose, amino acids, ions pass; proteins and cells do not

Filtration Pressures

• Outward pressures:

  • Hydrostatic pressure in glomerular capillaries (HPgc): Main force driving filtration

  • Colloid osmotic pressure in capsular space: Usually negligible

• Inward pressures:

  • Hydrostatic pressure in capsular space (HPcs)

  • Colloid osmotic pressure in glomerular capillaries (OPgc)

Net Filtration Pressure (NFP):

Glomerular Filtration Rate (GFR)

• Volume of filtrate formed per minute by both kidneys (~120-125 mL/min)

• Depends on NFP, surface area, and membrane permeability

Regulation of GFR

• Intrinsic controls (renal autoregulation):

  • Myogenic mechanism: Vascular smooth muscle responds to pressure changes

  • Tubuloglomerular feedback: Macula densa senses NaCl; adjusts afferent arteriole

• Extrinsic controls:

  • Sympathetic nervous system: Constricts afferent arterioles, reduces GFR

  • Renin-angiotensin-aldosterone mechanism: Increases blood pressure and GFR

Tubular Reabsorption

• Selective movement of substances from filtrate back into blood

• Routes:

  • Transcellular: Through cells

  • Paracellular: Between cells

• Active and passive transport mechanisms

• Sodium reabsorption:

  • Basolateral membrane: Na+/K+ ATPase pump

  • Apical membrane: Various symporters and antiporters

• Reabsorption of nutrients, water, ions:

  • Secondary active transport (e.g., glucose, amino acids)

  • Osmosis for water

  • Passive diffusion for some solutes

• Segmental reabsorption:

  • PCT: Most reabsorption occurs here

  • Nephron loop: Water in descending limb, Na+ and Cl- in ascending limb

  • DCT and collecting duct: Regulated by hormones

• Hormonal regulation:

  • ADH: Increases water reabsorption

  • Aldosterone: Increases Na+ reabsorption

  • ANP: Decreases Na+ reabsorption

  • PTH: Increases Ca2+ reabsorption

Tubular Secretion

• Transfer of substances from blood into tubular fluid

• Important for disposing of drugs, excess K+, controlling blood pH

Regulation of Urine Concentration and Volume

Medullary Osmotic Gradient

• Established by countercurrent mechanisms in juxtamedullary nephrons

• Countercurrent multiplier: Nephron loop creates gradient by differing permeability

• Countercurrent exchanger: Vasa recta preserves gradient by exchanging solutes and water

• Urea recycling: Enhances medullary osmotic gradient

• Diuretics: Increase urine volume by inhibiting Na+ reabsorption

Renal Function Evaluation

Urinalysis

• Examines physical and chemical properties of urine

Renal Clearance

• Measures volume of plasma cleared of a substance per minute

• Inulin used as standard marker

• C = clearance rate, U = concentration in urine, V = urine flow rate, P = concentration in plasma

• Helps assess GFR and renal function

Urine Composition and Characteristics

• Chemical composition: 95% water, 5% solutes (urea, Na+, K+, PO43-, etc.)

• Physical characteristics:

  • Color: Clear to deep yellow

  • Odor: Slightly aromatic

  • pH: Slightly acidic (pH 6)

  • Specific gravity: 1.001–1.035

Urinary Tract Structures

Ureters

• Mucosa: Transitional epithelium

• Muscularis: Smooth muscle layers

• Adventitia: Fibrous connective tissue

Urinary Bladder

• Trigone: Triangular area between ureter openings and urethra

• Urine storage capacity: 500–1000 mL

• Rugae: Folds allowing expansion

Urethra

• Internal urethral sphincter: Involuntary smooth muscle

• External urethral sphincter: Voluntary skeletal muscle

• Male vs Female: Male urethra longer, passes through prostate and penis; female shorter, more prone to infection

Micturition (Urination)

• Reflex controlled by spinal cord and higher brain centers

• Involves contraction of bladder detrusor muscle and relaxation of sphincters

Selected Homeostatic Imbalances

Example: In pyelonephritis, bacteria ascend from the bladder to infect the kidney, causing inflammation and potential kidney damage.

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