The Urinary System: Structure, Function, and Regulation

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The Urinary System

Introduction and Overview

The urinary system is essential for maintaining homeostasis by regulating fluid and electrolyte balance, acid–base balance, blood pressure, and the elimination of metabolic wastes. It consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys are the primary organs responsible for filtering blood and forming urine, while the other structures transport, store, and expel urine from the body.

Kidneys: Filter blood, remove waste, and form urine.
Ureters: Transport urine to the urinary bladder.
Urinary bladder: Stores urine until elimination.
Urethra: Eliminates urine from the body.

Key physiological processes in the kidneys:

Elimination of metabolic wastes (e.g., urea, uric acid)
Regulation of ion levels (Na+, K+, Ca2+, PO43−)
Regulation of acid–base balance (H+, HCO3−)
Blood pressure regulation (renin-angiotensin system)
Elimination of hormones and drugs
Hormone production (calcitriol, erythropoietin)
Gluconeogenesis during nutrient deprivation

Gross Anatomy of the Kidney

Location and Support

Located retroperitoneally; left kidney (T12–L3), right kidney ~2 cm lower.
Protected by several layers:
- Fibrous capsule: Dense irregular connective tissue, maintains shape and protects from trauma/infection.
- Perinephric fat: Adipose tissue for cushioning.
- Renal fascia: Anchors kidney to surrounding structures.
- Paranephric fat: Outermost adipose layer for additional cushioning.

Renal Ptosis and Hydronephrosis

Renal ptosis: Inferior displacement of the kidney, often due to loss of fat support (e.g., anorexia, elderly).
Can cause ureter kinking, leading to hydronephrosis (urine backup), potentially resulting in renal failure.

Kidney Variations and Anomalies

Renal agenesis: Failure of kidney development.
Pelvic kidney: Kidney remains in pelvic cavity.
Horseshoe kidney: Fusion of inferior portions of both kidneys.
Supernumerary kidneys: More than two kidneys develop.

Sectional Anatomy

Renal cortex: Outer region.
Renal medulla: Inner region, contains renal pyramids.
Renal columns: Extensions of cortex into medulla.
Renal pyramids: Subdivisions of medulla.
Corticomedullary junction: Where cortex meets medulla.
Renal papilla: Tip of pyramid.
Renal lobe: Pyramid + adjacent columns + cortex.
Renal sinus: Medial space for urine drainage (minor/major calyces, renal pelvis).

Innervation

Innervated by autonomic nervous system (renal plexus):
- Sympathetic (T10–T12): Decreases urine production.
- Parasympathetic (vagus nerve): Effects not well known.

Functional Anatomy of the Kidney

Nephron Structure

The nephron is the microscopic functional unit of the kidney, responsible for filtration and urine formation.

Renal corpuscle: Glomerulus (capillary tuft) + glomerular (Bowman’s) capsule.
Renal tubule: Proximal convoluted tubule (PCT), nephron loop (descending/ascending limbs), distal convoluted tubule (DCT).

Types of nephrons:

Cortical nephrons: Short loops, mostly in cortex (~85%).
Juxtamedullary nephrons: Long loops, extend deep into medulla, crucial for urine concentration.

Collecting Tubules and Ducts

Nephrons drain into collecting tubules → collecting ducts → papillary ducts.
Cell types:
- Principal cells: Respond to aldosterone and ADH.
- Intercalated cells (A & B): Regulate urine and blood pH.

Juxtaglomerular Apparatus (JGA)

Specialized region where DCT contacts afferent arteriole.
Granular (juxtaglomerular) cells: Release renin, regulate blood pressure.
Macula densa: Detects NaCl concentration, signals renin release.
Extraglomerular mesangial cells: Support communication within JGA.

Blood Flow and Filtrate Flow

Blood Flow Through the Kidney

Renal artery → segmental arteries → interlobar arteries → arcuate arteries → interlobular arteries → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries/vasa recta → interlobular veins → arcuate veins → interlobar veins → renal vein → inferior vena cava.

Filtrate, Tubular Fluid, and Urine Flow

Filtrate forms in glomerular capsule → PCT (tubular fluid) → nephron loop → DCT → collecting tubules → collecting ducts → papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder → urethra (urine).

Urine Formation

Overview

Glomerular filtration: Passive separation of water/solutes from blood plasma into capsular space.
Tubular reabsorption: Movement of substances from tubular fluid back into blood.
Tubular secretion: Active movement of substances from blood into tubular fluid.

Filtration Membrane

Composed of fenestrated endothelium, basement membrane, and podocyte layer (with filtration slits).
Allows passage of small molecules (water, glucose, ions), restricts large proteins and cells.
Mesangial cells: Support, phagocytosis, regulate filtration surface area.

Filtrate Formation and Composition

~180 L/day of filtrate produced.
Freely filtered: water, glucose, amino acids, ions, urea, hormones, vitamins, ketones.
Not filtered: formed elements, large proteins.
Limited filtration: intermediate-size proteins (repelled by charge/size).

Pressures in Glomerular Filtration

Glomerular hydrostatic pressure (HPg): Pushes fluid out of glomerulus (typically 60 mmHg).
Blood colloid osmotic pressure (OPg): Pulls fluid into glomerulus (typically 32 mmHg).
Capsular hydrostatic pressure (HPc): Opposes filtration (typically 18 mmHg).
Net filtration pressure (NFP):

Example:

Glomerular filtration rate (GFR): Volume of filtrate formed per minute (normal adult: 125 ml/min).

Regulation of GFR

Intrinsic (renal autoregulation): Maintains constant GFR via myogenic mechanism and tubuloglomerular feedback.
Extrinsic (neural/hormonal): Sympathetic stimulation decreases GFR; ANP increases GFR.

Reabsorption and Secretion in Tubules and Collecting Ducts

Transport Processes

Paracellular transport: Between cells.
Transcellular transport: Through cells (crosses luminal and basolateral membranes).
Most reabsorption occurs in PCT (microvilli increase surface area).

Transport Maximum and Renal Threshold

Transport maximum (Tm): Maximum reabsorption rate for a substance.
Renal threshold: Maximum plasma concentration before substance appears in urine.

Glucosuria

Glucose in urine (e.g., diabetes mellitus) when blood glucose exceeds Tm.
Acts as osmotic diuretic, increasing water loss and thirst.

Complete Reabsorption

Glucose: Reabsorbed in PCT via Na+/glucose symporters (secondary active transport), then facilitated diffusion into blood.
Small proteins: Reabsorbed by endocytosis, digested to amino acids, released into blood.

Renal Disease Consequences

Proteinuria: Excess protein in urine due to glomerular damage.
Increased plasma proteins if filtration is reduced.

Regulated Reabsorption

Sodium (Na+): Reabsorbed throughout tubule, especially PCT; regulated by aldosterone (increases reabsorption) and ANP (decreases reabsorption).
Water: Reabsorbed by osmosis; regulated by ADH (increases aquaporins in collecting ducts) and aldosterone.
Potassium (K+): Reabsorbed in PCT, secreted in collecting ducts (regulated by aldosterone).
Calcium and phosphate: Regulated by parathyroid hormone (PTH); PTH increases Ca2+ reabsorption, decreases PO43− reabsorption.
Bicarbonate (HCO3−) and hydrogen ions (H+): Regulate blood and urine pH; intercalated cells in collecting ducts adjust secretion/reabsorption as needed.

Elimination of Waste Products

Nitrogenous wastes: Urea (protein metabolism), uric acid (nucleic acid metabolism), creatinine (muscle metabolism).
Drugs and bioactive substances: Secreted mainly in PCT (e.g., antibiotics, hormones).

Establishing the Concentration Gradient

Gradient in medullary interstitium (Na+, Cl−) allows water reabsorption from collecting ducts (regulated by ADH).
Countercurrent multiplier: Nephron loop establishes gradient (descending limb: water out; ascending limb: salt out).
Countercurrent exchange: Vasa recta maintains gradient by exchanging water and solutes.
Urea recycling contributes to medullary osmolarity.

Diuretics

Increase urine output; used to treat hypertension.
Loop diuretics: Inhibit Na+ reabsorption in nephron loop.
Thiazides: Inhibit Na+ and water reabsorption in DCT.

Evaluating Kidney Function

Measuring Glomerular Filtration Rate (GFR)

Inulin clearance test: Inulin is freely filtered, not reabsorbed/secreted.
Normal GFR: 125 ml/min in adults.

Measuring Renal Plasma Clearance

Volume of plasma cleared of a substance per unit time.
Helps determine if a substance is reabsorbed or secreted.
Important for drug dosing and monitoring kidney function.

Renal Failure, Dialysis, and Transplant

Renal failure: Loss of ~90% kidney function.
Dialysis: Artificial filtration (peritoneal or hemodialysis).
Kidney transplant: Replacement with donor kidney.

Urine Characteristics, Transport, Storage, and Elimination

Characteristics of Urine

95% water, 5% solutes (salts, nitrogenous wastes, hormones, drugs, ketones).
pH: 4.5–8.0 (average 6.0).
Specific gravity: <1.010 (hydration), >1.020 (dehydration).
Color: Clear to dark yellow (urobilin content).
Odor: "Urinoid" (normal smell).

Urinary Tract Anatomy

Ureters

Muscular tubes from renal pelvis to bladder.
Layers: mucosa (transitional epithelium), muscularis (smooth muscle), adventitia (connective tissue).
Peristalsis propels urine; valves prevent backflow.
Innervation: Sympathetic (T11–L2), parasympathetic (vagus, pelvic splanchnic nerves).

Urinary Bladder

Expandable, muscular organ for urine storage.
Trigone: Immobile triangular area funnels urine to urethra.
Rugae: Mucosal folds for distension.
Detrusor muscle: Three layers of smooth muscle.
Adventitia (outer layer), peritoneum (superior surface only).

Urethra

Transports urine out of body.
Internal urethral sphincter: Involuntary, smooth muscle.
External urethral sphincter: Voluntary, skeletal muscle.
Female: ~4 cm, only urinary function.
Male: Prostatic, membranous, and spongy regions; also reproductive function.

Urinary Tract Disorders

Urinary Tract Infections (UTIs)

Caused by bacteria/fungi entering urinary tract.
More common in women.
Pyelonephritis: Infection involving kidneys.
Diagnosed by urinalysis.

Renal Calculi (Kidney Stones)

Formed from crystalline materials (e.g., calcium).
Risk factors: Hypercalciuria, dehydration.
Treatments: Lithotripsy (shock waves), ureteroscopy.

Micturition (Urination)

Controlled by autonomic and somatic nervous systems.
Storage reflex: Sympathetic stimulation relaxes detrusor, contracts internal sphincter.
Micturition reflex: Parasympathetic stimulation contracts detrusor, relaxes internal sphincter; voluntary relaxation of external sphincter allows urination.
Valsalva maneuver (abdominal muscle contraction) can assist urination.

Impaired Urination

Urinary incontinence: Inability to control urination.
Urinary retention: Inability to eliminate urine normally.

Summary Table: Main Functions and Structures of the Urinary System

Structure	Main Function(s)
Kidney	Filtration of blood, urine formation, regulation of fluid/electrolyte/acid-base balance, hormone production
Ureter	Transport urine from kidney to bladder
Urinary Bladder	Storage of urine
Urethra	Elimination of urine from body

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