The Urinary System: Structure, Function, and Regulation

The Urinary System

Overview and Gross Anatomy

The urinary system maintains homeostasis by regulating blood volume, blood pressure, body fluid composition, and removing metabolic wastes. It also performs metabolic functions such as detoxification and vitamin D activation.

• Functions:

  • Regulation of blood volume and pressure (via water loss and erythropoietin/EPO release)

  • Regulation of body fluid pH, electrolytes, and nutrient conservation

  • Removal of metabolic wastes from blood

  • Detoxification and activation of vitamin D

• Main Components: 2 kidneys, urinary tract (ureters, bladder, urethra)

Kidney Anatomy

• External Anatomy:

  • Location: Retroperitoneal

  • Hilum: Indentation for vessel attachment

  • Coverings: Renal capsule, adipose tissue, renal fascia

• Internal Anatomy:

  • Cortex (outer region)

  • Medulla (inner region): Contains renal pyramids (with nephrons) and renal columns

  • Renal lobe: Pyramid, column, and cortex section

  • Renal sinus: Minor calyces → major calyces → renal pelvis → ureter

• Blood and Nerve Supply:

  • Renal arteries → segmental → interlobar → arcuate → interlobular arteries

  • Afferent arterioles → glomerulus → efferent arterioles

  • Peritubular capillaries (along tubule), vasa recta (along nephron loop)

  • Renal plexus (sympathetic ANS): Regulates blood flow and renin release

Nephrons: Functional Unit of Filtration

• Parts of a Nephron:

  • Renal Corpuscle: Glomerular (Bowman's) capsule and glomerulus (capillary network)

  • Renal Tubule: Proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT)

  • Collecting ducts → papillary ducts

• Types of Nephrons:

  • Cortical (85%): Mostly in cortex, short loops

  • Juxtamedullary (15%): Long loops into medulla, important for water retention

• Juxtaglomerular Apparatus (JGA): Contains JG cells and macula densa; produces renin and EPO

Urine Collection and Release Structures

• Papillary ducts → calyces → ureters → bladder → urethra

Kidney Physiology I: Filtration

Three Main Processes

• Filtration: Movement of water and solutes from blood into nephron (glomerulus)

• Reabsorption: Return of useful substances to blood

• Secretion: Additional removal of substances from blood into filtrate

Glomerular Filtration

• Filtration Membrane: Fenestrated endothelium, basal lamina, podocytes (visceral epithelium)

• Filtrate: Water and small solutes (nutrients, electrolytes, wastes); initially similar to plasma

Driving Forces: Net Filtration Pressure (NFP)

• Glomerular Hydrostatic Pressure (GHP): Blood pressure in glomerulus

• Glomerular Colloid Osmotic Pressure (GCOP): Osmotic pull of plasma proteins

• Capsular Hydrostatic Pressure (CHP): Back pressure from filtrate in capsule

Equation:

$NFP = GHP - (GCOP + CHP)$

Normal values: $NFP = 50 - (30 + 10) = 10\ \text{mmHg}$

• Glomerular Filtration Rate (GFR): ~125 mL/min (180 L/day); 99% reabsorbed

• Measured clinically by creatinine clearance (24-hour urine collection)

Regulation of GFR

• Autoregulation: Myogenic (arteriole constriction/relaxation), tubuloglomerular feedback (macula densa)

• Hormonal: RAAS (renin-angiotensin-aldosterone system), ANP (atrial natriuretic peptide)

• Neural: Sympathetic stimulation (varies with stress level)

Regulatory Mechanism	Effect on GFR	Notes
Autoregulation (myogenic)	Maintains GFR despite BP changes	Afferent arteriole constricts/dilates
RAAS	Increases GFR (if BP low)	Renin → Angiotensin II → vasoconstriction, aldosterone, ADH
ANP	Increases GFR (if BP high)	Afferent dilation, efferent constriction
Sympathetic NS	Decreases GFR (high stress)	Constriction of arterioles

Kidney Physiology II: Tubular Reabsorption and Secretion

Nephron and Tubule Structures

• Nephron: Glomerulus, PCT, loop of Henle, DCT

• Types: Cortical and juxtamedullary

• Collecting ducts participate in reabsorption/secretion

Principles of Reabsorption and Secretion

• Reabsorption: Returns useful filtrate components to blood (mainly in PCT)

• Secretion: Moves solutes from blood into urine (mainly in DCT)

• Paracellular transport: Passive (diffusion); limited to water, some anions, urea

• Transcellular transport: Active or passive; includes carrier-mediated transport

• Transport Maximum (Tm): Maximum rate due to limited carrier proteins; excess appears in urine

Reabsorption and Secretion in the PCT

• Nearly all organic solutes (e.g., glucose) reabsorbed

• ~65% of water reabsorbed (osmosis)

• Sodium reabsorption via Na/K pump and facilitated diffusion

• Bicarbonate reabsorption via Na+/H+ transporter; helps regulate pH

• Secretion: Uric acid, ammonium, creatinine, some drugs

Reabsorption in the Nephron Loop: Countercurrent Multiplier

• Descending limb: Permeable to water, not NaCl

• Ascending limb: Impermeable to water, actively transports NaCl

• Countercurrent multiplier: Creates high osmolarity in medulla, allowing water reabsorption

• Vasa recta: Maintains gradient via countercurrent exchange

Reabsorption and Secretion in DCT/Collecting Ducts

• Last 15% of water and 10% of NaCl reabsorbed (hormonally regulated)

• Water reabsorption controlled by ADH and aldosterone

• H+ secretion for pH regulation

Kidney Physiology III: Regulation of Urinary Output

Osmolarity of Filtrate

• Filtrate starts isotonic (~300 mOsm)

• Countercurrent multiplier increases medullary osmolarity

• Longer nephron loops create greater gradients

Hormonal Control of DCT and Collecting Duct

• Aldosterone: Increases Na+ reabsorption, K+ secretion, and water retention

• ADH: Increases water permeability (aquaporins), concentrates urine

• ANP: Increases Na+ and water loss, decreases BP

Regulation of Urinary Output

• Autoregulation: Adjusts GFR in response to BP changes

• Hormones: RAAS, ADH, aldosterone, EPO, ANP, PTH/calcitonin (calcium regulation)

• Sympathetic NS: Reduces GFR during stress

• Renal thresholds: Maximum plasma concentration before substance appears in urine

Drug Interactions

• Diuretics: Increase urinary output (loop, thiazide, K-sparing types)

• RAAS inhibitors: ACE inhibitors, ARBs, aldosterone antagonists

Renal Clearance

• Estimates GFR by comparing blood and urine levels of a substance

• Creatinine commonly used; inulin is more accurate but requires injection

Micturition and Urination

Characteristics and Composition of Urine

• Color: Yellow (urochrome from bilirubin breakdown)

• pH: 4.5–8.0 (average 6)

• 95% water, 5% solutes (urea, ions, creatinine, uric acid)

• Specific gravity: 1.001–1.035

• Volume: 700–2000 mL/day (~1% of GFR)

Urinary Tract Structures

• Ureters: 30 cm muscular tubes, move urine by peristalsis, lined with transitional epithelium

• Bladder: Collapsible, muscular sac (detrusor muscle); trigone region at base

• Urethra: Drains bladder; internal sphincter (involuntary), external sphincter (voluntary); longer in males

Micturition Reflex

• Initiated by stretch receptors (~200 mL)

• Involves spinal reflex and voluntary control (pons, cortex)

• Relaxation of sphincters allows urination

• Affected by age, muscle tone, prostate size

Homeostatic Imbalances and Clinical Applications

Micturition Problems

• Incontinence: Weak sphincters

• Overactive bladder: Frequent urge

• Prostate enlargement: Hesitancy, urgency

• Urinary retention

Abnormal Urinary Output

• Polyuria: Excess urination (e.g., diabetes, high BP)

• Oliguria: Low urine output

• Hematuria: Blood in urine

• Hemoglobinuria, proteinuria, glycosuria

Urinary Tract Infections (UTIs)

• Bacterial infections, more common in females (shorter urethra)

• Can ascend to kidneys (pyelonephritis)

Kidney Stones (Renal Calculi)

• Composed mainly of calcium oxalate

• High-oxalate foods: Spinach, bran, celery, peanut butter, chocolate

Glomerulonephritis

• Damage to glomerulus (often traumatic or immune-mediated)

Renal Failure and Treatment

• Decreased GFR leads to accumulation of wastes

• BUN (Blood Urea Nitrogen) test monitors function

• Chronic: Gradual decline, managed by diet (low salt/protein)

• Acute: Sudden, often fatal

• Dialysis: Artificial filtration using diffusion gradients

Additional info: The urinary system is closely linked to fluid and electrolyte balance, acid-base regulation, and blood pressure control, which are covered in subsequent chapters.