The Urinary System: Structure, Function, and Regulation

The Urinary System

Overview and Gross Anatomy

The urinary system is essential for maintaining 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:

  • Regulates blood volume and pressure (via water loss and erythropoietin, EPO)

  • Maintains body fluid pH and electrolyte balance

  • Removes metabolic wastes from blood

  • Detoxifies substances and activates vitamin D

• 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: 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 tubules), vasa recta (along nephron loop)

  • Renal veins (drain blood)

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

Nephrons: The Functional Unit

Nephrons are the microscopic functional units of the kidney responsible for filtration and urine formation.

• 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 and papillary ducts

• Types of Nephrons:

  • Cortical nephrons: 85%, mostly in cortex, short loops

  • Juxtamedullary nephrons: 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

Processes of Urine Formation

Urine formation involves three main processes:

1. Filtration: Movement of water and solutes from blood into the nephron

2. Reabsorption: Return of useful substances to blood

3. Secretion: Active transport of additional wastes into the nephron

Glomerular Filtration

• Filtration Membrane Structure:

  • Fenestrated endothelium

  • Basal lamina

  • Visceral epithelium (podocytes)

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

• Net Filtration Pressure (NFP):

  • Formula: $NFP = GHP - (GCOP + CHP)$

  • Where:

    • GHP: Glomerular hydrostatic pressure (blood pressure in glomerulus)

    • GCOP: Glomerular colloid osmotic pressure (due to plasma proteins)

    • CHP: Capsular hydrostatic pressure (back pressure from filtrate)

  • Typical values: $NFP = 50 - (30 + 10) = 10$ 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:

  • Dilation of afferent/constriction of efferent arterioles increases GFR

  • Constriction of afferent/dilation of efferent arterioles decreases GFR

GFR Homeostatic Mechanisms

Mechanism	Description
Autoregulation	Myogenic response and tubuloglomerular feedback adjust afferent arteriole diameter to maintain GFR despite BP changes
Hormonal Regulation	RAAS increases GFR when BP is low; ANP increases GFR when BP is high
Neural Regulation	Sympathetic stimulation can increase or decrease GFR depending on intensity

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

• GFR: 180 L/day, 99% reabsorbed

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, limited (e.g., water, some anions, urea)

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

• Transport Maximum (TM): Maximum rate of transport 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 by osmosis

• Sodium reabsorption via Na/K pump and facilitated diffusion

• Bicarbonate reabsorption via Na+/H+ transporter and carbonic acid cycle

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

Reabsorption in the Nephron Loop: Countercurrent Multiplier

• Purpose: Creates a concentration gradient in the medulla to conserve water

• Structure:

  • Descending limb: Permeable to water, not NaCl

  • Ascending limb: Actively transports NaCl, impermeable to water

• Countercurrent Multiplier: Active NaCl transport increases medullary osmolarity, drawing water out of descending limb, amplifying the gradient

• Urea recycling: Adds to medullary osmolarity

• 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 variable (ADH, aldosterone)

• H+ secretion for pH regulation

• Collecting ducts: Final regulation of ions and water

Kidney Physiology III: Regulation of Urinary Output

Osmolarity of Filtrate

• Filtrate starts isotonic (~300 mOsm)

• Countercurrent multiplier creates high medullary osmolarity

• Longer nephron loops = greater concentration gradient

Hormonal Control of DCT and Collecting Duct

• Aldosterone: Increases Na+ reabsorption and K+ secretion; increases water reabsorption

• ADH: Increases water permeability via aquaporins; concentrates urine

• ANP: Increases Na+ secretion, blocks ADH/aldosterone, increases urine output

Regulation of Urinary Output

• Autoregulation: Adjusts GFR in response to BP changes

• Hormones: Renin-angiotensin, ADH, aldosterone, EPO, ANP, PTH/calcitonin

• Other factors: Sympathetic activity, renal thresholds, overall BP

• Drug interactions: Diuretics (loop, thiazide, K-sparing), RAAS inhibitors (ACE inhibitors, ARBs, aldosterone antagonists)

Summary Table: Hormonal Effects on Urinary Output

Hormone	Effect
ADH	Increases water reabsorption, concentrates urine, increases BP
Aldosterone	Increases Na+ (and water) reabsorption, increases BP
ANP	Increases Na+ and water loss, decreases BP
Renin-Angiotensin	Increases GFR, stimulates aldosterone and ADH
PTH/Calcitonin	Regulate Ca2+ reabsorption

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)

• Odor: Mild, varies with diet

• pH: 4.5–8.0 (average 6)

• Composition: 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

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

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

Micturition Reflex

• Initiated by stretch receptors (~200 mL)

• Reflex arc stimulates detrusor contraction and urge to urinate

• Central pathways (pons, cortex) allow voluntary control

• Failure to relax sphincters temporarily suppresses urge

• Age, muscle tone, and prostate size affect reflex

Homeostatic Imbalances and Clinical Applications

Micturition Problems

• Incontinence: Weak sphincter muscles

• Overactive bladder: Frequent urge

• Prostate enlargement: Hesitancy, urgency

• Urinary retention

Abnormal Urinary Output

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

• Oliguria: Decreased urination

• Hematuria: Blood in urine

• Hemoglobinuria: Hemoglobin in urine

• Proteinuria: Protein in urine

• Glycosuria: Glucose in urine

Urinary Tract Infections (UTIs)

• Bacterial infections ascending from urethra to bladder/kidneys

• More common in females (shorter urethra)

• Kidney infection: Pyelonephritis

Kidney Stones (Renal Calculi)

• Composed mainly of calcium oxalate

• Risk factors: High-oxalate foods (spinach, bran, chocolate, etc.)

Glomerulonephritis

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

Renal Failure and Treatment

• Decreased GFR leads to accumulation of wastes

• BUN test (Blood Urea Nitrogen) assesses function

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

• Acute renal failure: Sudden, often fatal

• Dialysis: Artificial filtration using diffusion gradients

Urinary System and Fluid Regulation

• Closely linked to fluid and electrolyte balance (see next chapter)

Example: If GFR drops from 99% to 98% reabsorption, urine output doubles from 1.8 L to 3.6 L/day, illustrating the sensitivity of urine volume to reabsorption rates.

Additional info: The notes above expand on the original outline by providing definitions, physiological mechanisms, and clinical context for each process and structure described.