The Urinary System: Structure, Function, and Regulation

The Urinary System

Introduction to the Urinary System

The urinary system is essential for maintaining homeostasis by regulating the composition and volume of blood and removing metabolic wastes. It consists of several organs that work together to filter blood, form urine, and eliminate it from the body.

• Overall Role: Removal of metabolic wastes, regulation of fluid and electrolyte balance, acid-base balance, and blood pressure.

• Organs: Kidneys (filter blood and form urine), ureters (transport urine), urinary bladder (stores urine), urethra (eliminates urine).

• Urinary Tract: Ureters, urinary bladder, and urethra.

• Filtrate vs. Urine: Filtrate is the fluid filtered from blood in the kidneys; urine is the final, modified fluid excreted from the body.

• Kidney Functions: Filtration, reabsorption, secretion, regulation of blood pressure, erythropoietin production, and activation of vitamin D.

Kidney Anatomy

Internal Structure and Blood Flow

• Regions: Renal cortex (outer), renal medulla (middle, contains pyramids), renal pelvis (collects urine).

• Functions: Cortex (filtration), medulla (concentration of urine), pelvis (urine collection).

• Nephron: Structural and functional unit; consists of renal corpuscle and renal tubule.

• Hilum: Medial indentation where vessels, nerves, and ureter enter/exit.

• Blood Flow: Afferent arteriole → glomerulus (capillaries) → efferent arteriole → peritubular capillaries → peritubular venules.

• Glomerulus: Filtration; Peritubular Capillaries: Reabsorption and secretion.

Renal Corpuscle Anatomy

Location, Structure, and Function

• Location: Renal cortex.

• Function: Filtration of blood plasma to form filtrate.

• Glomerular Capillaries: Fenestrated capillaries (allow passage of water and small solutes).

• Glomerular Capsule: Parietal layer (simple squamous epithelium, outer), visceral layer (podocytes, inner).

• Capsular Space: Collects filtrate.

• Podocytes: Specialized cells with filtration slits, crucial for selective filtration.

Renal Tubule Anatomy

Structure, Function, and Collecting System

• Location: Both cortex and medulla.

• Function: Modification of filtrate via reabsorption and secretion.

• Regions (in order): Proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT).

• Epithelium: PCT (cuboidal with many microvilli), nephron loop (thin: simple squamous; thick: cuboidal), DCT (cuboidal with fewer microvilli).

• Collecting System: Collecting duct (medulla), papillary duct (renal papilla).

• Filtrate Pathway: Renal corpuscle → PCT → nephron loop → DCT → collecting duct → papillary duct → renal pelvis.

• Juxtaglomerular Apparatus (JGA): Located at the junction of DCT and afferent arteriole; regulates blood pressure and GFR; contains macula densa (senses NaCl) and juxtaglomerular cells (release renin).

Types of Nephrons

Cortical vs. Juxtamedullary Nephrons

Feature	Cortical Nephrons	Juxtamedullary Nephrons
Abundance	~85%	~15%
Location	Cortex	Near cortex-medulla junction
Loop Length	Short	Long
Capillaries	Peritubular	Vasa recta
Urine Concentration	Limited	Major role

Mechanisms of Urine Formation

Filtration, Reabsorption, and Secretion

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

• Reabsorption: Return of useful substances from filtrate to blood (mainly PCT, also nephron loop, DCT, collecting duct).

• Secretion: Active transport of substances from blood into filtrate (mainly PCT, DCT, collecting duct).

• Excretion: Elimination of urine from the body (not the same as secretion).

Glomerular Filtration

Filtration Membrane and Net Filtration Pressure

• Filtration: Passive process driven by hydrostatic pressure.

• Filtration Membrane: Fenestrated endothelium, basement membrane (negatively charged), podocyte filtration slits.

• Charge: Basement membrane repels negatively charged proteins.

• Substances Filtered: Water, electrolytes, glucose, amino acids, small molecules.

• Substances Retained: Blood cells, large proteins.

• Glomerulonephritis: Inflammation leads to protein and blood cells in urine.

Net Filtration Pressure (NFP)

• Hydrostatic Pressure (HP): Pushes water out of capillaries.

• Osmotic Pressure (OP): Pulls water into capillaries.

• Direction: Water moves from high to low HP; from low to high OP.

• Calculation: $\text{NFP} = (\text{GHP}) - (\text{GCOP} + \text{CHP})$

• Glomerular Pressures: GHP (glomerular hydrostatic pressure), GCOP (glomerular colloid osmotic pressure), CHP (capsular hydrostatic pressure).

• Proteins: Albumin generates GCOP.

• Outcome: GHP usually "wins," causing net filtration.

Glomerular Filtration Rate (GFR)

Definition and Regulation

• GFR: Volume of filtrate formed per minute by both kidneys ($\approx 125\,\text{mL/min}$).

• Relationship: GFR is directly proportional to NFP.

• Regulation: GHP is most dynamically regulated.

• Renal Failure: Severely decreased GFR; leads to uremia (toxic buildup of wastes).

• Treatment: Dialysis or kidney transplant.

Regulation of GFR

Mechanisms and Effects

• Importance: Maintains fluid, electrolyte, and waste balance.

• Mechanisms: Autoregulation, hormonal regulation, neural regulation.

• Target: All mechanisms alter GHP.

• Arteriole Diameter: Vasoconstriction increases resistance, decreases flow; vasodilation decreases resistance, increases flow.

• Effects:

  • Afferent vasoconstriction: ↓ GHP, ↓ GFR

  • Efferent vasoconstriction: ↑ GHP, ↑ GFR

  • Afferent vasodilation: ↑ GHP, ↑ GFR

  • Efferent vasodilation: ↓ GHP, ↓ GFR

Autoregulation of GFR

Myogenic Mechanism

• Type: Local mechanism.

• Goal: Maintain stable GFR despite BP changes.

• Action: Changes afferent arteriole diameter in response to stretch.

• Stimulus: Increased BP stretches afferent arteriole → constriction; decreased BP → dilation.

• Effect: Returns GFR to normal.

Tubuloglomerular Feedback

• Stimulus: Increased GFR → more NaCl to macula densa.

• Response: Macula densa releases signals → afferent arteriole constricts → GFR decreases.

• Goal: Maintain GFR at set point.

Hormonal Regulation of GFR

Renin-Angiotensin-Aldosterone System (RAAS)

• Type: Systemic; affects GFR and blood pressure.

• Activation: Low BP, low GFR, or sympathetic stimulation.

• Initiation: Juxtaglomerular cells release renin.

• Pathway: Renin converts angiotensinogen (from liver) to angiotensin I; ACE (from lungs) converts it to angiotensin II.

• Effects: Angiotensin II constricts efferent arteriole (↑ GFR), stimulates aldosterone (Na+ reabsorption), ADH (water reabsorption), and increases systemic BP.

Atrial Natriuretic Peptide (ANP)

• Source: Heart atria.

• Goal: Lower blood volume and BP.

• Mechanism: Dilates afferent, constricts efferent arteriole → ↑ GFR.

• Effect: Increases urine output, decreases BP.

Neural Regulation of GFR

• Type: Systemic; affects GFR and BP.

• Division: Sympathetic nervous system.

• Action: Vasoconstriction (mainly afferent arteriole at high stimulation).

• Effect: Decreases GFR and increases BP (during stress or blood loss).

• Interaction: Stimulates RAAS.

Tubular Reabsorption and Secretion

Processes and Transport Mechanisms

• Reabsorption: Movement from filtrate to blood; recovers nutrients, water, ions. Occurs via passive (diffusion, osmosis) and active (ATP-dependent) transport.

• Secretion: Movement from blood to filtrate; removes wastes, drugs, excess ions. Mainly active transport.

• Passive vs. Active: Passive does not require energy; active requires ATP.

• Fates: Reabsorbed substances return to blood; secreted substances are excreted in urine.

• Homeostasis: Fluid/electrolyte, acid-base, and waste removal.

Transport Routes

• Paracellular: Between cells; passive only.

• Transcellular: Through cells; passive (diffusion, osmosis, facilitated diffusion) and active (primary, secondary) transport.

• Capillaries: Peritubular capillaries (cortical nephrons), vasa recta (juxtamedullary nephrons).

• Antiporter: Moves substances in opposite directions; Symporter: Moves substances in same direction.

Reabsorption and Secretion in Nephron Segments

Proximal Tubule

• Major Site: Most reabsorption occurs here; many microvilli increase surface area.

• Homeostasis: Fluid/electrolyte, acid-base, nutrient balance.

• Reabsorbed: Glucose, amino acids, Na+, Cl-, HCO3-, water.

• Sodium: Drives reabsorption of other substances (via Na+/K+ ATPase).

• Angiotensin II: Increases Na+ reabsorption.

• Secreted: H+, drugs, toxins.

Glucose Reabsorption

• Mechanism: Secondary active transport (SGLT symporter with Na+), then facilitated diffusion (GLUT transporter) into blood.

• Saturation: Transporters have a maximum rate; excess glucose appears in urine (glycosuria), often in diabetes mellitus.

Bicarbonate Ion Reabsorption

• Mechanism: Involves carbonic anhydrase enzyme; HCO3- converted to CO2 in filtrate, diffuses into cell, reconverted to HCO3- and transported to blood.

• Homeostasis: Maintains acid-base balance.

Water Reabsorption

• Requirements: Osmotic gradient and water channels (aquaporins).

• Obligatory: Not hormone-dependent (mainly in PCT and descending limb).

• Facultative: Hormone-dependent (ADH-regulated, mainly in collecting duct).

• Osmotic Gradient: Created by Na+ reabsorption.

Nephron Loop

• Descending Limb: Reabsorbs water (obligatory, via osmosis).

• Ascending Limb: Reabsorbs Na+, Cl- (active transport); impermeable to water.

• Osmotic Gradient: Multiplied by countercurrent mechanism.

Distal Tubule and Collecting System

• Reabsorption: Fine-tuning of Na+, Cl-, water, Ca2+ (hormone-regulated).

• Secretion: K+, H+, drugs, toxins.

• Hormones: Aldosterone (Na+ reabsorption, K+ secretion), ADH (water reabsorption via aquaporins), ANP (inhibits Na+ reabsorption).

• Diuretics: Drugs that increase urine output by inhibiting water reabsorption.

Acid-Base Balance

• Blood pH: 7.35–7.45 (slightly alkaline).

• Hydrogen Ions: High [H+] = low pH (acidic); low [H+] = high pH (alkaline).

• Kidney Processes: Secretion of H+ (lowers pH), reabsorption of HCO3- (raises pH).

• Acidosis: Kidneys increase H+ secretion, HCO3- reabsorption; urine becomes acidic.

• Alkalosis: Kidneys decrease H+ secretion, HCO3- reabsorption; urine becomes alkaline.

Regulation of Urine Concentration

Osmolarity and Hormonal Control

• Facultative Water Reabsorption: Determines urine concentration (regulated by ADH).

• Osmolarity Changes: Filtrate is isotonic in PCT, becomes concentrated in descending limb, diluted in ascending limb.

• Dilute Urine: Produced with low ADH, large volume, low osmolarity.

• Concentrated Urine: Produced with high ADH, small volume, high osmolarity.

• Hydration Status: Overhydration → dilute urine; dehydration → concentrated urine.

Formation of Dilute Urine

• Reason: Excess water intake.

• Mechanism: Little/no facultative water reabsorption; low ADH.

• Filtrate: Becomes more dilute as it passes through collecting duct.

Formation of Concentrated Urine

• Reason: Water conservation (dehydration).

• Requirements: Medullary osmotic gradient and ADH.

• Mechanisms: Countercurrent multiplier (nephron loop), urea recycling, countercurrent exchanger (vasa recta).

• Filtrate: Becomes more concentrated in collecting duct.

• Nephrolithiasis: Kidney stones; risk increases with concentrated urine.

Urine Composition and Urinalysis

Normal and Abnormal Findings

• Normal Composition: Water, urea, creatinine, ions (Na+, K+, Cl-, HCO3-).

• Abnormal Components: Glucose (diabetes), proteins (glomerular damage), blood (trauma/infection), ketones (starvation/diabetes).

• Color: Yellow (urochrome pigment from hemoglobin breakdown); darker color = more concentrated.

• Specific Gravity: Higher = more concentrated urine.

• pH: 4.5–8.0 (usually slightly acidic).

The Urinary Tract

Pathway and Structure

• Pathway: Kidneys → ureters → urinary bladder → urethra.

• Ureters: Transport urine; three layers (mucosa: transitional epithelium, muscularis: smooth muscle, adventitia: connective tissue).

• Urinary Bladder: Stores urine; three layers (mucosa: transitional epithelium, detrusor muscle: smooth muscle, adventitia); internal urethral sphincter controls outflow.

• Mucus: Protects bladder lining; deficiency increases infection risk.

• Location: Pelvic cavity; anterior to rectum (males), anterior to vagina and uterus (females).

• Urethra: Male (longer, dual function), female (shorter, more prone to infection).

• External Urethral Sphincter: Skeletal muscle, voluntary control, extension of pelvic floor muscles.

Micturition (Urination)

Control and Reflexes

• Internal Sphincter: Smooth muscle, involuntary, autonomic control.

• External Sphincter: Skeletal muscle, voluntary, somatic control.

• Receptors: Stretch receptors in bladder wall initiate reflex.

• Involuntary Reflex: Parasympathetic neurons contract detrusor, relax internal sphincter.

• Voluntary Control: Somatic neurons, cerebral cortex, external sphincter.

• Blockage: Loss of voluntary control leads to incontinence.

Summary: Integration of the Urinary System

• Kidneys filter blood, form urine, and regulate homeostasis.

• Urine is transported by ureters, stored in the bladder, and eliminated via the urethra.

• Multiple regulatory mechanisms ensure proper urine formation and excretion.