The Urinary System: Structure, Function, and Physiology

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The Urinary System

Overview and Functions

The urinary system is essential for maintaining the body's internal environment by regulating water, solute concentrations, and removing metabolic wastes. It consists of the kidneys, ureters, urinary bladder, and urethra.

Regulation of water and solutes: Kidneys control total water volume and solute concentration.
Ion balance: Maintains ion concentrations in extracellular fluid.
Acid-base balance: Ensures long-term acid-base equilibrium.
Excretion: Removes metabolic wastes, toxins, and drugs.
Hormone production: Produces erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).
Vitamin D activation: Converts vitamin D to its active form.
Gluconeogenesis: Generates glucose during prolonged fasting.

Anatomy of the urinary system organs

Components of the Urinary System

Kidneys: Major excretory organs.
Ureters: Transport urine from kidneys to bladder.
Urinary bladder: Temporary storage for urine.
Urethra: Conducts urine out of the body.

Dissection of urinary system organs (male)

Gross Anatomy of the Kidneys

Location and External Anatomy

The kidneys are retroperitoneal, located in the superior lumbar region between T12 and L5. The right kidney is slightly lower due to the liver. Each kidney is topped by an adrenal gland.

Renal hilum: Entry/exit for ureters, blood vessels, lymphatics, and nerves.
Supportive tissue layers:
- Renal fascia: Anchoring outer layer.
- Perirenal fat capsule: Cushions the kidney.
- Fibrous capsule: Prevents infection spread.

Kidney location relative to thoracic cage Cross-section showing supportive tissue layers around kidney

Internal Gross Anatomy

The kidney has three main regions:

Renal cortex: Superficial, granular region.
Renal medulla: Deep region with cone-shaped pyramids.
Renal pelvis: Funnel-shaped tube continuous with ureter.

Urine flow: Renal pyramid → minor calyx → major calyx → renal pelvis → ureter.

Frontal section of kidney showing cortex, medulla, calyces, and pelvis Diagram of kidney showing cortex, medulla, calyces, pelvis, and vessels Internal anatomy of the kidney

Blood and Nerve Supply

Renal Circulation

Kidneys receive about one-fourth of cardiac output per minute.

Arterial flow: Renal → segmental → interlobar → arcuate → cortical radiate (interlobular).
Venous flow: Cortical radiate → arcuate → interlobar → renal veins.
Nerve supply: Sympathetic fibers from renal plexus.

Blood vessels of the kidney Path of blood flow through renal blood vessels

Nephrons: The Functional Units

Structure of Nephrons

Nephrons are the structural and functional units that form urine. Each kidney contains over one million nephrons.

Renal corpuscle: Includes the glomerulus and glomerular (Bowman's) capsule.
Renal tubule: Includes proximal convoluted tubule, nephron loop, distal convoluted tubule, and collecting duct.

Nephron structure and urine formation

Renal Corpuscle

Glomerulus: Tuft of fenestrated capillaries for efficient filtrate formation.
Bowman's capsule: Cup-shaped structure surrounding the glomerulus.

Bowman's capsule and glomerulus

Renal Tubule and Collecting Duct

Proximal convoluted tubule (PCT): Cuboidal cells with dense microvilli for reabsorption and secretion.
Nephron loop (Loop of Henle): U-shaped, with descending and ascending limbs.
Distal convoluted tubule (DCT): Cuboidal cells, mainly for secretion.
Collecting duct: Receives filtrate from DCT.

Proximal convoluted tubule cells Histology of renal tubule regions Nephron loop structure Distal convoluted tubule cells Renal cortical tissue

Classes of Nephrons

Cortical nephrons: 85% of nephrons, mostly in cortex.
Juxtamedullary nephrons: Long loops deeply invade medulla, crucial for concentrated urine production.

Cortical and juxtamedullary nephrons

Nephron Capillary Beds

Types of Capillary Beds

Glomerulus: Specialized for filtration, fed and drained by arterioles.
Peritubular capillaries: Low-pressure, porous, adapted for absorption.
Vasa recta: Long vessels parallel to nephron loops in juxtamedullary nephrons, important for urine concentration.

Nephron capillary beds Glomerulus capillary bed Peritubular capillary beds Vasa recta capillary beds Blood vessels of the renal cortex

Juxtaglomerular Complex (JGC)

Structure and Function

The JGC regulates filtrate formation and blood pressure. It consists of:

Macula densa: Chemoreceptors sensing NaCl content.
Granular cells: Mechanoreceptors sensing blood pressure, secrete renin.
Extraglomerular mesangial cells: Pass signals between macula densa and granular cells.

Juxtaglomerular complex structure Macula densa cells Granular cells (juxtaglomerular cells) Extraglomerular mesangial cells

Physiology of the Kidney

Urine Formation

The kidneys process 180L of fluid daily, forming only 1.5L of urine. Three main processes are involved:

Glomerular filtration: Produces cell- and protein-free filtrate.
Tubular reabsorption: Returns 99% of substances to blood.
Tubular secretion: Moves substances from blood to filtrate.

Three major renal processes

Glomerular Filtration

Passive process: Driven by hydrostatic pressure.
Filtration membrane: Three layers: fenestrated endothelium, basement membrane, and podocyte foot processes.
Allows passage: Water, glucose, amino acids, nitrogenous wastes; blocks cells and most proteins.

Glomerular filtration process Filtration membrane structure Filtration membrane and macromolecule handling

Pressures Affecting Filtration

Outward pressure: Hydrostatic pressure in glomerular capillaries (HPgc), ~55 mmHg.
Inward pressures: Hydrostatic pressure in capsular space (HPcs), 15 mmHg; colloid osmotic pressure in capillaries (OPgc), 30 mmHg.
Net filtration pressure (NFP):

Pressures affecting glomerular filtration

Glomerular Filtration Rate (GFR)

Definition: Volume of filtrate formed per minute by both kidneys (120–125 ml/min).
Factors: NFP, surface area, membrane permeability.

Regulation of Glomerular Filtration

Intrinsic controls: Renal autoregulation (myogenic and tubuloglomerular feedback mechanisms).
Extrinsic controls: Neural and hormonal mechanisms (renin-angiotensin-aldosterone system).

Regulation of glomerular filtration rate Summary table of GFR regulation

Tubular Reabsorption and Secretion

Tubular Reabsorption

Tubular reabsorption reclaims most of the filtrate and returns it to the blood.

Routes: Transcellular (through cells) and paracellular (between cells).
Active and passive transport: Used for water and solutes.

Transcellular and paracellular routes of reabsorption Transcellular route of reabsorption Paracellular route of reabsorption

Transport Maximum

Definition: Maximum rate of reabsorption for each substance, determined by number of carriers.
Clinical relevance: Hyperglycemia can exceed Tm, causing glucose in urine.

Reabsorptive Capabilities of Renal Tubules

PCT: Site of most reabsorption (all nutrients, 65% Na+ and water).
Nephron loop: Descending limb allows water reabsorption; ascending limb allows solute reabsorption.
DCT and collecting duct: Reabsorption regulated by hormones (ADH, aldosterone, atrial natriuretic peptide, parathyroid hormone).

Reabsorptive capabilities of renal tubules Nephron loop reabsorption ADH effect on collecting duct Aldosterone effect on collecting duct Hormonal regulation of reabsorption Reabsorption capabilities of different segments

Tubular Secretion

Tubular secretion moves substances from blood into filtrate, mainly in the PCT.

Functions: Disposal of drugs, elimination of reabsorbed wastes, removal of excess K+, regulation of blood pH.

Tubular secretion process Functions of tubular secretion

Regulation of Urine Concentration and Volume

Countercurrent Mechanisms

The kidneys use countercurrent mechanisms to maintain a medullary osmotic gradient, allowing concentration or dilution of urine.

Countercurrent multiplier: Interaction of filtrate flow in nephron loops.
Countercurrent exchanger: Blood flow in vasa recta preserves gradient.

Countercurrent mechanism overview Medullary osmotic gradient Countercurrent exchanger in vasa recta Countercurrent exchanger preserves gradient

Formation of Dilute or Concentrated Urine

Overhydration: Produces dilute urine (low ADH).
Dehydration: Produces concentrated urine (high ADH).

Mechanism for forming dilute or concentrated urine

Urea Recycling

Urea contributes to the medullary osmotic gradient by recycling between nephron loop and collecting duct.

Clinical Evaluation of Kidney Function

Urinalysis and Renal Clearance

Urinalysis: Examines urine for disease or illegal substances.
Renal clearance: Volume of plasma cleared of a substance per unit time. Used to assess GFR and detect renal disease.
Formula: where C = clearance rate, U = urine concentration, V = urine flow rate, P = plasma concentration.

Clinical Disorders

Chronic renal disease: GFR < 60 ml/min for 3 months.
Renal failure: GFR < 15 ml/min; causes uremia, treated by dialysis or transplant.

Urine: Composition and Characteristics

Chemical Composition

95% water, 5% solutes (urea, uric acid, creatinine, ions).
Abnormal components may indicate pathology.

Urine composition

Physical Characteristics

Color: Pale to deep yellow; abnormal colors may indicate disease.
Odor: Slightly aromatic; changes with disease or diet.
pH: Slightly acidic (~6), varies with diet.
Specific gravity: 1.001–1.035.

Transport, Storage, and Elimination of Urine

Ureters

Ureters are muscular tubes that convey urine from kidneys to bladder.

Three layers: Mucosa (transitional epithelium), muscularis (smooth muscle), adventitia (fibrous connective tissue).
Peristalsis: Propels urine; rate adjusted to urine formation.

Ureters anatomy Layers of ureter wall

Clinical Disorders: Renal Calculi

Kidney stones: Crystallized salts; can block ureter and cause pain.
Treatment: Lithotripsy, hydration, surgical removal.

Renal calculi (kidney stones)

Urinary Bladder

Muscular sac: Temporary storage for urine.
Trigone: Area outlined by ureter and urethra openings; infection-prone.
Wall layers: Mucosa, detrusor muscle, adventitia.
Capacity: Moderately full bladder holds ~500 ml; can expand further.

Urinary bladder anatomy Layers of bladder wall Urine storage capacity of bladder

Urethra

Muscular tube: Drains bladder.
Lining: Varies from transitional to stratified squamous epithelium.
Sphincters: Internal (involuntary) and external (voluntary).
Female urethra: Short, tightly bound to anterior vaginal wall.
Male urethra: Longer, carries urine and semen; divided into prostatic, membranous, and spongy regions.

Urethra anatomy Urethral sphincters

Clinical Disorders: Urinary Tract Infections (UTIs)

Causes: Improper hygiene, short female urethra.
Symptoms: Pain, frequent urination, blood, cramping.
Treatment: Antibiotics.

Micturition (Urination)

Mechanism

Micturition involves contraction of detrusor muscle, opening of internal and external sphincters.

Storage phase: Bladder fills with urine.
Voiding phase: Bladder contracts, sphincters open, urine expelled.

Control of micturition

Reflexive and Voluntary Control

Infants: Reflexive urination via spinal stretch receptors.
Adults: Pontine centers mature, allowing voluntary control.

Clinical Problems

Incontinence: Loss of bladder control; stress or overflow types.
Retention: Inability to expel urine; may require catheterization.

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