The Urinary System

Organs of the Urinary System

The urinary system is responsible for filtering blood, removing waste, and regulating fluid and electrolyte balance. Its main organs include:

• Kidneys: Located retroperitoneally on either side of the vertebral column, they filter blood and produce urine.

• Ureters: Muscular tubes that transport urine from the kidneys to the urinary bladder.

• Urinary Bladder: A hollow, muscular organ that stores urine until elimination.

• Urethra: A tube that carries urine from the bladder to the exterior of the body.

Functions of the Kidneys

• Excretion of metabolic wastes

• Regulation of blood volume and pressure

• Regulation of electrolyte balance

• Regulation of acid-base balance

• Regulation of erythropoiesis (via erythropoietin secretion)

• Detoxification of blood

External Structure of the Kidney

• Location: Upper posterior abdominal wall, protected by the lower ribs.

• Support Structures: Renal fascia, perirenal fat capsule, and fibrous capsule.

• Coverings: Each kidney is surrounded by a tough fibrous capsule for protection.

The Nephron: Basic Unit of Filtration

The nephron is the microscopic structural and functional unit of the kidney, responsible for filtering blood and forming urine.

• Components: Renal corpuscle (glomerulus + Bowman's capsule) and renal tubule (proximal convoluted tubule, nephron loop, distal convoluted tubule).

• Function: Filtration, reabsorption, and secretion.

Kidney Structure: Macro and Microscopic Levels

• Macroscopic: Cortex (outer region), medulla (inner region with renal pyramids), renal pelvis (collects urine).

• Microscopic: Nephrons, collecting ducts, blood vessels.

Blood Flow Through the Kidneys

• Arterial Pathway: Renal artery → segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries → afferent arterioles.

• Capillary Beds: Glomerulus (filtration), peritubular capillaries/vasa recta (reabsorption and secretion).

• Venous Pathway: Cortical radiate veins → arcuate veins → interlobar veins → renal vein.

Filtrate Pathway Through the Nephron and Collecting System

• Glomerular capsule → proximal convoluted tubule → nephron loop (descending and ascending limbs) → distal convoluted tubule → collecting duct → papillary duct.

Cortical vs. Juxtamedullary Nephrons

• Cortical nephrons: Located mostly in the cortex; short nephron loops; majority of nephrons.

• Juxtamedullary nephrons: Long nephron loops extending deep into the medulla; important for concentrating urine.

Filtrate, Tubular Fluid, and Urine

• Filtrate: Fluid filtered from blood in the glomerulus.

• Tubular fluid: Filtrate as it passes through the renal tubule.

• Urine: Final product after reabsorption and secretion, collected in the renal pelvis.

Urine Formation: Three Major Processes

• Glomerular Filtration: Movement of water and solutes from blood into the glomerular capsule.

• Tubular Reabsorption: Movement of substances from the filtrate back into the blood.

• Tubular Secretion: Movement of substances from blood into the tubular fluid.

Filtration Membrane Structure

• Composed of fenestrated endothelium, basement membrane, and podocyte filtration slits.

Glomerular Filtration Rate (GFR)

• Definition: Volume of filtrate produced per minute by both kidneys.

• Average Value: Approximately 125 mL/min in adults.

Net Filtration Pressure (NFP)

• Determined by hydrostatic and colloid osmotic pressures:

Factors Affecting GFR

• Blood pressure, blood volume, constriction/dilation of afferent and efferent arterioles.

Regulation of GFR

• Myogenic mechanism: Smooth muscle response to changes in pressure.

• Tubuloglomerular feedback: Macula densa cells sense NaCl concentration.

• Renin-angiotensin-aldosterone system (RAAS): Increases GFR via vasoconstriction and aldosterone release.

• Atrial natriuretic peptide (ANP): Increases GFR by dilating afferent arterioles.

• Sympathetic nervous system: Decreases GFR during stress.

Pathways of Filtrate and Urine

• Filtrate: Glomerulus → Bowman's capsule → PCT → nephron loop → DCT → collecting duct.

• Urine: Papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder → urethra.

Tubular Reabsorption and Secretion

• Reabsorption: Water, glucose, amino acids, and ions reabsorbed in PCT, nephron loop, DCT, and collecting duct by passive and active transport.

• Secretion: Occurs mainly in PCT and DCT; substances like H+, K+, and drugs are secreted into the tubule.

Hormonal Regulation of Reabsorption and Secretion

• RAAS: Increases Na+ and water reabsorption, increases blood pressure.

• Antidiuretic hormone (ADH): Increases water reabsorption in collecting ducts.

• ANP: Inhibits Na+ reabsorption, increases urine output.

Regulation of Urine Concentration and Volume

• Permeability of renal tubule: Varies along its length, allowing for concentration or dilution of urine.

• Conditions: High ADH = concentrated urine; low ADH = dilute urine.

Countercurrent Mechanism

• Nephron loop: Creates osmotic gradient in medulla.

• Vasa recta: Maintains gradient by countercurrent exchange.

Physical and Chemical Properties of Urine

• Normal urine: Clear, yellow, slightly acidic (pH ~6), specific gravity 1.001–1.035.

• Excretion rate: Determined by filtration, reabsorption, and secretion rates.

• Renal clearance: Used to estimate GFR.

Where U = concentration in urine, V = urine flow rate, P = concentration in plasma.

Urine Transport, Storage, and Elimination

• Ureters: Transport urine via peristalsis.

• Urinary bladder: Stores urine; detrusor muscle contracts during urination.

• Urethra: Conducts urine out of the body; longer in males than females.

• Micturition reflex: Involuntary and voluntary control of urination.

Fluid, Electrolyte, and Acid-Base Homeostasis

Fluid Homeostasis

• Fluid compartments: Intracellular fluid (ICF, ~2/3 of body water) and extracellular fluid (ECF, ~1/3; includes interstitial fluid and plasma).

• Electrolyte concentrations: Na+ and Cl- high in ECF; K+ and phosphate high in ICF.

• Osmotic pressure: Generated by solute concentration differences; drives water movement.

• Hydrostatic pressure: Pushes water out of compartments.

• Water gain: Ingestion, metabolic water production.

• Water loss: Urine, sweat, feces, respiration.

• Regulation: Thirst mechanism, ADH, aldosterone, ANP.

• Dehydration: Water loss exceeds intake; overhydration: Excess water intake or retention.

Electrolyte Homeostasis

• Major electrolytes: Sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl-), phosphate (PO43-).

• Functions: Nerve impulse transmission, muscle contraction, acid-base balance.

• Hormonal regulation: Aldosterone (increases Na+ reabsorption), parathyroid hormone (increases Ca2+), calcitonin (decreases Ca2+).

• Calcium and phosphate: Inversely regulated; PTH increases Ca2+ and decreases phosphate reabsorption.

Acid-Base Homeostasis

• Blood pH: Normally 7.35–7.45; maintained by buffer systems, respiratory, and renal mechanisms.

• Buffer systems: Bicarbonate, phosphate, and protein buffers stabilize pH.

• PCO2 and bicarbonate: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

• Respiratory regulation: Hypoventilation increases CO2 (lowers pH); hyperventilation decreases CO2 (raises pH).

• Renal regulation: Kidneys secrete H+ and reabsorb HCO3- to adjust pH.

• Compensation: Respiratory or metabolic disturbances are corrected by the other system.

Homeostatic Responses of Body Systems

• Cardiovascular system: Adjusts blood pressure and volume.

• Endocrine system: Releases hormones (ADH, aldosterone, ANP) to regulate fluid and electrolyte balance.

• Urinary system: Adjusts excretion of water, electrolytes, and acids/bases to maintain homeostasis.

Example Table: Major Electrolyte Concentrations in Body Fluids

Additional info: Some explanations and the table were expanded for clarity and completeness based on standard Anatomy & Physiology content.