The Urinary System and Fluid, Electrolyte, and Acid-Base Homeostasis

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, just below the rib cage.

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

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

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

Functions of the Kidneys

• Excretion of metabolic wastes (e.g., urea, creatinine, uric acid)

• Regulation of blood volume and pressure

• Regulation of electrolyte balance

• Regulation of acid-base balance

• Regulation of erythropoiesis (via erythropoietin secretion)

• Activation of vitamin D

External Structure of the Kidney

• Location: Retroperitoneal, between T12 and L3 vertebrae.

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

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

Nephron: The Functional Unit

The nephron is the microscopic structural and functional unit of the kidney, responsible for filtration and urine formation.

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

• Function: Filters blood, reabsorbs needed substances, and secretes wastes.

Kidney Structure: Macro and Microscopic Levels

• Macroscopic: Cortex (outer), medulla (inner), renal pyramids, renal pelvis.

• Microscopic: Nephrons, collecting ducts, capillary networks.

Blood Flow Through the Kidneys

• Arterial Pathway (largest to smallest): 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 (smallest to largest): 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

Feature	Cortical Nephrons	Juxtamedullary Nephrons
Location	Mostly in cortex	Near cortex-medulla junction
Nephron Loop	Short	Long, extends deep into medulla
Function	Majority of filtration	Concentrating urine

Filtrate, Tubular Fluid, and Urine

• Filtrate: Fluid filtered from blood at 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

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

2. Tubular Reabsorption: Movement of substances from tubular fluid back into blood.

3. Tubular Secretion: Movement of substances from blood into 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: About 125 mL/min in adults.

Net Filtration Pressure (NFP)

• Determined by hydrostatic and colloid osmotic pressures:

$\text{NFP} = (\text{Glomerular hydrostatic pressure}) - (\text{Capsular hydrostatic pressure} + \text{Blood colloid osmotic pressure})$

Factors Affecting GFR

• Increased blood pressure increases GFR; dehydration or low blood pressure decreases GFR.

Regulation of GFR

• Myogenic Mechanism: Smooth muscle response to changes in blood pressure.

• Tubuloglomerular Feedback: Macula densa cells sense NaCl and adjust afferent arteriole diameter.

• Renin-Angiotensin-Aldosterone System (RAAS): Increases blood pressure and GFR.

• Atrial Natriuretic Peptide (ANP): Decreases blood pressure and GFR.

• Sympathetic Nervous System: Can decrease 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 (osmosis, diffusion) and active (pumps, transporters) processes.

• 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, raises blood pressure.

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

• ANP: Inhibits Na+ reabsorption, lowers blood pressure.

Regulation of Urine Concentration and Volume

• Permeability: Different segments of the nephron have varying permeability to water and solutes, 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.

• Function: Allows kidneys to produce concentrated urine.

Physical and Chemical Properties of Urine

• Normal urine: Clear, pale 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.

$\text{Renal Clearance} = \frac{U \times V}{P}$ Where U = concentration in urine, V = urine flow rate, P = plasma concentration.

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.

• Bladder Histology: Transitional epithelium allows stretching.

• Micturition Reflex: Stretch receptors trigger bladder contraction and relaxation of internal urethral sphincter.

• Voluntary Control: External urethral sphincter under somatic control.

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:

  • ICF: High K+, low Na+

  • ECF: High Na+, low K+

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

• Hydrostatic Pressure: Physical force of fluid; opposes osmotic pressure.

• Water Gain/Loss: Intake (drinking, food, metabolism); loss (urine, sweat, feces, respiration).

• Regulation: Thirst mechanism, ADH, aldosterone, and ANP control intake and output.

• 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 (regulates Ca2+), calcitonin, and ANP.

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

Acid-Base Homeostasis

• Blood pH: Normal range 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-

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

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

• Renal Regulation: Kidneys secrete H+ and reabsorb/generate HCO3- to adjust pH.

• Compensation: Respiratory or metabolic disturbances are corrected by the other system (e.g., metabolic acidosis compensated by hyperventilation).

Integration of Homeostatic Mechanisms

• Cardiovascular System: Adjusts blood pressure and volume.

• Endocrine System: Hormones (ADH, aldosterone, ANP) regulate fluid and electrolyte balance.

• Urinary System: Excretes or conserves water, electrolytes, and acids/bases to maintain homeostasis.

Example

• During dehydration, ADH is released, increasing water reabsorption in the kidneys, while aldosterone promotes Na+ reabsorption, helping restore fluid balance.