Urinary System and Fluid, Electrolyte, and Acid-Base Balance: Study Guide

Urinary System

Anatomical Structures and Functions of the Kidney

The kidneys are paired organs responsible for filtering blood, removing waste, and regulating fluid and electrolyte balance. They are located retroperitoneally and are composed of an outer cortex and inner medulla.

• Renal cortex: Outer region containing glomeruli and convoluted tubules.

• Renal medulla: Inner region with renal pyramids and nephron loops.

• Renal pelvis: Funnel-shaped structure collecting urine into the ureter.

• Major functions: Excretion of metabolic wastes, regulation of blood volume and pressure, electrolyte balance, and acid-base homeostasis.

Other Urinary Organs

• Ureters: Muscular tubes transporting urine from kidneys to bladder via peristalsis.

• Urinary bladder: Hollow, muscular organ for temporary urine storage; lined by transitional epithelium.

• Urethra: Tube carrying urine from bladder to exterior; length and function differ between sexes.

• Micturition: The process of urination, involving voluntary and involuntary muscle control.

Nephron Structure and Function

The nephron is the functional unit of the kidney, responsible for urine formation through filtration, reabsorption, and secretion.

• Renal corpuscle: Includes the glomerulus (capillary tuft) and Bowman's capsule (surrounds glomerulus).

• Renal tubule: Consists of the proximal convoluted tubule (PCT), nephron loop (loop of Henle), distal convoluted tubule (DCT), and collecting duct.

Processes in the Nephron

• Filtration: Occurs in the glomerulus; blood plasma is filtered into Bowman's capsule.

• Reabsorption: Movement of substances from filtrate back into blood, mainly in PCT.

• Secretion: Transfer of additional wastes from blood into filtrate, mainly in DCT and collecting duct.

Glomerulus and Filtration Membrane

• Glomerulus: Fenestrated capillaries allowing passage of water and small solutes.

• Visceral layer of Bowman's capsule: Composed of podocytes with filtration slits.

• Filtration membrane: Three layers (fenestrated endothelium, basement membrane, podocyte slits) determine permeability.

Afferent and Efferent Arterioles, NFP, and GFR

• Afferent arteriole: Brings blood to glomerulus; wider diameter increases glomerular pressure.

• Efferent arteriole: Drains blood from glomerulus; narrower diameter maintains high pressure.

• Net Filtration Pressure (NFP): The pressure driving filtration; calculated as:

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

• Glomerular Filtration Rate (GFR): Volume of filtrate formed per minute; regulated by arteriole diameter, blood pressure, and feedback mechanisms.

Proximal and Distal Convoluted Tubules

• PCT: Major site of reabsorption (glucose, amino acids, ions, water).

• DCT: Site of regulated reabsorption and secretion, influenced by hormones (aldosterone, ADH).

Loop of Henle (Nephron Loop)

• Descending limb: Permeable to water, not solutes; water reabsorbed.

• Ascending limb: Impermeable to water; active transport of Na+, K+, Cl-.

• Cortical nephrons: Short loops, mainly in cortex.

• Juxtamedullary nephrons: Long loops extending deep into medulla; crucial for urine concentration.

Collecting Duct

• Principal cells: Regulate water and Na+ balance (responsive to ADH and aldosterone).

• Intercalated cells: Regulate acid-base balance (secrete H+ or HCO3-).

• Regulation: ADH increases water reabsorption; aldosterone increases Na+ reabsorption.

Juxtaglomerular Apparatus (JGA)

• Macula densa: Senses NaCl concentration in DCT.

• Granular (juxtaglomerular) cells: Secrete renin in response to low blood pressure.

• Extraglomerular mesangial cells: Support communication between macula densa and granular cells.

Peritubular and Vasa Recta Capillaries

• Peritubular capillaries: Surround PCT and DCT; involved in reabsorption and secretion.

• Vasa recta: Surround nephron loops of juxtamedullary nephrons; maintain medullary osmotic gradient.

Hydrostatic and Osmotic Pressures

• Hydrostatic pressure: Drives filtration out of glomerulus.

• Osmotic pressure: Opposes filtration; due to plasma proteins drawing water back into capillaries.

• Balance: Determines net movement of water and solutes.

Hormonal Regulation of Kidney Function

• Renin: Released by JGA; initiates angiotensin II formation.

• Angiotensin II: Vasoconstrictor; stimulates aldosterone and ADH release.

• Aldosterone: Increases Na+ reabsorption in DCT and collecting duct.

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

• Atrial natriuretic peptide (ANP): Inhibits Na+ reabsorption, lowers blood pressure.

Countercurrent Mechanisms and Urine Concentration

• Countercurrent multiplier: Loop of Henle creates osmotic gradient in medulla.

• Countercurrent exchanger: Vasa recta preserves gradient by passive exchange.

• Result: Allows production of concentrated or dilute urine depending on hydration status.

Urine Composition

• Normal constituents: Water, urea, creatinine, uric acid, ions (Na+, K+, Cl-).

• Abnormal constituents: Glucose, proteins, blood, ketones, bile pigments (indicate pathology).

Fluids, Electrolytes, and Acid-Base Balance

Fluid Compartments and Electrolytes

Body fluids are distributed between intracellular fluid (ICF) and extracellular fluid (ECF) compartments, each with characteristic electrolytes.

• ICF: High in K+, Mg2+, phosphate, and proteins.

• ECF: High in Na+, Cl-, HCO3-.

Water Balance and Imbalances

• Water gain: Ingestion (food, drink), metabolic water.

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

• Imbalances: Dehydration (water loss exceeds intake), overhydration (excess water intake).

• Diuretics: Substances that increase urine output (e.g., caffeine, furosemide).

Acid-Base Disorders

• Respiratory acidosis: CO2 retention (hypoventilation); compensated by renal HCO3- retention.

• Respiratory alkalosis: CO2 loss (hyperventilation); compensated by renal HCO3- excretion.

• Metabolic acidosis: Excess acid or HCO3- loss; compensated by increased ventilation.

• Metabolic alkalosis: Excess HCO3- or acid loss; compensated by decreased ventilation.

Regulation of Major Electrolytes

• Sodium (Na+): Regulated by aldosterone, ANP, and ADH.

• Potassium (K+): Regulated by aldosterone (promotes secretion in DCT/collecting duct).

• Calcium (Ca2+): Regulated by parathyroid hormone (PTH) and calcitonin.

• Bicarbonate (HCO3-): Regulated by kidneys (reabsorption/secretion) and respiratory system.

Buffering Systems and Acid-Base Balance

• Chemical buffers: Bicarbonate, phosphate, and protein buffer systems act immediately to resist pH changes.

• Respiratory mechanism: Adjusts CO2 exhalation to regulate pH (minutes).

• Renal mechanism: Adjusts H+ secretion and HCO3- reabsorption (hours to days).

• Order of effectiveness: Chemical buffers > respiratory > renal (most powerful, but slowest).

Table: Comparison of Acid-Base Disorders

Disorder	Primary Cause	Compensation
Respiratory Acidosis	CO2 retention (hypoventilation)	Renal HCO3- retention
Respiratory Alkalosis	CO2 loss (hyperventilation)	Renal HCO3- excretion
Metabolic Acidosis	Acid gain or HCO3- loss	Increased ventilation (CO2 loss)
Metabolic Alkalosis	HCO3- gain or acid loss	Decreased ventilation (CO2 retention)

Example: Regulation of Blood Pressure by the Kidney

When blood pressure drops, the JGA releases renin, leading to angiotensin II formation. Angiotensin II constricts blood vessels and stimulates aldosterone and ADH release, increasing Na+ and water reabsorption, thus raising blood pressure.

Additional info: The above notes expand on the review outline by providing definitions, mechanisms, and examples for each key point, ensuring a comprehensive and self-contained study guide for exam preparation.