The Urinary System
NURS 231 - The Urinary System Study Notes
Topic Overview
Course: NURS 231
Topic: The Urinary System
Pages Covered: Chapter 25, Some of Chapter 26
Learning Objectives
Main Functions of the Kidneys
Location: Pages 974-975
Basic Anatomy of the Urinary System
Focus: Summary from notes (not detailed textbook information)
Location: Pages 975-983
Process of Filtration
Details: Explain glomerular filtration and how it is regulated
Intrinsic Controls & Extrinsic Controls
Location: Pages 984-989
Reabsorption
Definition: Describe how and where various substances are reabsorbed
Location: Pages 989-994
Secretion
Location: Pages 994-995
Acid-Base Balance
Kidneys' role in regulating acid-base balance
Details: Summary from notes only
Location: Pages 1029-1035
Regulation of Urine Concentration and Volume
Mechanism: Countercurrent mechanism
Location: Pages 995-1000
Urine Transport, Storage, and Elimination
Location: Pages 1002-1005
Alterations in Normal Physiology
Location: In-class examples
Overview of the Urinary System
Main Function: Urine production
Components:
Kidneys
Ureters: Transport urine from kidneys to urinary bladder
Urinary Bladder: Storage for urine
Urethra: Passage from bladder to outside body
Functions of the Kidneys
Homeostasis: Maintain body’s internal environment
Key Functions:
Excretion of metabolic wastes, toxins, drugs
Regulation of water volume and solute concentration in water
Regulation of ion concentrations in extracellular fluid (ECF)
Long-term acid-base balance
Production of erythropoietin (regulates RBC production)
Renin production (regulates blood pressure)
Conversion of vitamin D to its active form
Gluconeogenesis (if required)
Basic Anatomy of the Kidneys
Gross Anatomy:
Renal Cortex: Outer superficial region
Renal Medulla: Deep region beneath the cortex
Renal Pelvis: Collects urine, empties into ureter
Functional Unit: Nephron
Approximately 1 million nephrons per kidney
Microscopic Anatomy of the Kidneys
Nephron Components:
Renal Corpuscle:
Glomerulus: Highly porous capillaries for filtrate formation
Glomerular (Bowman’s) Capsule: Surrounds glomerulus, allowing entry into proximal convoluted tubule
Renal Tubule:
Proximal Convoluted Tubule (PCT): Closest to corpuscle
Nephron Loop (Loop of Henle): Descends into and ascends from the medulla
Distal Convoluted Tubule (DCT): Further from corpuscle
Collecting Duct: Drains contents from the distal convoluted tubule
Kidney Physiology
Fluid Processed: 180 L daily; only 1.5 L forms urine
Filtrate: Essentially blood plasma minus proteins
Filtering Capacity: Kidneys filter body's entire plasma volume approximately 60 times daily
Oxygen Consumption: 20-25% of oxygen utilized at rest
Main Processes in Kidney Function
Glomerular Filtration: Produces filtrate
Tubular Reabsorption: Returns 99% of substances from filtrate to blood
Tubular Secretion: Moves substances from blood to filtrate
Glomerular Filtration
Definition: Passive process requiring no metabolic energy
Mechanism: Hydrostatic pressure pushes fluids and solutes through filtration membranes
Filtration Allowed: Water and solutes smaller than plasma proteins pass; cells do not pass through
Filtration Membrane Structure
Components:
Fenestrated Capillary Endothelium
Basement Membrane
Podocyte Foot Processes: Feature filtration slits
Pressures Affecting Glomerular Filtration
Outward Pressures: Promote filtrate formation
Hydrostatic pressure in glomerular capillaries (HPgc) at 55 mm Hg
Inward Pressures: Inhibit filtration formation
Hydrostatic pressure in capsular space (HPcs) at 15 mm Hg
Colloid osmotic pressure (OPgc) in capillaries at 30 mm Hg
Net Filtration Pressure (NFP): Combination of these pressures indicating overall filtration effect
Glomerular Filtration Rate (GFR)
Definition: Volume of filtrate formed per minute by all glomeruli in both kidneys, normal range: 120-125 ml/min
Factors Affecting GFR:
Net Filtration Pressure (NFP)
Total surface area for filtration
Filtration membrane permeability
Control Mechanisms:
Intrinsic: Renal autoregulation maintaining GFR
Extrinsic: Systematic blood pressure maintenance via nervous/endocrine systems
Regulation of GFR
Intrinsic Mechanisms:
Myogenic Mechanism: Adjusts afferent arteriole diameter based on blood pressure changes
Tubuloglomerular Feedback: Adjusts GFR based on NaCl concentration in filtrate
Extrinsic Controls:
Sympathetic Nervous System: Constriction of arterioles under low blood pressure conditions increases pressure and decreases GFR
Renin-Angiotensin-Aldosterone Mechanism: Includes hormone release pathways affecting blood pressure and filtration
Tubular Reabsorption
Definition: Process quickly reclaiming tubular contents into the blood:
Routes:
Transcellular Route: Apical to basolateral membrane transport through cells
Paracellular Route: Movement via tight junctions between neighboring cells
Key Reabsorbed Substances:
Sodium (Na+): Most abundant cation in filtrate, transported actively
Nutrients: Glucose, amino acids, vitamins via secondary active transport
Water: Reabsorbed mainly via osmosis aided by aquaporins
Reabsorption Capabilities
Proximal Convoluted Tubule (PCT): Site for most nutrient reabsorption; 65% of Na+ and water; half of urea reabsorbed
Nephron Loop:
Descending Limb: Permeable to water, not solutes
Ascending Limb: Permeable to solutes, not water
Distal Convoluted Tubule and Collecting Duct:
Regulated by hormones (e.g., ADH, Aldosterone) affecting Na+ and water reabsorption, thus impacting blood pressure and fluid volume
Tubular Secretion
Definition: Works inversely to reabsorption
Substances Secreted: K+, H+, NH₄⁺, creatinine, organic acids/bases
Functions:
Disposal of substances, including drugs
Eliminating undesirable substances passively reabsorbed
Excess K+ regulation and blood pH control via H+ or bicarbonate () alterations
Acid-Base Balance Regulation by Kidneys
Mechanisms:
Lungs eliminate CO₂ to adjust carbonic acid levels
Kidneys regulate bicarbonate (HCO₃⁻) and H+, adjusting blood concentration as needed:
Conserving or generating HCO₃⁻: Directly linked to H+ loss
Excreting HCO₃⁻: Requires retaining H+
Body maintains pH by reallocating H⁺ in buffer systems and generating new HCO₃⁻ as necessary
Urine Concentration and Volume Regulation
Kidneys adjust urine concentration to maintain osmotic balance based on hydration levels.
Countercurrent Mechanisms: Key in concentrating/diluting urine, involving:
Countercurrent Multiplier: Flow interactions in nephron loop
Countercurrent Exchanger: Blood flow dynamics via vasa recta, preserving osmotic gradients
Mechanisms for Urine Formation
Dilute Urine: Large volumes with low concentration in hydration; reduced ADH levels
Concentrated Urine: Small volumes with high concentration during dehydration; maximal ADH levels released
Diuretics
Effects: Enhance urinary output by altering reabsorption mechanisms:
ADH inhibitors (e.g., alcohol)
Na+ reabsorption inhibitors (e.g., caffeine)
Osmotic diuretics pulling water via substances not reabsorbed (e.g., glucose in diabetes)
Urine Transport, Storage, and Elimination
Urine flows from renal pelvis through ureters to urinary bladder.
Micturition (Urination)
Contraction: Urinary bladder smooth muscle (detrusor) by automatic nervous system (ANS)
Internal Urethral Sphincter Opening: Managed by ANS
External Urethral Sphincter Opening: Managed by somatic nervous system
Control of Micturition
Bladder Filling: Triggers stretch receptor activation leading to contraction of detrusor through spinal reflexes
Regulation: By higher brain centers for micturition regulation appropriate to the body's condition