Urinary System and Fluid & Electrolyte Balance: Study Notes

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Urinary System

Overview of the Urinary System

The urinary system is responsible for filtering blood, removing waste, regulating fluid and electrolyte balance, and maintaining acid-base homeostasis. Its main organs include the kidneys, ureters, urinary bladder, and urethra.

Organs: Kidneys, ureters, urinary bladder, urethra
Main function: Excretion of metabolic wastes, regulation of blood volume and pressure, electrolyte and pH balance, hormone production

Flow of Urine Through the Urinary System

Urine is formed in the kidneys and follows a specific path to exit the body.

Glomerulus
Renal corpuscle
Proximal convoluted tubule (PCT)
Loop of Henle (nephron loop)
Distal convoluted tubule (DCT)
Collecting duct
Papillary duct
Minor calyx
Major calyx
Renal pelvis
Ureter
Bladder
Urethra

Renal Corpuscle Structure and Function

The renal corpuscle is the initial filtering component of the nephron, consisting of the glomerulus and Bowman's capsule.

Glomerulus: A tuft of fenestrated capillaries that filters blood; receives blood from the afferent arteriole and drains into the efferent arteriole.
Bowman's capsule: Double-walled capsule surrounding the glomerulus, with a parietal (simple squamous epithelium) and visceral (podocytes) layer.
Function: Site of glomerular filtration; small molecules (water, ions, glucose) pass into Bowman's capsule, while large molecules are retained in the blood.

Blood Flow in the Kidney

Blood enters and leaves the kidney through a series of vessels, ensuring filtration and reabsorption.

Inflow: Aorta → Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Cortical radiate arteries → Afferent arteriole → Glomerulus
Outflow: Efferent arteriole → Vasa recta/peritubular capillaries → Cortical radiate veins → Arcuate veins → Interlobar veins → Renal vein → Inferior vena cava

Types of Capillaries in the Kidney

Peritubular capillaries: Surround PCT and DCT of cortical nephrons; involved in reabsorption and secretion; low pressure, located in cortex.
Vasa recta: Surround nephron loop of juxtamedullary nephrons; important for concentrating urine.

Filtration Pressures in the Glomerulus

Glomerular filtration is driven by several pressures:

Glomerular hydrostatic pressure (GHP): Main force pushing fluid out of glomerulus (about 55 mmHg).
Capsular hydrostatic pressure (CHP): Pressure of fluid in Bowman's capsule opposing filtration (about 15 mmHg).
Blood colloid osmotic pressure (BCOP): Osmotic pull of plasma proteins drawing water back into blood (about 30 mmHg).

Net Filtration Pressure (NFP) Formula:

Glomerular Filtration Rate (GFR)

Definition: The rate at which the kidneys filter blood through all glomeruli per minute.
Clinical importance: Indicator of kidney function.

Juxtaglomerular Complex

Components: Macula densa, juxtaglomerular cells, extraglomerular mesangial cells.
Function: Regulates blood pressure and GFR via renin secretion.

Nephron Structure and Function

Nephron: Functional unit of the kidney; majority are cortical nephrons.
Order of filtration: Fenestrated endothelium → Basement membrane → Podocyte filtration slits → Capsular space
Functions of nephron segments:
- PCT: Bulk reabsorption of water, Na+, glucose, amino acids; secretion of waste.
- Descending limb: Water reabsorption; filtrate becomes concentrated.
- Ascending limb: Na+/K+/Cl- reabsorption; filtrate becomes dilute.
- DCT: Hormone-regulated reabsorption/secretion of Na+, Ca2+, K+, H+.

Hormonal Regulation

Aldosterone: Increases sodium reabsorption in DCT and collecting duct.
ADH (Antidiuretic hormone): Increases water reabsorption in DCT and collecting duct by increasing permeability; also stimulates thirst.
Angiotensin II: Stimulates aldosterone and ADH, increases Na+ reabsorption, thirst, and vasoconstriction.
Natriuretic peptides: Lower blood volume and pressure by promoting sodium and water excretion.
PTH (Parathyroid hormone): Increases blood calcium levels.

Transport Mechanisms in the Nephron

Filtration: Driven by pressure.
Reabsorption: Passive, secondary active, and primary active transport.
Secretion: Active transport.
Types of movement:
- Simple diffusion: High to low concentration, no energy or transporter.
- Facilitated diffusion: High to low concentration via protein channel, no energy.
- Active transport: Low to high concentration, requires ATP.
- Osmosis: Water movement from low to high solute concentration.
- Cotransport: Two substances move in same direction via same transporter.
- Countertransport: Two substances move in opposite directions via same transporter.
Sodium transport: Uses pumps, cotransporters, channels; Na+/K+ pump is essential.
Glucose transport: SGLT (Sodium-glucose linked transporter).
Chloride movement (thick ascending limb): Salt leaves, water stays in.
Hydrogen ion movement (PCT): Active transport.

Urine Storage and Micturition

Urinary bladder: Temporary storage of urine; lined by transitional epithelium.
Micturition: Reflex and voluntary process of expelling urine.
Pontine storage center: Promotes urine storage by inhibiting micturition reflex, relaxing detrusor muscle, and contracting sphincters.
Trigone: Smooth triangular region in bladder formed by ureteral and internal urethral openings.

Effects of Aging on the Urinary System

Decreased kidney function and urine-concentrating ability
Weakened bladder and sphincter muscles
Increased residual urine, risk of incontinence, and UTIs
Reduced drug clearance by kidneys and liver

Fluids, Electrolytes, and Acid-Base Balance

Body Fluid Compartments

Intracellular fluid (ICF): Fluid inside cells; 2/3 of total body water.
Extracellular fluid (ECF): Fluid outside cells; 1/3 of total body water. Includes interstitial fluid, plasma, and transcellular fluid.
Exchange between ECF compartments: Occurs across capillary walls via filtration and reabsorption.

Percentage of water in adults:

Females: 50%
Males: 60%

Average daily water intake and loss: 2500 mL

Electrolytes and Their Roles

Electrolytes: Charged ions in body fluids that conduct electricity and regulate nerve, muscle, fluid, and pH balance.
Principal ions of ECF: Major cation: Na+; Major anions: Cl-, HCO3-
Principal ions of ICF: Major cation: K+; Major anions: phosphate, negatively charged proteins
Most dangerous electrolyte imbalance: Potassium (K+)

Acid-Base Balance

Normal ECF pH: 7.35–7.45
Acidosis: pH < 7.35
Alkalosis: pH > 7.45
Buffers: Substances that minimize pH changes by binding or releasing H+ as needed.
Carbonic acid-bicarbonate buffer system: Controls pH by shifting between CO2, carbonic acid, and bicarbonate.
Organs maintaining acid-base balance: Lungs, kidneys, liver, GI tract, bones

Types of Acids

Volatile acids: Removed by lungs (e.g., CO2)
Fixed acids: Removed by kidneys (e.g., sulfuric acid)
Inorganic acids: From breakdown of inorganic compounds

Acid-Base Disorders and Compensation

Respiratory acidosis: Caused by hypoventilation; kidneys retain HCO3- and excrete H+ to compensate.
Respiratory alkalosis: Caused by hyperventilation; kidneys excrete HCO3- and retain H+ to compensate.
Metabolic acidosis: Caused by kidney damage, uncontrolled diabetes (ketoacidosis), or diarrhea; kidneys excrete H+ and retain HCO3-.
Metabolic alkalosis: Caused by vomiting or excessive sodium bicarbonate intake; kidneys excrete HCO3- and retain H+.
Acute vs. chronic respiratory disorders: Acute: sudden, no time for compensation; Chronic: long-term, kidneys compensate.

Common Electrolyte and Acid-Base Disorders

Disorder	Description
Hypercalcemia	High blood calcium
Hypocalcemia	Low blood calcium
Hyperkalemia	High blood potassium
Hypokalemia	Low blood potassium
Hypernatremia	High blood sodium
Hyponatremia	Low blood sodium
Hypercapnia	High blood CO2
Hypocapnia	Low blood CO2

Hormonal and Physiological Regulation

ADH: Most significant effect on blood osmolarity; increases water reabsorption and stimulates thirst.
Angiotensin II: Increases ECF volume by stimulating aldosterone, ADH, thirst, and vasoconstriction.
Natriuretic peptides: Lower blood volume and pressure.
PTH: Increases blood calcium levels.

Clinical and Pathological Considerations

Hyponatremia: Can result from drinking pure water during strenuous exercise.
Ketoacidosis: Metabolic acidosis due to uncontrolled diabetes mellitus.
Metabolic alkalosis: Can result from excessive vomiting or sodium bicarbonate intake.
Kidney damage: Can cause metabolic acidosis.
Drug clearance: Reduced in elderly due to decreased kidney and liver function.

Summary Table: Body Fluid Compartments and Principal Ions

Compartment	Location	Fraction of Total Body Water	Major Cation	Major Anion(s)
ICF	Inside cells	2/3	K+	Phosphate, proteins
ECF	Outside cells	1/3	Na+	Cl-, HCO3-

Key Definitions

Buffer: Substance/system that minimizes pH changes by binding or releasing H+.
Electrolyte: Charged ion in body fluids, essential for electrical activity and fluid balance.
Nephron: Functional unit of the kidney.
GFR: Rate of glomerular filtration per minute.

Example: Net Filtration Pressure Calculation

Given: GHP = 55 mmHg, CHP = 15 mmHg, BCOP = 30 mmHg

Additional info:

Most abundant organic waste in urine is urea.
Exchange between plasma and interstitial fluid occurs across capillary walls.
ICF and ECF are separated by the plasma membrane; ECF is further divided by capillary walls.