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 elimination.

• 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

• Production of hormones (e.g., erythropoietin, renin)

• Activation of vitamin D

External Structure of the Kidney

• Location: On the 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 functional unit of the kidney, responsible for filtering blood and forming urine.

• Each kidney contains about 1 million nephrons.

• Consists of a renal corpuscle (glomerulus + Bowman's capsule) and a renal tubule (proximal tubule, nephron loop, distal tubule).

Kidney Structure: Macro and Microscopic Levels

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

• Microscopic: Nephrons, collecting ducts, blood vessels.

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.

Path of Filtrate and Urine

• Filtrate Pathway: Glomerulus → Bowman's capsule → proximal tubule → nephron loop (loop of Henle) → distal tubule → collecting duct.

• Urine Pathway: Collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → bladder → urethra.

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 at the glomerulus.

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

• Urine: Final fluid after reabsorption and secretion, ready for excretion.

Processes in Urine Formation

• Glomerular Filtration: Movement of water and solutes from blood into Bowman's capsule.

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

• 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:

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

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; high blood pressure leads to vasoconstriction

• Tubuloglomerular feedback: Macula densa cells sense NaCl and adjust afferent arteriole diameter; increased NaCl leads to afferent vasoconstriction

• 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.

Tubular Reabsorption and Secretion

• Reabsorption: Water, glucose, amino acids, and ions reabsorbed by passive (osmosis, diffusion) and active (pumps, transporters) processes, mainly in the proximal tubule.

• Secretion: Occurs mainly in the distal tubule and collecting duct; removes drugs, H+, K+ from blood.

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 sections allows for concentration or dilution of urine.

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

Countercurrent Mechanism

• Nephron loop: Creates osmotic gradient in medulla.

• Vasa recta: Maintains gradient via countercurrent exchange.

Physical and Chemical Properties of Urine

• Color: Pale yellow to amber.

• pH: 4.5–8.0 (average 6.0).

• Specific gravity: 1.001–1.035.

Renal Clearance and GFR Measurement

• Renal clearance rate of inulin or creatinine can estimate GFR.

$\text{Renal Clearance} = \frac{U \times V}{P}$ 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: 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), extracellular fluid (ECF, ~1/3; includes interstitial fluid and plasma).

• Major electrolytes:

  • ICF: K+, Mg2+, phosphate

  • ECF: Na+, Cl-, Ca2+, bicarbonate

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

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

• 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

• Functions:

  • Na+: Main ECF cation, regulates fluid balance.

  • K+: Main ICF cation, important for membrane potential.

  • Ca2+: Muscle contraction, neurotransmission, blood clotting.

  • Cl-: Main ECF anion, balances Na+.

  • Phosphate: Buffering, bone structure.

• 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: Normal range 7.35–7.45.

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

• PCO2 and bicarbonate:

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

• Respiratory system: Alters CO2 exhalation to adjust pH.

• Hypoventilation: Increases CO2, lowers pH (acidosis).

• Hyperventilation: Decreases CO2, raises pH (alkalosis).

• Kidneys: Secrete H+, reabsorb/generate HCO3- to regulate pH.

• Compensation: Respiratory or renal adjustments to correct pH imbalances.

Integration of Homeostatic Mechanisms

• Cardiovascular system: Adjusts blood pressure and flow to maintain fluid balance.

• Endocrine system: Hormones (ADH, aldosterone, ANP, PTH) regulate water, electrolyte, and pH balance.

• Urinary system: Excretes or conserves water, electrolytes, and H+/HCO3- as needed.

Summary Table: Major Electrolytes in Body Fluids

Electrolyte	Main Location	Function	Hormonal Regulation
Na+	ECF	Fluid balance, nerve impulse	Aldosterone, ANP
K+	ICF	Membrane potential	Aldosterone
Ca2+	ECF	Muscle contraction, clotting	PTH, calcitonin
Cl-	ECF	Osmotic balance	Follows Na+
Phosphate	ICF, bone	Buffering, bone structure	PTH

Example: Homeostatic Response to Dehydration

• Decreased blood volume stimulates ADH and aldosterone release.

• Kidneys reabsorb more water and Na+, reducing urine output.

• Thirst mechanism increases water intake.

Additional info: Some explanations and examples have been expanded for clarity and completeness.