Kidney Functional Anatomy & Urine Formation

Summary of Kidney Functions

  • Kidneys provide multiple, simultaneous homeostatic services:
    • Excretion of metabolic wastes: urea, uric acid, creatinine, bilirubin, excess H^+.
    • Removal of foreign chemicals: drugs, toxins, pesticides, food additives.
    • Hormonal activities (secrete, metabolize, and excrete):
    • Renal erythropoietic factor (stimulates erythropoietin/erythropoetin).
    • 1,25‐dihydroxycholecalciferol (active vitamin D_3).
    • Renin (initiates the renin–angiotensin cascade).
    • Metabolism/excretion of most peptide hormones (e.g., insulin, angiotensin II).
    • Regulation of acid–base balance (non-volatile acid excretion, bicarbonate conservation).
    • Gluconeogenesis from amino acids during prolonged fasting.
    • Control of arterial pressure via extracellular-fluid balance, renin–angiotensin system, prostaglandins, kallikrein–kinin system.
    • Regulation of water & electrolyte excretion: Na^+, K^+, H^+, Ca^{2+}, PO_4^{3-}, Mg^{2+}.

Excretion of Metabolic Waste Products

  • Products and their origins:
    • Urea – protein (amino-acid) catabolism.
    • Uric acid – nucleic-acid catabolism.
    • Creatinine – muscle creatine phosphate metabolism.
    • Bilirubin – hemoglobin breakdown.

Elimination of Foreign Chemicals

  • Same tubular mechanisms that clear wastes also remove xenobiotics:
    • Pesticides (e.g., organophosphates), food additives, drugs (antibiotics, analgesics), industrial toxins.
    • Usually poorly reabsorbed and often actively secreted → high urinary clearance.

Renal Endocrine Functions

  • Erythropoietin production
    • Trigger: renal cortical hypoxia ↓O_2 delivery → juxtaglomerular interstitial cells ↑erythropoietin.
    • Effect: ↑red-cell production in bone marrow.
  • Activation of Vitamin D
    • 25-OH vitamin D → 1,25-(OH)$2$ vitamin D3 in proximal-tubule cells.
    • Active form regulates intestinal/renal Ca^{2+} & PO_4^{3-} absorption.
  • Renin release (see juxtaglomerular apparatus).

Acid–Base Regulation

  • Kidneys are the only route for removal of non-volatile (fixed) acids (sulfuric, phosphoric).
  • Adjust plasma [HCO_3^-] by:
    • Reabsorbing filtered bicarbonate.
    • Generating new HCO_3^- via ammonium and titratable-acid secretion.

Gluconeogenesis

  • During prolonged fasting, cortex converts amino-acid carbon skeletons → glucose.
  • Complements hepatic gluconeogenesis; can supply up to 40\% of endogenous glucose.

Regulation of Arterial Pressure

  • Fast: renin–angiotensin–aldosterone system (RAAS) alters vascular tone & Na^+ handling.
  • Local: prostaglandins, kallikrein–kinin vasodilators.
  • Long term: Na^+ & water balance → extracellular-fluid volume.

Water & Electrolyte Balance

  • Fine-tuned excretion/retention of:
    • Sodium & water (linked osmotically).
    • Potassium (aldosterone-sensitive secretion).
    • Hydrogen ions (acid–base).
    • Calcium, phosphate, magnesium (PTH & vitamin D dependent).

Physiologic Anatomy of the Kidneys and Urinary Tract

  • Location: posterior abdominal wall, retro-peritoneal; weight ≈ 150\,\text{g} each.
  • Enclosed by a tough fibrous capsule.
  • Hilum (medial): entry/exit for renal artery/vein, lymphatics, nerves, ureter.
  • Renal blood flow ≈ 22\% of cardiac output ≈ 1100\,\text{mL min}^{-1}.
    • Two sequential capillary beds:
    • Glomerular capillaries (high \approx 60\,\text{mm Hg}) – filtration.
    • Peritubular capillaries (low \approx 13\,\text{mm Hg}) – reabsorption/secretion.

The Nephron – Functional Unit

  • 8\times10^5–10^6 nephrons per kidney; no regeneration → ↓number with age (≈10\% loss per decade after 40 yr).
  • Components:
    1. Renal corpuscle = glomerulus + Bowman’s capsule → produces protein-free filtrate.
    2. Renal tubule processes filtrate → urine; segments:
    • Proximal convoluted tubule (PCT).
    • Loop of Henle (descending thin, ascending thin/thick).
    • Distal convoluted tubule (DCT).
    • Collecting system (collecting tubule & collecting duct).

Cortical vs. Juxtamedullary Nephrons

  • Cortical nephrons (≈70!–!80\%):
    • Short loops of Henle penetrate small distance into medulla.
    • Surrounded by dense peritubular capillary network.
  • Juxtamedullary nephrons (≈20!–!30\%):
    • Long loops of Henle extend deep into medulla → critical for urine concentration.
    • Efferent arterioles form vasa recta (hair-pin capillaries) paralleling loops → counter-current exchange.

Juxtaglomerular Apparatus (JGA)

  • Interface where thick ascending limb → early DCT contacts afferent & efferent arterioles.
  • Cells & roles:
    • Macula densa (tubular epithelium): senses tubular [NaCl] → signals renin release & GFR adjustment.
    • Juxtaglomerular (granular) cells (arteriolar wall): modified smooth muscle → synthesize, store, release renin when stimulated by:
    1. ↓renal perfusion pressure.
    2. Catecholamines / sympathetic activity.
    3. Macula densa signal.
    • Mesangial cells (extraglomerular): contractile & phagocytic → modulate filtration coefficient K_f.

Urine Transport: Ureters → Bladder

  • Filtrate exits collecting ducts → minor → major calyces → renal pelvis.
  • Stretch of calyces initiates peristaltic waves (enhanced by parasympathetic, inhibited by sympathetic) that propel urine down ureters.
  • Smooth-muscle ureter wall contains intrinsic pacemaker & autonomic innervation.
  • Detrusor tone compresses ureteral tunnels, preventing vesicoureteral reflux; inadequate sub-mucosal tunnel length → reflux → hydronephrosis.
  • Ureterorenal reflex: ureteral obstruction (stone) → pain → sympathetic renal arteriolar constriction → ↓GFR of affected kidney, limiting pressure build-up.

Bladder Structure

  • Smooth-muscle chamber: body (urine reservoir) + neck (funnel into urethra).
  • Detrusor muscle: interlacing fibers in all directions; contraction raises intravesical pressure to 40!–!60\,\text{mm Hg}.
  • Trigone: triangular posterior wall region between ureteral orifices and bladder neck; mucosa firmly attached (smooth appearance).
  • Internal sphincter (bladder neck): thickened detrusor + elastic tissue; smooth muscle with basal tone keeps neck closed until threshold pressure exceeded.
  • External sphincter (urogenital diaphragm): voluntary skeletal muscle innervated by pudendal nerve (somatic) → conscious control.

Neural Control of Bladder & Urethra

  • Parasympathetic (pelvic nerves; S2–S3)
    • Afferents: bladder & posterior urethra stretch receptors.
    • Efferents: detrusor contraction, internal sphincter relaxation.
  • Sympathetic (hypogastric; T11–L2)
    • Primarily vascular modulation; minor role in detrusor relaxation/internal sphincter contraction.
  • Somatic (pudendal nerve)
    • Innervates external sphincter; voluntary maintenance or release.

Cystometrogram & Bladder Filling

  • Intravesical pressure–volume curve:
    • Empty bladder P \approx 0.
    • 30!–!50\,\text{mL} → P=5!–!10\,\text{cm H}_2\text{O}.
    • Compliance keeps 200!–!300\,\text{mL} additional with minimal \Delta P.
    • Beyond 300!–!400\,\text{mL} → steep pressure rise.
  • Superimposed micturition waves (acute pressure spikes up to 100\,\text{cm H}_2\text{O}) produced by micturition reflex.

Micturition Reflex Loop

  • Stretch → pelvic afferents → sacral spinal cord → parasympathetic efferents → detrusor contraction + internal sphincter relaxation.
  • Self-regenerative positive feedback until fatigued; single cycle = rapid pressure rise → sustained contraction → relaxation back to baseline.
  • Partially filled bladder: contractions subside → storage.
  • Progressive filling: frequency & intensity ↑ until cortical decision or reflex override triggers voiding.

Brain Modulation

  • Pontine centers (strong facilitation & inhibition).
  • Cortical centers (frontal/medial): predominantly inhibitory but can become excitatory.
  • Functions:
    1. Tonic inhibition of reflex during inappropriate times.
    2. Voluntary initiation: cortical facilitation of sacral center + voluntary abdominal compression → extra stretch, reflex amplification, external-sphincter relaxation.

Voiding Disorders

  • Atonic (sensory) bladder: destruction of afferents (e.g., dorsal-root tabes dorsalis, crush injury) → no reflex → over-distension, overflow dribbling, inability to start/maintain stream.
  • Automatic bladder: spinal cord lesion above sacral levels; initial spinal shock suppresses reflex, but with time reflex returns without cortical control → periodic, unpredictable voiding.
  • Uninhibited neurogenic bladder: partial cord/brain-stem damage removes cortical inhibition but leaves facilitation → small bladder volumes trigger powerful reflex → frequent urination.

Overview of Urine Formation

  • Three renal processes:
    1. Glomerular filtration (GF): bulk fluid transfer into Bowman’s capsule, protein-free.
    2. Tubular reabsorption (TR): return of water/solutes to peritubular capillaries.
    3. Tubular secretion (TS): selective transfer from capillaries into tubule lumen.
  • Overall relationship:
    \text{Urinary Excretion Rate} = \text{GF} - \text{TR} + \text{TS}

Patterns of Handling (Examples)

  • Filtration only: substance neither reabsorbed nor secreted (e.g., creatinine) → excretion = filtration.
  • Filtration + partial reabsorption: most electrolytes (Na^+, Cl^-); excretion < filtration.
  • Filtration + complete reabsorption: nutrients (glucose, amino acids) → excretion =0.
  • Filtration + secretion: organic acids/bases (PAH, drugs) → excretion > filtration.

Quantitative Emphasis

  • TR is usually far larger than urinary excretion; small % changes in GF or TR markedly alter output.
    • Example: If GFR ↓ to 10\% of normal and TR unchanged → urine volume would rise from 1.5\,\text{L day}^{-1} to 19.5\,\text{L day}^{-1}.
  • Large filtration + adjustable reabsorption advantages:
    1. Rapid clearance of poorly reabsorbed wastes.
    2. Multiple passes (entire extracellular fluid filtered several times daily) → tight control of composition.
    3. Ability to mould urine composition precisely to moment-to-moment needs.

Practical / Clinical Connections & Implications

  • Declining nephron number with aging necessitates adaptive hypertrophy; drugs cleared renally may require dose reduction in elderly.
  • Measurement of creatinine clearance exploits “filtration only” property to estimate GFR.
  • Chronic vesicoureteral reflux predisposes to recurrent pyelonephritis and scarring.
  • Recognizing neurogenic bladder patterns guides localization of neurologic lesions and catheterization schedules.
  • Disruption of JGA signaling (e.g., NSAID inhibition of prostaglandins, ACE-inhibitor blockade of RAAS) has therapeutic and side-effect profiles tied to renal hemodynamics.