Urinary System – Comprehensive Study Notes

Learning Objectives

  • Enumerate major aims of Chapter 42 (Lessons 42.1 & 42.2):
    • Identify & locate every organ of the urinary system.
    • Describe kidney position in the abdomen, as well as structures seen in a coronal section.
    • Trace renal arterial flow, from renal artery through specialized micro-vasculature.
    • Compare structure & function of ureters, urinary bladder, urethra.
    • Explain micturition reflex and voluntary control pathways.
    • Name every part of a nephron and relate it to urine formation steps.
    • Discuss the processes of filtration, reabsorption, secretion, counter-current concentration/dilution.
    • Relate hormonal & autoregulatory mechanisms to urine volume regulation.
    • List normal urine characteristics and catalogue urinary disorders.

Overview of the Urinary System

  • Kidneys = principal organs; overall task → regulate blood-plasma composition to keep internal fluid environment in dynamic constancy (homeostasis).
  • Systemic functions:
    • Filter ~180\,\text{L} of plasma-like fluid daily, returning most to blood.
    • Excrete wastes, toxins, excess ions, water → final urine volume usually 1!–!2\,\text{L day}^{-1}.

Location & Gross Anatomy of Urinary Organs

  • Kidneys lie retroperitoneally on either side of vertebral column between T{12} & L3.
  • Protective coverings:
    • Renal fasciae: tough connective tissue anchoring kidneys.
    • Perirenal (renal) fat pad: thick adipose cushion.
  • Remaining structures (posterior abdominal wall → pelvic floor): ureters, bladder in pelvis, urethra exiting body.

Kidney Anatomy: External & Internal

  • Shape: bean-shaped/oval with medial hilum (indentation).
  • Size: 11\,\text{cm} \times 7\,\text{cm} \times 3\,\text{cm}.
  • Internal regions:
    • Renal cortex (outer reddish-brown granular layer).
    • Renal medulla (inner, composed of 8–18 renal pyramids).
    • Renal columns: cortical tissue separating pyramids.
    • Papilla: apex of each pyramid ⇢ drains into minor calyx.
    • Calyces merge → renal pelvis → narrows as ureter.

Renal Blood Supply

  • Renal artery (branch of abdominal aorta) → segmental → interlobar → arcuate → interlobular arteries.
  • Afferent arteriole enters each nephron’s glomerulus; efferent arteriole exits → peritubular capillaries or vasa recta (depending on nephron type).
  • Kidneys receive ~20\,\% of resting cardiac output; high vascularization essential for filtration.

Gross Urinary Tract Structures

  • Ureters: 25–30 cm muscular tubes (transitional epithelium, smooth muscle, adventitia) conducting urine from pelvis to bladder via peristalsis.
  • Urinary bladder:
    • Hollow organ mainly of detrusor (smooth muscle) + rugae.
    • Functions as temporary reservoir; can distend greatly before stimulating stretch receptors.
  • Urethra:
    • Female: ~3\,\text{cm}, anterior to vagina, opens at vestibule.
    • Male: ~20\,\text{cm}; prostatic, membranous, spongy segments; shared with reproductive system; terminates at urinary meatus.

Micturition (Urination)

  • As bladder fills, stretch → spinal cord (S2–S4) → parasympathetic efferents:
    • Detrusor contracts.
    • Internal urethral sphincter (smooth muscle) relaxes.
  • External urethral sphincter (skeletal muscle) initially contracts (voluntary control via pudendal nerve); conscious relaxation → voiding.
  • Reflex integrated with higher brain centers → allows postponement.

Nephron Structure

  • Each kidney ≈ 1!–!1.3\,\text{million} nephrons.
  • Two major regions per nephron:
    • Renal corpuscle = glomerulus + Bowman (glomerular) capsule.
    • Renal tubule = PCT → Henle loop (thin descending, thin ascending, thick ascending) → DCT → connecting tubule → collecting duct (shared by many nephrons).
  • Collecting ducts of a pyramid converge → papillary duct → minor calyx.

Renal Corpuscle & Filtration Membrane

  • Glomerulus: fenestrated capillaries allowing protein-free plasma passage.
  • Mesangial cells between capillaries support, contract, and regulate filtration surface area.
  • Bowman capsule layers:
    • Parietal layer = simple squamous epithelium.
    • Visceral layer = podocytes; foot processes (pedicels) interdigitate forming filtration slits spanned by slit diaphragms.
  • Filtration barrier components:
    • Capillary endothelium with fenestrations.
    • Fused basement membrane.
    • Podocyte slit diaphragm.
    • Permits water & small solutes (<\sim70 kDa) while retaining formed elements & most proteins.

Renal Tubule Segments

  • Proximal convoluted tubule (PCT): simple cuboidal with dense microvilli (“brush border”); primary site for reabsorption.
  • Henle loop:
    • Thin descending limb: permeable to water, less to solutes.
    • Ascending limb: impermeable to water; thick segment actively pumps \text{Na}^+, \text{K}^+, \text{Cl}^- (via NKCC2 cotransporter).
  • Distal convoluted tubule (DCT): fewer microvilli; site of fine-tuning reabsorption/secretion; contains macula densa cells.
  • Juxtaglomerular apparatus (JGA): macula densa + juxtaglomerular (granular) cells + extraglomerular mesangial cells; senses flow/NaCl to adjust GFR & renin release.

Types of Nephrons & Blood Supply

  • Cortical nephrons (~85\%): short loops, peritubular capillaries surround PCT/DCT.
  • Juxtamedullary nephrons (~15\%): long loops extend deep into medulla; efferent arterioles form vasa recta → crucial for countercurrent exchange.

Urine Formation: Overview

  • Three sequential processes:
    1. Filtration (glomerulus → capsular space).
    2. Tubular reabsorption (tubule → peritubular blood).
    3. Tubular secretion (peritubular blood → tubule).
  • Net daily outcome: Of 180\,\text{L} filtrate, ~178\,\text{L} reabsorbed, leaving 1!–!2\,\text{L} urine.

Glomerular Filtration

  • Driven by effective filtration pressure (EFP):
    \text{EFP}= (P{\text{GC}} - P{\text{BS}}) - (\pi{\text{GC}} - \pi{\text{BS}})
    where P{\text{GC}} = glomerular capillary hydrostatic, P{\text{BS}} = capsular hydrostatic, \pi = colloid osmotic pressures.
  • Normal glomerular filtration rate (GFR) ≈ 125\,\text{mL min}^{-1} (≈180\,\text{L day}^{-1}), proportional to systemic mean arterial pressure unless autoregulated.

Tubular Reabsorption

  • PCT reabsorbs ~65\% filtrate volume.
    • \text{Na}^+ actively pumped via \text{Na}^+/\text{K}^+-ATPase at basolateral membrane.
    • Glucose, amino acids co-transported with sodium (secondary active transport).
    • Anions (Cl$^-$, \text{HCO}3^-, \text{PO}4^{3-}) follow electrochemically.
    • Water follows osmotically through aquaporin-1 channels.
  • Obligatory vs facultative water reabsorption:
    • Obligatory in PCT & thin descending limb.
    • Facultative in DCT/CD under \text{ADH} control.

Countercurrent Mechanisms & Medullary Concentration

  • Countercurrent multiplier (Henle loop of juxtamedullary nephron):
    • Thick ascending limb pumps \text{NaCl} into medullary IF, raising osmolality (up to \sim1200\,\text{mOsm kg}^{-1}).
    • Descending limb equilibrates with surrounding IF by losing water.
  • Countercurrent exchange (vasa recta):
    • Blood flows opposite direction, picks up water extruded by descending limb while preserving medullary gradient (prevents wash-out).
  • Combined effect enables kidneys to produce urine from \approx50\,\text{mOsm} (very dilute) to \approx1200\,\text{mOsm} (highly concentrated) depending on hydration & \text{ADH}.

Reabsorption in DCT & Collecting Duct

  • DCT actively reabsorbs \text{Na}^+ (stimulated by aldosterone); accompanying anion/ water movement fine-tunes extracellular fluid (ECF) volume.
  • Collecting duct (especially medullary segment):
    • Water permeability regulated by \text{ADH} inserting Aquaporin-2 channels.
    • Urea recycling: High CD permeability (in presence of ADH) allows urea to exit into medullary IF, reinforcing osmolality.

Tubular Secretion

  • Primary secreted ions: \text{K}^+ (aldosterone-dependent), \text{H}^+ (acid-base balance), \text{NH}_4^+.
  • Mechanism augments removal of drugs, metabolites, excess ions.

Regulation of Urine Volume

  • Hormonal controls:
    • \text{ADH} (posterior pituitary): increases water reabsorption → concentrates urine.
    • Aldosterone (adrenal cortex): increases \text{Na}^+ reabsorption/\text{K}^+ secretion → secondary water retention.
    • Atrial natriuretic hormone (ANH): antagonizes aldosterone, promoting natriuresis & diuresis.
  • Autoregulatory controls:
    • Myogenic mechanism: afferent arteriole smooth muscle contracts when stretched → stabilizes GFR.
    • Tubuloglomerular feedback via JGA/macula densa → releases adenosine to constrict afferent arteriole if NaCl high; may stimulate renin when NaCl low.

Urine Composition

  • Normal components:
    • Water (~95\%).
    • Nitrogenous wastes: urea (~25\,\text{g day}^{-1}), uric acid, creatinine, ammonia.
    • Electrolytes: \text{Na}^+, \text{K}^+, \text{Cl}^-, \text{HCO}3^-, \text{PO}4^{3-}, \text{SO}_4^{2-}.
    • Pigments: urochrome (yellow color from bilirubin metabolism).
    • Trace hormones & vitamins.
  • Abnormal findings (indicative of pathology): blood (hematuria), glucose (glycosuria), albumin (proteinuria), casts, calculi, bacteria.

Urinary Disorders

  • Vascular: Renal hypertension (renal artery stenosis → activates renin-angiotensin system).
  • Obstructive:
    • Renal calculi (kidney stones) causing colic, hydronephrosis.
    • Tumors (renal cell carcinoma, bladder cancer), enlarged prostate, congenital strictures.
  • Functional: Neurogenic or overactive bladder—loss of voluntary control, urgency.
  • Infections: Urethritis, cystitis, (pyelo)nephritis.
  • Glomerular: Glomerulonephritis, nephrotic syndrome (massive proteinuria, edema).
  • Kidney failure:
    • Acute vs chronic; chronic renal failure involves progressive nephron loss → uremia, requires dialysis or transplant.

Key Numbers, Equations & Statistics

  • Kidney dimensions: 11 \times 7 \times 3\,\text{cm}.
  • Female urethra length: \approx3\,\text{cm}; Male urethra length: \approx20\,\text{cm}.
  • Filtrate volume: \sim180\,\text{L day}^{-1}; typical urine output: 1!–!2\,\text{L day}^{-1}.
  • GFR equation (simplified): \text{GFR}=Kf \times \text{EFP}, where Kf = filtration coefficient.
  • Osmotic range of renal medulla: 300!–!1200\,\text{mOsm kg}^{-1}.

Clinical & Real-World Connections

  • Importance of maintaining GFR: decline (
  • Diuretics exploit nephron physiology: loop diuretics inhibit NKCC2, thiazides block NaCl transport in DCT, potassium-sparing antagonize aldosterone or ENaC.
  • Blood pressure meds (ACE inhibitors, ARBs) protect glomerulus by lowering efferent arteriolar resistance.
  • Ethical aspect: dialysis allocation, organ transplantation equity.
  • Hydration advice: dilute urine reduces stone & infection risk; high-protein diets raise urea load → renal stress in compromised patients.