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.