Identify, in diagrams, photomicrographs and electron micrographs, the parts of a nephron and its associated blood vessels and structures, limited to: glomerulus, Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, c

Homeostasis in Mammals – The Nephron (Cambridge IGCSE / A‑Level)

Learning Objective

Identify, in diagrams, photomicrographs and electron micrographs, the six structures listed in the syllabus and their associated blood vessels:

  • Glomerulus

  • Bowman’s capsule

  • Proximal convoluted tubule (PCT)

  • Loop of Henle (descending & ascending limbs)

  • Distal convoluted tubule (DCT)

  • Collecting duct

1. Overview – Complete Nephron Diagram

Complete labelled schematic of a nephron showing glomerulus, Bowman’s capsule, PCT, Loop of Henle (descending & ascending limbs), DCT, collecting duct, afferent & efferent arterioles, peritubular capillaries and vasa recta

Figure 1 – Fully labelled schematic of a single nephron. All six required structures are highlighted together; the afferent arteriole enters the glomerulus, the efferent arteriole exits and gives rise to peritubular capillaries (cortex) or vasa recta (medulla).

2. Detailed Description of Each Segment

2.1 Glomerulus

  • Location: Within Bowman’s capsule, renal cortex.

  • Microscopic hallmarks: Fenestrated endothelial cells, thin basement membrane, podocyte foot processes with filtration slits.

  • Primary function (syllabus wording): High‑pressure filtration of plasma (proteins retained).

  • Associated vessels: Afferent arteriole → glomerular capillaries → efferent arteriole.

Light‑microscope photomicrograph of a glomerulus showing capillary loops and Bowman’s space

Figure 2 – Light‑microscope view of a glomerulus.

2.2 Bowman’s Capsule

  • Structure: Double‑walled cup; inner visceral layer (podocytes) and outer parietal layer (simple squamous epithelium).

  • Primary function: Collects the primary filtrate from the glomerulus.

2.3 Proximal Convoluted Tubule (PCT)

  • Location: Immediately distal to Bowman’s capsule, cortex.

  • Microscopic hallmarks: Cuboidal cells, dense brush‑border of microvilli, abundant mitochondria, lateral intercellular clefts.

  • Primary function: Re‑absorption of ~65 % of filtered Na⁺, water, glucose, amino acids and HCO₃⁻; secretion of H⁺ and organic acids.

  • Associated vessels: Peritubular capillaries derived from the efferent arteriole.

Electron micrograph of PCT cells showing a dense brush border and numerous mitochondria

Figure 3 – Electron micrograph of PCT cells.

2.4 Loop of Henle

  • Segments:

    • Descending limb – thin squamous epithelium, highly water‑permeable, no microvilli.

    • Ascending limb – thick stratified epithelium, water‑impermeable, active Na⁺/K⁺/2Cl⁻ transport, abundant mitochondria.

  • Primary function: Generates the medullary osmotic gradient (counter‑current multiplier) essential for urine concentration.

  • Quantitative note: < 5 % of filtered Na⁺ is re‑absorbed in the Loop of Henle.

  • Associated vessels: Vasa recta (counter‑current exchange system).

2.5 Distal Convoluted Tubule (DCT)

  • Location: Cortical region, after the ascending limb.

  • Microscopic hallmarks: Cuboidal cells with few microvilli, prominent basal infoldings, many mitochondria.

  • Primary function: Regulated re‑absorption of Na⁺ and Ca²⁺; secretion of K⁺ and H⁺.

  • Associated vessels: Peritubular capillaries.

2.6 Collecting Duct

  • Location: Receives filtrate from many nephrons; traverses cortex → medulla → papilla.

  • Microscopic hallmarks: Principal cells (apical aquaporin‑2 vesicles) and intercalated cells (apical H⁺‑ATPase / HCO₃⁻ exchangers).

  • Primary function: Final water re‑absorption (up to 99 % under ADH) and urea recycling.

  • Associated vessels: Vasa recta.

Photomicrograph of a cortical collecting duct showing principal cells

Figure 4 – Photomicrograph of a cortical collecting duct.

3. Primary Functions – One‑Column Table (Syllabus‑Focused)

StructurePrimary Function (as required by the syllabus)
GlomerulusHigh‑pressure filtration of plasma (proteins retained)
Bowman’s capsuleCollects the primary filtrate from the glomerulus
Proximal convoluted tubule (PCT)Re‑absorbs ~65 % of Na⁺, water, glucose, amino acids and HCO₃⁻; secretes H⁺ and organic acids
Loop of HenleCreates the medullary osmotic gradient (counter‑current multiplier) for urine concentration
Distal convoluted tubule (DCT)Regulated re‑absorption of Na⁺ and Ca²⁺; secretion of K⁺ and H⁺
Collecting ductFinal water re‑absorption (ADH‑dependent) and urea recycling

4. Summary of Structure, Location & Vascular Relationships

StructureLocation in NephronAssociated Blood Vessels / Structures
GlomerulusWithin Bowman’s capsule (cortex)Afferent arteriole → glomerular capillaries → efferent arteriole
Bowman’s capsuleSurrounds glomerulus (cortex)Visceral (podocytes) & parietal layers; Bowman’s space
PCTImmediately distal to Bowman’s capsule (cortex)Peritubular capillaries (from efferent arteriole)
Loop of HenleDescends into medulla, returns to cortexVasa recta (counter‑current exchanger)
DCTAfter ascending limb (cortex)Peritubular capillaries
Collecting ductTravels cortex → medulla → papilla; receives many nephronsVasa recta

5. Quantitative Re‑absorption Data (Cambridge‑style)

SegmentApprox. % of Filtered Na⁺ Re‑absorbedApprox. % of Filtered Water Re‑absorbed
PCT≈ 65 %≈ 65 %
Loop of Henle< 5 %≈ 15 % (mostly in descending limb)
DCT≈ 5 %≈ 5 %
Collecting duct (ADH present)Variable (≈ 10 % of remaining Na⁺)Up to 99 % of the water that reaches it

6. Diagnostic Microscopic Features (Identify in Diagrams/Photomicrographs)

StructureKey Microscopic Feature
GlomerulusFenestrated endothelium, thin basement membrane, podocyte foot processes with filtration slits
Bowman’s capsuleVisceral layer (podocytes) vs. parietal layer (simple squamous); Bowman’s space
PCTDense brush‑border microvilli, numerous mitochondria, lateral clefts
Descending limbThin squamous epithelium, no microvilli, high water permeability
Ascending limbThick stratified epithelium, abundant mitochondria, water‑impermeable
DCTFew microvilli, prominent basal infoldings, mitochondria for active transport
Collecting ductPrincipal cells with apical aquaporin‑2 vesicles; intercalated cells with H⁺‑ATPase

7. Flow of Filtrate → Urine (Quick‑Reference Flow‑Chart)

  1. Glomerular filtration → Bowman’s capsule (primary filtrate)

  2. Re‑absorption of ~65 % of solutes & water in the PCT

  3. Concentration & dilution steps in the Loop of Henle

  4. Regulated re‑absorption/secretion in the DCT

  5. Final adjustment in the Collecting duct (ADH‑dependent water re‑absorption, urea recycling)

  6. Urine exits via the papillary duct → minor calyx → renal pelvis.

8. Hormonal Regulation of Nephron Segments

8.1 Antidiuretic Hormone (ADH)

TargetMembrane EffectPhysiological Outcome
Collecting duct (principal cells)Insertion of aquaporin‑2 water channels into the apical membraneIncreased water re‑absorption → concentrated urine, reduced urine volume

8.2 Aldosterone (RAAS)

TargetTransporter Up‑regulatedOutcome
DCT & collecting duct (principal cells)Na⁺/K⁺‑ATPase (basolateral) and ENaC (apical)Enhanced Na⁺ re‑absorption, K⁺ secretion → expanded extracellular fluid volume, ↑ blood pressure

8.3 Parathyroid Hormone (PTH)

TargetTransporter/Channel StimulatedOutcome
DCT (early)Active Ca²⁺‑ATPase (basolateral) & Ca²⁺ channelsIncreased Ca²⁺ re‑absorption → maintenance of blood calcium levels

8.4 Renin–Angiotensin–Aldosterone System (RAAS)

  • Trigger: Low arterial pressure or low Na⁺ delivery to the macula densa.

  • Sequence: Juxtaglomerular cells release renin → angiotensinogen → angiotensin I → ACE converts to angiotensin II → vasoconstriction + aldosterone release.

  • Nephron effects: Angiotensin II constricts the efferent arteriole (↑ glomerular filtration pressure) and stimulates aldosterone‑mediated Na⁺ re‑absorption in DCT & collecting duct.

9. Counter‑Current Mechanisms (Physics Made Simple)

9.1 Counter‑Current Multiplier (Loop of Henle)

  1. Descending limb: Water leaves the tubular fluid (high permeability) following the osmotic gradient; solutes remain, making the filtrate increasingly hypotonic.

  2. Ascending limb (thick segment): Active Na⁺/K⁺/2Cl⁻ transport out of the tubular fluid; water‑impermeable, so the filtrate becomes progressively more dilute.

  3. Result: A steep medullary osmotic gradient (up to ~1200 mOsm kg⁻¹) is established, enabling the collecting duct to re‑absorb water under ADH control.

9.2 Counter‑Current Exchange (Vasa Recta)

  • Descending vasa recta lose water and gain solutes as they descend into the hyper‑osmotic medulla.

  • Ascending vasa recta regain water and lose solutes on the way back to the cortex.

  • This exchange preserves the medullary gradient while supplying blood to the nephron.

10. Key Points for Examination (Cambridge Checklist)

  1. Direction of blood flow: afferent → glomerulus → efferent → peritubular capillaries (cortex) or vasa recta (medulla).

  2. Identify each of the six structures in a single complete‑nephron diagram (Figure 1).

  3. Link structural adaptations to function:

    • Fenestrations & podocyte slits → filtration (glomerulus).

    • Brush‑border microvilli → maximal re‑absorption (PCT).

    • Thin epithelium, high water permeability → water loss (descending limb).

    • Thick epithelium, active Na⁺/K⁺/Cl⁻ transport, water‑impermeable → solute removal (ascending limb).

    • Aquaporin‑2 insertion → water re‑absorption (collecting duct, ADH).

  4. Recall quantitative figures: ~65 % Na⁺ & water in PCT; < 5 % in Loop of Henle; up to 99 % water re‑absorbed in collecting duct under ADH.

  5. Describe both counter‑current mechanisms and state the role of the vasa recta.

  6. Match hormones to their specific nephron segment and the transporter they regulate (ADH – AQP2; Aldosterone – ENaC/Na⁺/K⁺‑ATPase; PTH – Ca²⁺‑ATPase).