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

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Homeostasis in Mammals: Nephron Structure

Homeostasis in Mammals – The Nephron

Learning Objective

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 (PCT)

  • Loop of Henle (descending and ascending limbs)

  • Distal convoluted tubule (DCT)

  • Collecting duct

Overview of Nephron Anatomy

The nephron is the functional unit of the mammalian kidney. Each kidney contains approximately one million nephrons, each consisting of a vascular component (the glomerulus) and a tubular component that modifies the filtrate to form urine.

Suggested diagram: Schematic of a complete nephron showing the glomerulus, Bowman’s capsule, PCT, Loop of Henle, DCT and collecting duct with associated arterioles and venules.

1. Glomerulus

The glomerulus is a tuft of capillaries arising from the afferent arteriole and draining into the efferent arteriole. Its primary role is the ultrafiltration of plasma.

  • Structure: Fenestrated endothelial cells, a basement membrane, and podocytes with filtration slits.

  • Function: Generates the primary filtrate by allowing water, ions, and small molecules to pass while retaining cells and large proteins.

  • Key parameter: Filtration pressure ≈ \$10\ \text{kPa}\$.

Suggested photomicrograph: Light‑microscope view of a glomerular tuft with visible capillary loops.

2. Bowman’s Capsule

Bowman’s capsule surrounds the glomerulus and collects the filtrate. It consists of two layers:

  1. Visceral layer – formed by podocytes.

  2. Parietal layer – simple squamous epithelium.

The space between the layers is the Bowman’s space (urinary space) where filtrate accumulates before entering the proximal tubule.

Suggested electron micrograph: Podocyte foot processes and filtration slits in the visceral layer of Bowman’s capsule.

3. Proximal Convoluted Tubule (PCT)

The PCT is the first segment of the tubular system and is highly specialized for reabsorption.

  • Microscopic features: Brush border of microvilli, abundant mitochondria.

  • Reabsorption: \overline{65} % of filtered Na⁺, water, glucose, amino acids, and bicarbonate.

  • Secretion: Organic acids, drugs, and H⁺ ions.

Suggested diagram: Cross‑section of PCT showing microvilli and tight junctions.

4. Loop of Henle

The Loop of Henle creates a concentration gradient in the renal medulla, essential for water reabsorption.

  • Descending limb – permeable to water, impermeable to solutes.

  • Ascending limb – impermeable to water, actively transports Na⁺, K⁺, and Cl⁻ out of the tubular fluid.

  • Result: Counter‑current multiplier system establishing an osmotic gradient up to \$1200\ \text{mOsm/kg}\$ in the inner medulla.

Suggested diagram: Counter‑current multiplier mechanism of the Loop of Henle.

5. Distal Convoluted Tubule (DCT)

The DCT fine‑tunes electrolyte balance under hormonal control.

  • Reabsorption: Na⁺ and Cl⁻ (via Na⁺‑Cl⁻ cotransporter).

  • Secretion: K⁺ and H⁺ (regulated by aldosterone).

  • Calcium reabsorption is stimulated by parathyroid hormone (PTH).

Suggested photomicrograph: DCT cells with fewer microvilli than PCT.

6. Collecting Duct

The collecting duct determines the final urine volume and concentration.

  • Permeability to water is regulated by antidiuretic hormone (ADH).

  • Principal cells: Reabsorb Na⁺ (aldosterone) and secrete K⁺.

  • Intercalated cells: Regulate acid‑base balance by secreting H⁺ or reabsorbing HCO₃⁻.

Suggested diagram: Collecting duct showing principal and intercalated cells, and ADH‑mediated insertion of aquaporin‑2 channels.

Summary Table of Nephron Segments

SegmentKey Structural FeaturesPrimary FunctionsAssociated \cdot essels
GlomerulusFenestrated capillaries, basement membrane, podocytesUltrafiltration of plasmaAfferent arteriole → Efferent arteriole
Bowman’s capsuleVisceral (podocytes) & parietal layers, Bowman’s spaceCollects primary filtrateSurrounds glomerulus
Proximal convoluted tubuleBrush border, abundant mitochondriaReabsorption of \overline{65} % filtrate (Na⁺, water, glucose, amino acids)Peritubular capillaries
Loop of HenleThin descending limb, thick ascending limbCreates medullary osmotic gradient (counter‑current multiplier)Vasa recta (counter‑current exchange)
Distal convoluted tubuleFewer microvilli, hormone‑responsive transportersFine‑tuning of Na⁺, Cl⁻, K⁺, Ca²⁺; acid‑base regulationPeritubular capillaries
Collecting ductPrincipal & intercalated cells, ADH‑regulated aquaporinsRegulates final urine volume & composition; water reabsorptionVasa recta, renal pelvis veins

Key Points for Examination

  1. Label all parts of the nephron in a schematic diagram.

  2. Explain how the glomerular filtration rate (GFR) is influenced by afferent and efferent arteriolar tone.

  3. Describe the role of the Loop of Henle in producing hyperosmotic medullary interstitium.

  4. Identify hormonal control mechanisms (ADH, aldosterone, PTH) acting on the distal tubule and collecting duct.

  5. Compare reabsorption capacities of the PCT and DCT.