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 · 8 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 Structure

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 together with the afferent/efferent arterioles, peritubular capillaries and vasa recta.

Detailed Description of Each Segment

1. Glomerulus

The glomerulus is a tuft of fenestrated capillaries enclosed within Bowman’s capsule. Blood pressure forces plasma (minus proteins) through the capillary walls into the capsule, forming the primary filtrate.

  • Associated vessels: Afferent arteriole (supplies blood) and efferent arteriole (drains blood).

  • Key features: Fenestrated endothelium, basement membrane, podocyte foot processes with filtration slits.

Suggested photomicrograph: Light‑microscope view of a glomerulus with visible capillary loops and Bowman’s space.

2. Bowman’s Capsule

The Bowman’s capsule is a double‑walled, cup‑shaped structure that surrounds the glomerulus. The inner visceral layer is formed by podocytes, while the outer parietal layer is simple squamous epithelium.

  • Function: Collects the filtrate that passes through the glomerular filtration barrier.

3. Proximal Convoluted Tubule (PCT)

The PCT is a highly coiled segment lined by cuboidal epithelial cells with a dense brush border of microvilli, increasing surface area for reabsorption.

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

  • Associated vessels: Peritubular capillaries that arise from the efferent arteriole and surround the PCT.

Suggested electron micrograph: High‑magnification view of PCT cells showing abundant microvilli and mitochondria.

4. Loop of Henle

The Loop of Henle has a descending limb that is permeable to water but not solutes, and an ascending limb that is impermeable to water but actively transports Na⁺, K⁺ and Cl⁻ out of the tubular fluid.

  • Function: Generates a medullary osmotic gradient essential for urine concentration.

  • Associated vessels: Vasa recta – a series of straight capillaries that run parallel to the loop in the renal medulla, providing a counter‑current exchange system.

5. Distal Convoluted Tubule (DCT)

The DCT is shorter and less coiled than the PCT, lined by cuboidal cells with fewer microvilli. It is the site of regulated reabsorption and secretion under hormonal control.

  • Key processes: Reabsorption of Na⁺ (via Na⁺/Cl⁻ cotransporter), Ca²⁺ (under parathyroid hormone), and secretion of K⁺ and H⁺.

  • Associated vessels: Peritubular capillaries continue to surround the DCT.

6. Collecting Duct

The collecting duct receives filtrate from multiple nephrons. Its permeability to water is regulated by antidiuretic hormone (ADH), allowing final adjustment of urine volume and concentration.

  • Function: Final concentration of urine; transport of urea into the medullary interstitium.

  • Associated vessels: Vasa recta, which supply blood to the medullary interstitium and participate in the counter‑current exchange.

Suggested photomicrograph: Section of a cortical collecting duct showing principal cells with aquaporin channels.

Summary Table

StructureLocation in NephronMain FunctionAssociated Blood \cdot essels / Structures
GlomerulusWithin Bowman’s capsule (renal cortex)Filtration of plasmaAfferent arteriole (inflow), Efferent arteriole (outflow)
Bowman’s capsuleSurrounds glomerulusCollects primary filtrateVisceral layer (podocytes), Parietal layer
Proximal convoluted tubule (PCT)Immediately distal to Bowman’s capsuleReabsorption of nutrients, ions, water, bicarbonatePeritubular capillaries (derived from efferent arteriole)
Loop of HenleDescends into renal medulla and returns to cortexCreates medullary osmotic gradient (counter‑current multiplier)Vasa recta (counter‑current exchanger)
Distal convoluted tubule (DCT)After ascending limb, in cortical regionRegulated reabsorption/secretion (Na⁺, Ca²⁺, K⁺, H⁺)Peritubular capillaries
Collecting ductReceives fluid from multiple nephrons; traverses cortex & medullaFinal urine concentration; water reabsorption under ADHVasa recta; interstitial cells producing urea

Key Points for Examination

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

  2. Identify structural adaptations that relate to function:

    • Fenestrated capillaries and podocyte slit diaphragms in the glomerulus for filtration.

    • Brush border microvilli in the PCT for maximal reabsorption.

    • Thin walls of the descending limb for water permeability; thick walls of the ascending limb for active ion transport.

    • Aquaporin‑2 channels in principal cells of the collecting duct regulated by ADH.

  3. Link hormonal control to specific segments:

    • ADH → collecting duct water permeability.

    • Parathyroid hormone → Ca²⁺ reabsorption in DCT.

    • Aldosterone → Na⁺ reabsorption and K⁺ secretion in DCT and collecting duct.

  4. Understand the counter‑current mechanisms:

    • Multiplication in the Loop of Henle.

    • Exchange in the vasa recta.