Relate the detailed structure of the Bowman's capsule and the proximal convoluted tubule (PCT) to their specific functions in the formation of urine.
1. Bowman's Capsule – Structure and Function
The Bowman's capsule is the initial filtering unit of the nephron. Its key structural components are:
Parietal layer of simple squamous epithelium – forms the outer wall of the capsule.
Visceral layer (podocytes) of simple squamous epithelium – lines the interior and creates filtration slits.
Filtration slits (fenestrae) and slit diaphragms – allow passage of plasma water and small solutes while retaining cells and large proteins.
Glomerular capillaries – a tuft of high‑pressure capillaries surrounded by the visceral layer.
Bowman's space (capsular lumen) – collects the filtrate before it enters the renal tubule.
The functional outcome of this architecture is the production of primary urine by ultrafiltration. The driving forces can be expressed as:
\$\$
GFR = Kf \times (P{GC} - P{BS} - \pi{GC})
\$\$
where \$Kf\$ is the filtration coefficient, \$P{GC}\$ is glomerular capillary hydrostatic pressure, \$P{BS}\$ is Bowman's space hydrostatic pressure, and \$\pi{GC}\$ is the oncotic pressure of glomerular blood.
Suggested diagram: Cross‑section of Bowman's capsule showing parietal layer, podocytes, filtration slits, and glomerular capillaries.
2. Proximal Convoluted Tubule – Structure and Function
The PCT follows the Bowman's capsule and is specialized for reabsorption and secretion. Its distinctive structural features include:
Simple cuboidal epithelium with a brush border – dense microvilli increase surface area \overline{20}‑30 µm² per cell.
Abundant mitochondria – supply ATP for active transport mechanisms.
Basolateral infoldings and numerous transport proteins – facilitate Na⁺/K⁺‑ATPase activity and cotransport.
Peritubular capillaries and vasa recta – lie close to the tubule, creating a counter‑current exchange system.
Key functional processes:
Reabsorption of \overline{65} % of filtered Na⁺, water, glucose, amino acids, and bicarbonate.
Secretion of organic acids, drugs, and excess ions into the tubular lumen.
Maintenance of acid‑base balance via bicarbonate reclamation.
where \$P_{Na^+}\$ denotes the permeability coefficient mediated by Na⁺/K⁺‑ATPase.
Suggested diagram: Longitudinal view of a proximal convoluted tubule cell highlighting microvilli, mitochondria, and basolateral infoldings.
3. Integration – From Filtration to Reabsorption
The transition from Bowman's capsule to the PCT illustrates a seamless hand‑off of fluid:
Primary filtrate, isotonic to plasma, enters Bowman's space.
As it passes into the PCT, \overline{65} % of the filtrate’s solutes and water are reclaimed, rendering the tubular fluid hypotonic relative to plasma.
Active transport of Na⁺ creates an osmotic gradient that drives water reabsorption through aquaporins (AQP1) in the PCT.
The close proximity of peritubular capillaries ensures that reabsorbed substances are rapidly returned to the systemic circulation, preserving homeostasis.
4. Summary Table – Structure ↔ Function
Nephron Segment
Key Structural Feature
Primary Functional Role in Urine Formation
Bowman's capsule (parietal layer)
Simple squamous epithelium forming a rigid capsule
Provides a sealed space (Bowman's space) to collect filtrate.
Bowman's capsule (visceral layer – podocytes)
Podocyte foot processes with slit diaphragms
Selective filtration: permits water, ions, small molecules; blocks cells & large proteins.
Glomerular capillaries
High hydrostatic pressure, fenestrated endothelium
Maximises reabsorption of Na⁺, glucose, amino acids, and water.
PCT (basolateral mitochondria)
Abundant mitochondria
Supply ATP for Na⁺/K⁺‑ATPase and secondary active transport.
PCT (basolateral infoldings)
Extensive membrane infoldings with transport proteins
Facilitate rapid movement of reabsorbed solutes into peritubular capillaries.
5. Clinical Correlation
Damage to any of these structures disrupts homeostasis:
Glomerulonephritis – inflammation of the visceral layer destroys slit diaphragms, allowing proteins to leak into urine (proteinuria).
Proximal tubular necrosis – loss of brush‑border microvilli reduces reabsorption, leading to glucosuria and electrolyte imbalances.
6. Quick Revision Questions
Explain how the structure of podocyte foot processes contributes to selective filtration.
Why is a high density of mitochondria essential in PCT cells?
Calculate the theoretical GFR if \$Kf = 12.5\,\text{mL/min/mmHg}\$, \$P{GC}=45\,\text{mmHg}\$, \$P{BS}=15\,\text{mmHg}\$, and \$\pi{GC}=25\,\text{mmHg}\$.