Nephron Structure and Function

• The nephron is the functional unit of the kidney responsible for filtering blood and producing urine.

Components of the Nephron

• Renal Corpuscle: Initial filtering component, composed of the glomerulus and Bowman's capsule.
• Glomerulus: A network of capillaries that filters blood based on size and charge.
• Bowman's Capsule: A cup-shaped structure surrounding the glomerulus that collects the filtrate.
• Proximal Convoluted Tubule (PCT): Highly coiled tubule where most reabsorption of water, ions, and nutrients occurs.
• Loop of Henle: A U-shaped structure that establishes a concentration gradient in the renal medulla.
  • Descending Limb: Permeable to water, allowing water to be reabsorbed into the medullary interstitium.
  • Ascending Limb: Impermeable to water, actively transports Na+, Cl-, and K+ into the medullary interstitium.
• Distal Convoluted Tubule (DCT): Further reabsorption of ions and water occurs here, regulated by hormones.
• Collecting Duct: Receives filtrate from multiple nephrons and transports it to the renal pelvis.
  • Medullary Collecting Duct: Final site of water reabsorption, influenced by antidiuretic hormone (ADH).

Detailed Structure and Function

• Renal Corpuscle:
  • Glomerulus:
    • Afferent Arteriole: Carries blood into the glomerulus.
    • Efferent Arteriole: Carries blood out of the glomerulus.
    • Glomerular Filtration Membrane: Consists of fenestrated endothelium, basement membrane, and podocytes.
      • Fenestrated Endothelium: Allows passage of small solutes but prevents passage of blood cells and large proteins.
      • Basement Membrane: A meshwork of proteins that restricts the passage of large proteins.
      • Podocytes: Cells with foot processes (pedicels) that create filtration slits, further restricting protein passage.
  • Bowman's Capsule:
    • Parietal Layer: Outer layer of simple squamous epithelium.
    • Visceral Layer: Inner layer formed by podocytes that surround the glomerular capillaries.
    • Bowman's Space: Space between the parietal and visceral layers where filtrate collects.
• Proximal Convoluted Tubule (PCT):
  • Structure:
    • Simple cuboidal epithelium with a brush border of microvilli to increase surface area for reabsorption.
    • Abundant mitochondria to provide energy for active transport processes.
  • Function:
    • Reabsorption: ~65% of filtered Na+, water, Cl−, K+, glucose, amino acids, bicarbonate, phosphate, and urea.
    • Secretion: H+, NH3, organic acids, and bases.
• Loop of Henle:
  • Descending Limb:
    • Thin Segment: Simple squamous epithelium, highly permeable to water and moderately permeable to solutes.
    • Water Reabsorption: Due to the high osmolarity of the medullary interstitium, water moves out of the descending limb into the interstitium.
  • Ascending Limb:
    • Thin Segment: Simple squamous epithelium, impermeable to water, permeable to some solutes.
    • Thick Segment: Simple cuboidal to columnar epithelium with active transport of Na+, K+, and Cl−.
    • Function:
      • Impermeable to Water: Prevents water reabsorption.
      • NaCl Reabsorption: Active transport of Na+, K+, and Cl− into the medullary interstitium, contributing to the medullary osmotic gradient.
• Distal Convoluted Tubule (DCT):
  • Structure:
    • Simple cuboidal epithelium with fewer microvilli than PCT.
    • Fewer mitochondria compared to PCT.
  • Function:
    • Reabsorption: Na+, Cl−, and water (regulated by hormones such as aldosterone and ADH).
    • Secretion: K+, H+, NH3, and some drugs.
    • Macula Densa: Specialized cells in the DCT that monitor NaCl concentration in the filtrate and regulate glomerular filtration rate (GFR) through the tubuloglomerular feedback mechanism.
• Collecting Duct:
  • Structure:
    • Principal Cells: Respond to ADH and aldosterone; involved in Na+ reabsorption and K+ secretion.
    • Intercalated Cells: Involved in acid-base balance by secreting H+ or HCO3−.
  • Function:
    • Water Reabsorption: Regulated by ADH, which increases the permeability of the collecting duct to water, allowing water to move into the hypertonic medullary interstitium.
    • Urea Recycling: Increases medullary osmolarity.
    • Final adjustments to the urine's osmolality and electrolyte balance.

Types of Nephrons

• Cortical Nephrons:
  • Short loops of Henle that barely penetrate the medulla.
  • Glomeruli located in the outer cortex.
  • Responsible for solute and water reabsorption and secretion.
• Juxtamedullary Nephrons:
  • Long loops of Henle that extend deep into the medulla.
  • Glomeruli located near the cortex-medulla junction.
  • Important for establishing the medullary osmotic gradient and concentrating urine.

Blood Supply to the Nephron

• Afferent Arteriole: Branches from the interlobular artery and delivers blood to the glomerulus.
• Glomerulus: Capillary network where filtration occurs.
• Efferent Arteriole: Carries blood away from the glomerulus.
  • Peritubular Capillaries: Surround the PCT and DCT in cortical nephrons, facilitating reabsorption and secretion.
  • Vasa Recta: Specialized peritubular capillaries that surround the loop of Henle in juxtamedullary nephrons, maintaining the medullary osmotic gradient.

Juxtaglomerular Apparatus (JGA)

• A structure near the glomerulus where the afferent arteriole and the DCT come into contact.
• Components:
  • Macula Densa: Cells in the DCT that monitor NaCl concentration.
  • Juxtaglomerular (JG) Cells: Modified smooth muscle cells in the afferent arteriole that secrete renin in response to low blood pressure or low NaCl concentration in the DCT.
  • Extraglomerular Mesangial Cells: Cells located between the macula densa and the afferent arteriole, involved in communication and regulation of GFR.
• Function: Regulates blood pressure and GFR through the renin-angiotensin-aldosterone system (RAAS) and tubuloglomerular feedback.