Nephrons

Nephron Structure

• Functional units of kidneys that perform filtration and concentrate urine
• Made of renal corpuscle and tubular segments
  • Renal Corpuscle: the initial filtration portion, comprised of the glomerulus* and Bowman’s Capsule
    • glomerulus: tuft of capillaries where the initial filtration (ie glomerular filtration) happens
    • Bowman’s Capsule: where the filtrate goes
• Span both the cortex and medulla of the kidney
  • The cortex contains the convoluted tubules
  • The medulla contains the Loop of Henle
• Each nephron is surrounded by peritubular capillaries in the cortex and the vasa recta in the medulla
• Allow exchange of water and other molecules between blood and filtrate (the would-be urine)

Nephron Function

• The filtrate (would-be urine) is now in the Bowman’s capsule and traveling through the nephron
• Main function: 
  • Reabsorb any ions/nutrients if needed before they’re excreted
  • Secrete more waste into the filtrate (urine) in case it needs to excreted
• Remember that in the next few steps, we’re dealing with 3 environments
  • Filtrate inside the tubule
  • Interstitial fluid in the kidneys
  • Blood in the vasa recta and peritubular capillaries

1. Proximal Convoluted Tubule

• Associated vessels: peritubular capillaries
• Located in the Renal Cortex
• Here, the bulk of the filtrate from the Bowman’s Capsule gets reclaimed into the peritubular capillaries
  • Amino acids, vitamins, glucose, and water get reabsorbed into the blood via diffusion
• Dump the HUNK
  • PCT also secretes H+, urea, Na+, and K+ into the filtrate
  • Uses H+ to regulate pH of the filtrate through the Bicarbonate Buffer

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Loop of Henle

• Located in the Renal medulla
  • Remember that the renal medulla is extremely salty
• Associated vessels: vasa recta

2. Loop of Henle - Descending Limb

Associated vessels: vasa recta

• Only permeable to water and not to salt and other ions
  • Water reabsorbed from the filtrate into the interstitial fluid passively
    • Remember that the interstitial fluid in the medulla is salty, and water flows down the concentration gradient

3. Loop of Henle - Thin Ascending Limb

Associated vessels: vasa recta

• Right now, the filtrate contains more solute than the interstitial fluid, AND this region is impermeable to water
  • So, ions move out of the filtrate passively into the interstitial fluid

4. Loop of Henle - Thick Ascending Limb

Associated vessels: vasa recta

• Creates the salty environment of the renal medulla
• Actively pumps out ions from the filtrate into the interstitial fluid
• The limb is impermeable to water, which means water won’t follow out, and the interstitium remains hypertonic

With the vasa recta, the Loop of Henle creates a countercurrent multiplier system.

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5. Distal Convoluted Tubule

• Associated vessels: peritubular capillaries
• Maintains the pH
• Regulated reabsorption
  • Aldosterone acts to increase sodium reabsorption by increasing Na+/K+ pump activity
    • Sodium goes out of filtrate and into interstitial fluid while potassium goes into the filtrate
  • Parathyroid Hormone (PTH) acts to increase Ca2+ absorption