Lecture Notes on Kidney Function and Filtration

Key Concepts

Secretion and Reabsorption

• Secretion: Removal of waste from the peritubular capillaries into the nephrons.
  • Involves moving substances from the blood into the tubules.
  • Works alongside reabsorption, which puts substances back into the blood.

Filtration, Secretion, and Reabsorption

• Filtration and secretion: Remove things from the blood.
• Reabsorption: Returns substances into the blood.
• Excretion Rate: Important in drug testing to determine how fast a drug is excreted.
  • Fast excretion may require more doses per day (e.g., penicillin).
  • Can be modified by tagging with proteins to slow it down.

Capillary Beds in the Kidney

• Three types:
  • Glomerulus
  • Peritubular Capillaries
  • Vasa Recta: Involved in maintaining the osmolarity of the medullary region.
    • Important for urine concentration.
    • Resembles 'strings of pearls' in structure.

Proximal Convoluted Tubule (PCT)

• Major site for:
  • Reabsorption: Nearly all glucose, amino acids, and significant portions of water and sodium are reabsorbed here.
  • Secretion: A lot occurs here as well.
• Goal: Retrieve valuable substances from filtrate quickly to prevent loss in urine.
• Rest of the tubule system fine-tunes the filtrate to match body's current conditions.

Filtration Mechanism

• Occurs at the glomerulus.
• Types of Capillaries:
  • Continuous Capillaries: Most common, found throughout the body.
  • Fenestrated Capillaries: Have small holes, used for filtration (kidneys, brain, choroid plexuses).
  • Sinusoids: Large holes, found in liver, spleen, bone marrow.
• Fenestrated Capillaries:
  • Allow passage of most substances except proteins and formed elements.
  • Surrounded by a basement membrane and connective tissue.
  • Blood must pass through three filters during filtration.