Overview
This lecture focused on the nephron, the kidney's functional unit, detailing its structure, function, and key processes involved in urine formation: filtration, reabsorption, and secretion.
Nephron Anatomy & Structure
- The nephron is the functional unit of the kidney, responsible for cleaning blood and making urine.
- Each kidney has over a million nephrons, each consisting of a renal corpuscle (in the cortex) and renal tubule (extending into the medulla).
- The renal corpuscle is where the cardiovascular and urinary systems meet, consisting of the glomerulus (capillary bed) and Bowman's (glomerular) capsule.
- Afferent arteriole brings blood into the glomerulus; efferent arteriole carries it out after filtration.
- Bowman's capsule has two layers: parietal (lining capsule) and visceral (with podocytes covering capillaries).
- The renal tubule pathway: Bowman's capsule → proximal convoluted tubule (PCT) → loop of Henle (descending and ascending limbs) → distal convoluted tubule (DCT) → collecting duct.
Filtration, Reabsorption, and Secretion (FRS)
- Urine formation involves three key steps: Filtration, Reabsorption, Secretion (FRS).
- Filtration occurs in the glomerulus/Bowman's capsule—blood pressure forces small molecules out, creating filtrate.
- Reabsorption mainly occurs in the PCT, moving valuable substances (glucose, amino acids, ions) from filtrate back into the blood.
- Secretion primarily occurs in the DCT, moving waste (e.g., K+, H+) from the blood into the filtrate for excretion.
Function and Flow Details
- On the left side (Bowman's capsule, PCT, descending limb), the nephron is permeable to water—water can leave freely.
- On the right side (ascending limb, DCT), water is not permeable—water is trapped until the collecting duct.
- Peritubular capillaries surround the nephron, enabling exchange between the blood and filtrate via diffusion and blood pressure.
- Filtrate only becomes "urine" upon entering the collecting duct, after all reabsorption and secretion is complete.
Key Terms & Definitions
- Nephron — Smallest functional unit of the kidney, responsible for filtering blood and forming urine.
- Renal corpuscle — Structure of glomerulus and Bowman's capsule where filtration occurs.
- Glomerulus — Capillary bed where blood is filtered.
- Bowman's capsule — Cup-like structure collecting filtrate from glomerulus.
- Afferent arteriole — Blood vessel bringing blood into the glomerulus.
- Efferent arteriole — Blood vessel carrying filtered blood out of the glomerulus.
- Podocytes — Specialized cells on Bowman's capsule's visceral layer, forming filtration slits.
- Peritubular capillaries — Capillaries surrounding nephron tubules for reabsorption/secretion.
- Filtration — Movement of fluid and small molecules from blood into Bowman's capsule.
- Reabsorption — Movement of useful substances from filtrate back to blood.
- Secretion — Movement of excess/waste substances from blood into filtrate.
Action Items / Next Steps
- Draw and label a nephron diagram, including all major parts and note water permeability along sections.
- Memorize the Filtration-Reabsorption-Secretion (FRS) sequence and definitions.
- Be able to explain the role of peritubular capillaries in nephron function.