Kidney Function and Urine Formation

Overview of Kidney Function

• Kidneys have millions of functional units called nephrons.
• Nephrons are vital in urine formation.
• Four main steps in urine formation:
  1. Filtration
  2. Reabsorption
  3. Secretion
  4. Excretion

Key Terminology

• Reabsorption: Movement of water and solutes from the nephron back into circulation.
• Secretion: Movement of solutes from circulation back into the nephron tubular.

Anatomy of the Nephron

• Afferent arteriole: Brings blood/plasma to the nephron's glomerulus.
• Efferent arteriole: Exits the nephron's head and forms Vasa Recta (capillaries following the nephron tube).
• Nephron Segments:
  • Bowman's capsule
  • Proximal convoluted tubule (PCT)
  • Loop of Henle
  • Distal convoluted tubule (DCT)
  • Collecting duct

Reabsorption in Different Segments

• Proximal Convoluted Tubule (PCT):
  • Reabsorbs: Sodium, Chloride, Potassium, Glucose, Amino acids, Urea, Bicarbonate, and Water.
• Loop of Henle:
  • Descending limb: Water reabsorption.
  • Ascending limb: Sodium, Chloride, and Potassium reabsorption.
• Distal Convoluted Tubule (DCT):
  • Reabsorbs: Sodium, Chloride, Potassium, Calcium, Magnesium, Bicarbonate.
• Collecting Duct:
  • Reabsorbs: Sodium, Chloride, Urea, and Water.

Secretion in Different Segments

• PCT:
  • Secretes: Creatinine, Certain drugs, Hydrogen ions (acidic).
• DCT:
  • Secretes: Hydrogen ions, Potassium.

Detailed Reabsorption and Secretion Mechanisms

PCT and Ascending Loop of Henle

• PCT Mechanisms:
  • Sodium reabsorbed with glucose or amino acids.
  • Sodium/Hydrogen exchanger and Sodium-Potassium ATPase pump are crucial.
  • Bicarbonate reabsorption involves a chemical reaction with carbonic anhydrase.
• Ascending Loop of Henle:
  • Reabsorbs using a transporter for Potassium, Chloride, Sodium.
  • Sodium-Potassium ATPase pump on the basal surface.

DCT and Collecting Duct

• DCT Mechanisms:
  • Sodium and Chloride reabsorbed with a symporter.
  • Sodium exchanged for Potassium at the basal surface.
  • Magnesium and Calcium reabsorbed via paracellular routes.
• Collecting Duct Mechanisms:
  • Sodium reabsorbed in exchange for Hydrogen ions.
  • Bicarbonate reabsorption using bicarbonate-chloride exchanger.
  • Aldosterone influences Sodium and Potassium exchange.
  • Aquaporins controlled by Antidiuretic Hormone (ADH) for water reabsorption.

Urine Composition

• Primarily water.
• Contains nitrogenous waste, metabolites, and low numbers of red/white blood cells (higher in case of infections/pathology).