Clinical Microscopy: Renal Physiology

Renal Anatomy and Physiology

• Afferent arterioles carry blood to the glomerulus; efferent arterioles carry blood away.
• Kidneys are two bean-shaped organs located under the diaphragm, each weighing approximately 150 g in adult males and 135 g in adult females.
• Urethra is the conduit for urine excretion; measuring 20 cm in males and 3-4 cm in females.
• Nephrons, the kidney's basic functional units, number between 1 to 1.5 million per kidney.

Circulation and Capillary Systems

• Peritubular capillaries surround the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT) for substance reabsorption.
• Vasa recta neighboring the loops of Henle in juxtamedullary nephrons facilitate water and salt exchange.

Kidney Functions

• Major renal functions include:
  • Glomerular filtration
  • Tubular reabsorption
  • Tubular secretion

Glomerular Filtration

• The glomerulus consists of a capillary tuft comprising eight lobes, acting as a nonselective filter for plasma substances.
• Specific gravity (SG) is crucial for measuring urine concentration, with an SG threshold of ≥ 1.026 indicating potential issues.
• Creatinine serves as a fundamental reference point for assessing kidney function.

Daily Urine Output

• Average daily urine output is around 1,200 mL, with normal ranges between 600-2,000 mL.
• Nocturnal urine output typically follows a day-to-night ratio of 1:2 to 1:3.

Urine Output Abnormalities

• Polyuria is characterized by increased daily urine output:
  • Children: >2.5-3.0 mL/kg/day
  • Adults: >2.5 L/day
• Oliguria denotes decreased urine output, with infants producing less than 100 mL.

Urinary Sediment and Artifacts

• Common urinary sediment artifacts include:
  • Starch granules (from gloves): characterized by Maltese crosses and dimples.
  • Other artifacts include oil droplets, air bubbles, pollen grains, hair, and fibers, often showing birefringence under a polarizing microscope.

Clinical Associations

• Key pathologies associated with urine findings:
  • Glomerulonephritis: May cause RBC casts indicating nephron bleeding.
  • Pyelonephritis: Present with WBC casts, indicating inflammation.
  • Congestive heart failure and renal diseases often reflect in urinary sediment and abnormalities.

Specific Gravity and Concentration

• Specific gravity is influenced by solute number and density; useful for screening purposes.
• The Mosenthal test compares day and night urine samples to evaluate kidney concentrating ability.

Pathologic Indicators

• Pathologic conditions that affect urine include glomerulonephritis, pyelonephritis, and heart failure as indicated by casts and specific gravity changes.

Test your knowledge on clinical microscopy, particularly focusing on renal physiology. This quiz covers key aspects such as the roles of afferent and efferent arterioles, as well as the structure and function of the kidneys. Prepare to assess your understanding of these critical components in renal health.