Nephrons: Functional Unit of the Kidney

Overview

• Nephrons are the functional units of the kidney, with approximately one million per kidney.
• Focus on the glomerular structure, a specialized vascular structure that connects the cardiovascular and renal systems.
• Important for controlling blood pressure, blood composition, and urine composition.

Important Processes

• Ultrafiltration
  • Production of filtrate from blood plasma using the glomerulus.
  • Acts as a filter, retaining large or negatively charged items in the blood.
• Reabsorption and Secretion
  • Reabsorption: Returns specific items from filtrate back to the blood.
  • Secretion: Transfers items from blood into the filtrate.

Glomerular Function

• Structure: Tuft of capillaries with a negative charge, repelling similar charges.
• Allowances: Most dissolved items in plasma can pass, size and charge are key factors.
• Protections: Red blood cells, white blood cells, platelets, and large proteins like albumin should not pass.
• Damage Indicators: Presence of these items in urine indicates potential damage, often from hypertension or trauma.

Nephron Structure

• Components:
  • Bowman's Capsule: Collects primary filtrate.
  • Proximal Convoluted Tubule (PCT): Reabsorbs 100% of glucose, most amino acids, and some water/ions.
  • Loop of Henle: Divided into descending and ascending limbs.
    • Descending Limb: Reabsorbs water via aquaporins.
    • Ascending Limb: Reabsorbs sodium and chloride, impermeable to water.
  • Distal Convoluted Tubule (DCT): Final adjustments to filtrate composition.
  • Collecting Duct: Delivers urine to renal pelvis; ADH can modify water reabsorption.

Nephron Types

• Juxtamedullary Nephrons: 15% of nephrons, significant for water reabsorption.
• Cortical Nephrons: 85% of nephrons, shorter loop of Henle.

Blood Supply

• Afferent Arteriole: Brings blood to the glomerulus.
• Efferent Arteriole: Drains the glomerulus, smaller lumen compared to afferent arteriole.
• Peritubular Capillaries: Surround PCT for reabsorption and secretion.

Physiological Functions

• Blood Pressure Regulation: Adjustments in afferent/efferent arteriole diameter control pressure and filtrate formation.
• Blood Filtration: High blood pressure in glomerulus promotes filtrate production.
• Urine Production: Nephrons produce about 1.5 liters of urine per day after reabsorption processes.
• pH Maintenance: Kidneys regulate blood pH slowly, lungs provide rapid adjustments.

Hormonal Influence

• ADH: Modifies water reabsorption in the collecting duct.
• Aldosterone: Increases sodium reabsorption, affecting water retention.

Key Metrics

• Glomerular Filtration Rate (GFR): Measures urine production per unit time.
• Filtrate Osmolality: Changes throughout the nephron, indicative of reabsorption/secretion efficiency.

Clinical Relevance

• Damage Indicators: Protein or blood cells in urine indicate glomerular damage.
• Pharmaceutical Targets: Nephron processes are targets for drugs affecting blood pressure and fluid balance.