In-depth Notes on Renal System and Functionality

Importance of Glomerular Function

• Glomerular Filtration Rate (GFR) is a critical measure reflecting kidney function.
• GFR is influenced by:
  • Blood pressure in the glomerulus
  • Osmotic pressure of plasma
  • Blood flow to the glomeruli
  • Permeability of the renal corpuscle.
• Average GFR value: ~105-125 mL/min.

Homeostasis and the Renal System

• The kidney plays a vital role in homeostasis, maintaining balance in bodily functions.
• Its adaptive function allows it to respond intelligently to various stimuli for the organism's survival.

Functions of the Renal System

• Main tasks include:
  • Regulation of blood volume, pressure, ionic concentration, pH, osmolarity, and glucose concentration.
  • Excretion of waste products.
  • Hormone production:
  • Calcitriol: Active form of Vitamin D.
  • Erythropoietin: Stimulates red blood cell production.

Renal Corpuscle and Ultrafiltration

• Comprises:
  • Glomerulus (capillary network)
  • Bowman’s capsule (double-walled epithelial cup).
  • Filtration membrane made from endothelial cells, basal lamina, and podocytes.
• Ultrafiltration:
  • A passive process relying on high blood pressure (BP) within the glomerulus.
• Net Filtration Pressure (NFP) formula: NFP = GBHP - (CHP + BCOP)
  • Where:
  • GBHP: glomerular blood hydrostatic pressure = 55 mmHg
  • CHP: capsular hydrostatic pressure = 15 mmHg
  • BCOP: blood colloid osmotic pressure = 30 mmHg
  • Example of calculations:
    NFP = 55 - (15 + 30) = 10 mmHg

Regulation of GFR

• Intrinsic Mechanisms:
  • Myogenic mechanism: Smooth muscle contracts in response to stretch.
  • Tubuloglomerular feedback: Macula densa cells monitor filtrate flow and adjust renal arterioles.
• Extrinsic Mechanisms:
  • Sympathetic stimulation leads to constriction of afferent arterioles during low blood pressure situations.
  • Renin secretion from juxtaglomerular cells helps with blood pressure control and vasoconstriction.

Renal Tubule and Reabsorption

• Major structure includes proximal and distal convoluted tubules and the loop of Henle.
• Reabsorption:
  • Around 99% of the fluid is reabsorbed.
  • Key reabsorptions occur in the proximal tubule (67% of Na+, Cl−, and virtually all glucose and amino acids).
• Mechanisms in proximal tubule:
  • Na+ reabsorption occurs via Na+–H+ antiporter and symporter mechanisms.
  • Example of Na+ reabsorption processes specified with transport pathways.

Loop of Henle and Concentration Mechanism

• Reabsorption rates:
  • Thick ascending limb: Active transport of NaCl.
  • Descending limb: Permeable to H2O but not to solutes.
  • Overall: 25% NaCl and 15% water reabsorbed.
• Countercurrent multiplication utilized for urine concentration.

Late Distal Tubule and Collecting Duct Regulation

• Reabsorb variable amounts of Na+, secretion of K+, and H+.
• Principal cells: Secretes K+ and reabsorbs NaCl depending on hormones (Aldosterone).
• Intercalated cells: Regulates H+ and HCO3−.

Summary of Tubular Reabsorption

• Mechanisms and Hormones: Affect reabsorption throughout nephron segments.
  • Proximal tubule: 67% Na+ reabsorbed (Angiotensin II influence).
  • Loop of Henle: 25% NaCl, primarily through NKCC2.
  • Distal tubule and collecting duct: Adjust water reabsorption in response to ADH (Vasopressin) and thirst signals.

Regulation of Water Balance

• ADH mechanism: Acts on collecting tubules to insert aquaporins, allowing water reabsorption.
• Osmoregulation impacts plasma osmolality, primarily influenced by Na+ levels.
• Diuretic implications: Low ADH leads to increased urine production; implications of alcohol consumption inhibiting ADH release.