Glomerular Filtration and Mechanisms

Intro to Net Filtration Pressure and GFR

• Net Filtration Pressure (NFP):
  • Defined as the pressure that drives fluid out of the capillaries and into the Bowman's capsule in the kidneys.
  • Related to blood pressure; changes in systemic blood pressure directly affect NFP and Glomerular Filtration Rate (GFR).
• Glomerular Filtration Rate (GFR):
  • The volume of fluid filtered by the kidneys per unit time.
  • Directly proportional to NFP, and therefore to blood pressure.

Importance of GFR

• GFR resembles a balance (Goldilocks principle):
  • Must be neither too high nor too low, but just right for optimal filtration.
  • Constant filtration is essential for maintaining homeostasis in the body by filtering out waste while reabsorbing necessary substances.
• Effects of Altered GFR:
  • High GFR:
  • Rapid movement of fluid: important substances (e.g., glucose, amino acids) may be lost in urine due to insufficient reabsorption time.
  • Low GFR:
  • Slow flow may lead to excessive reabsorption of waste products, negatively affecting bodily functions.

Mechanisms Regulating GFR

• Two Main Control Systems
  • Intrinsic (Autoregulation):
  • Local mechanisms acting at the nephron (the functional unit of the kidney).
  • Extrinsic Factors:
  • Neural and hormonal signals responding to systemic changes in blood pressure.
• Intrinsic Mechanisms:
  • Fine-tuning blood flow and pressure within normal limits (80-180 mmHg) to maintain constant GFR.

Afferent and Efferent Arterioles Role

• Afferent Arteriole:
  • Responds to systemic blood pressure.
  • If blood pressure rises, it constricts to reduce blood flow to glomerulus, lowering GFR.
  • If blood pressure drops, it dilates to increase blood flow, increasing GFR.
• Efferent Arteriole:
  • Constriction leads to increased back pressure in glomerulus, enhancing filtration when needed.

Juxtaglomerular Apparatus

• Components:
  • Located near glomerulus; consists of granular cells (secrete renin) and macula densa cells (detect sodium concentration in distal convoluted tubule).
• Functions:
  • Granular cells respond to sympathetic nervous system stimulation by contracting and releasing renin.
  • Macula densa senses sodium levels to adjust GFR accordingly.

Extrinsic Mechanisms of GFR Control

• Sympathetic Nervous System Activation:
  • When blood pressure is severely low, priority shifts to sustaining systemic pressure rather than GFR.
  • Inhibits filtration, reduces urine production to conserve blood volume.
• Renin-Angiotensin Mechanism:
  • Renin is released from granular cells; initiates a cascade that leads to the production of angiotensin II, a potent vasoconstrictor that helps restore blood pressure.

Hormonal Controls of Blood Pressure and GFR

• Aldosterone: Promotes sodium reabsorption, enhancing water reabsorption and increasing blood volume.
• Antidiuretic Hormone (ADH): Increases water reabsorption in the collecting ducts, concentrating urine and increasing blood volume.
• Atrial Natriuretic Peptide (ANP): Released when blood pressure is high; promotes urine production to reduce blood volume and pressure.

Summary of Kidney Filtration Processes

• Filtration, Reabsorption, and Secretion:
  • Filtration: Initial removal of substances from blood into nephron.
  • Reabsorption: Essential nutrients and water are taken back into blood.
  • Secretion: Additional wastes are sent to urine from blood.

Concentration Gradient in Kidney

• Role of Countercurrent Mechanism:
  • Establishes and maintains concentration gradients to optimize water and salt reabsorption, crucial for urine concentration.
• Juxtamedullary Nephrons:
  • Ability to form concentrated urine due to their location and length of loops of Henle within the medullary gradient.

Regulation of Water and Electrolytes

• Role of Aquaporins:
  • Integral proteins that facilitate water reabsorption; regulated by ADH.
• Transport Maximum and Renal Threshold:
  • Indicates the limits of reabsorption of substances like glucose; high glucose levels can overwhelm reabsorption capacity leading to glucose in urine.

Conclusion

• Maintaining GFR within the right parameters is vital for homeostasis through fluid balance, waste elimination, and regulation of blood pressure.