RS 3

Introduction to GFR Regulation

• Objective of study: To understand the mechanisms involved in the regulation of the Glomerular Filtration Rate (GFR) during urine formation.

Glomerular Filtration Overview

• Key areas to consider:

  • Structural integrity

  • Capillary blood pressure within the glomerulus

  • Regulation of glomerular filtration pressure

Important Parameters of GFR

• GFR is influenced by three main factors:

  1. Glomerular Capillary Blood Pressure (GCP)

  2. Capillary Surface Area

  3. Membrane Permeability

• Formula: GFR = K_f (GCP - CP - COP)

  • where

  • K_f (filtration coefficient) is influenced by the glomerular surface area and membrane permeability.

Normal Conditions Affecting GFR

• Under normal conditions, both Capsular Pressure (CP) and Colloid Osmotic Pressure (COP) are relatively stable. Therefore:

  • GFR largely depends on GCP.

  • Adjustment of GFR occurs through changes in diameters of the afferent and efferent arterioles.

Mechanisms for Regulating GFR

• Three basic mechanisms regulate GCP:

  1. Autoregulation

  2. Sympathetic Control

  3. Hormonal Regulation

Autoregulation of GFR

• Maintains constant GCP despite variations in systemic arterial pressure (ideal range: 80 to 180 mmHg).

• Key Processes:

  • Myogenic Response:

  • Vascular smooth muscle contracts when stretched due to high blood pressure, reducing blood flow through the arteriole.

  • Results in depolarization and increased intracellular calcium leading to contraction.

  • Tubuloglomerular Feedback (TGF):

  • Involves the juxtaglomerular apparatus (JGA) which senses changes in distal tubule flow.

  • Macula densa cells release signaling molecules in response to changes in sodium (Na+) and chloride (Cl-) concentrations in tubule fluid.

    • Response: Increased flow leads to reduced nitric oxide secretion, causing vasoconstriction of afferent arterioles and vice versa.

Sympathetic Control of GFR

• Response to stress or decreased blood volume:

  • Mediated by the sympathetic nervous system, leading to:

  1. Vasoconstriction of afferent arterioles.

  2. Enhanced sodium reabsorption in proximal tubules.

  3. Increased renin secretion from the juxtaglomerular apparatus, which raises blood pressure.

Hormonal Regulation of GFR

• Renin-Angiotensin-Aldosterone System (RAAS):

  • Important for Na+ balance, blood volume, and blood pressure regulation.

  • Process:

  1. Renin is secreted by granular cells in response to low blood volume or pressure.

  2. Renin converts angiotensinogen (from the liver) to angiotensin I, which is converted to angiotensin II primarily in the lungs (by ACE).

  3. Effects of Angiotensin II:

     • Potent vasoconstrictor affecting both afferent and efferent arterioles.

     • Stimulates aldosterone release, promoting Na+ and water reabsorption, leading to increased blood volume and pressure.

• Atrial Natriuretic Peptide (ANP):

  • Released from the right atrium in response to increased blood volume.

  • Opposes RAAS effects by:

  • Inhibiting Na+ and water reabsorption.

  • Promoting vasodilation and increasing glomerular filtration surface area.

Other Factors Affecting GFR

• Prostaglandin E2 (PGE2):

  • Minor impact on GFR in normal conditions; under stress or dehydration, it can cause vasodilation of afferent arterioles, counteracting intense vasoconstriction effects.

Sample Questions for Revision

1. Which parameter primarily regulates GFR?

2. Afferent arteriolar vasoconstriction due to intense sympathetic stimulation will:

   • a) Increase GFR,

   • b) Decrease blood flow to peritubular capillaries, etc.

3. Tubuloglomerular feedback involves macula densa cells releasing _ affecting the afferent arteriole.

4. Renin secretion in kidneys can be triggered by a decrease in which factor?

5. Identify components of the Juxtaglomerular Apparatus (JGA).

6. What does angiotensin II stimulate in terms of hormonal effects?

Answers to Sample Questions

1. d

2. c

3. b

4. d

5. b

6. b