Understanding Renal Blood Flow Regulation

Nov 16, 2024

Regulation of Renal Blood Flow

Overview

  • Blood flow rate through kidneys is crucial for filtration.
  • Determines solute retention/disposal, water retention/disposal, blood osmolarity, and blood pressure.

Sympathetic Nerves

  • Kidneys innervated by sympathetic neurons via celiac plexus and splanchnic nerves.
  • Vasodilation: Reduced sympathetic stimulation → increased kidney blood flow during rest.
  • Vasoconstriction: Increased action potentials → constricted arteriolar smooth muscle → reduced glomerular flow.
    • Stress increases sympathetic activity → vasoconstriction of afferent arterioles (via norepinephrine and epinephrine).
    • Blood redirected to organs with urgent needs.
  • Renin Release: Falls in blood pressure stimulate renin release → increases angiotensin II → promotes aldosterone for Na+ and water retention.
  • Small changes in afferent arterial pressure significantly affect GFR.

Autoregulation

  • Kidneys regulate blood flow amid varying blood pressures.
  • Ensures stable filtration rate through:

Arteriole Myogenic Mechanism

  • Relies on smooth muscle cell response to stretching.
  • Increased Blood Pressure: Afferent arteriole smooth muscle contracts to resist pressure, maintaining flow.
  • Decreased Blood Pressure: Smooth muscle relaxes, reducing resistance for consistent flow.

Tubuloglomerular Feedback

  • Involves Juxtaglomerular Apparatus (JGA) using ATP, adenosine, and nitric oxide (NO).
  • Macula Densa Cells: Detects changes in fluid rate and Na+ concentration in DCT.
    • High GFR: Less NaCl reabsorbed → higher filtrate osmolarity → macula densa releases ATP/adenosine → afferent arteriole constriction → reduced GFR.
    • Low GFR: More Na+ reabsorbed → less ATP/adenosine → afferent arteriole dilation → increased GFR.
  • Role of NO: Causes vasodilation, countering ATP/adenosine effects to fine-tune GFR.

Summary Table

| Change in GFR | NaCl Absorption | Role of ATP and adenosine/Role of NO | Effect on GFR | |---------------|-----------------|-------------------------------------|----------------| | Increased GFR | Tubular NaCl increases | ATP and adenosine increase, causing vasoconstriction | Vasoconstriction slows GFR | | Decreased GFR | Tubular NaCl decreases | ATP and adenosine decrease, causing vasodilation | Vasodilation increases GFR | | Increased GFR | Tubular NaCl increases | NO increases, causing vasodilation | Vasodilation increases GFR | | Decreased GFR | Tubular NaCl decreases | NO decreases, causing vasoconstriction | Vasoconstriction decreases GFR |