Physiology of Renal Blood Flow

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Blood Flow[edit]

  • 1.2-1.3L blood per minute goes to the kidneys - 25% of cardiac output
  • Measured using the Fick principle - amount of a given substance taken up per unit of time divided by A-V difference
  • Glomerular capillary pressure is about 40% of systemic arterial pressure, with a further big drop through the efferent arteriole resulting in a capillary pressure of about 8mmHg

Regulation of Renal Blood Flow

  • Noradrenaline constricts renal vessels, mostly aimed at the interlobular arteries and afferent arterioles
  • Dopamine is made in the kidney and causes renal vasodilatation and natriuresis
  • Angiotensin II exerts a constrictor effect on afferent and efferent arterioles
  • Prostaglandins - increase blood flow in the renal cortex and decrease blood flow in renal medulla
  • ACh causes renal vasodilation
  • High protein diet raises glomerular capillary pressure and increases renal blood flow

Renal Nerve Functions

  • Renal nerve stimulation causes:
  • Increased renin secretion by noradrenaline action on β1-adrenergic receptors on juxtaglomerular cells
  • Increased Na+ reabsorption by direct action of noradrenaline on renal tubular cells
  • Proximal and distal tubules and the thick ascending limb of the loop of Henle are richly innervated. Stimulation causes:
  • Increase in sensitivity of the granular cells in the JGA
  • Increased renin secretion
  • Increased Na+ reabsorption
  • At high levels of stimulation, renal vasoconstriction with decreased glomerular filtration and renal blood flow
  • Mediated by α1 adrenergic receptors, and less so by α2 adrenergic receptors
  • Arterial baroreceptor firing resulting from systemic blood pressure falls produces renal vasoconstriction as part of the overall vasoconstrictor response
  • Renal blood flow is also decreased during exercise and in rising from the supine position

Renal Blood Flow Autoregulation

  • At moderate pressures. renal vascular resistance varies with blood pressure to produce a relatively constant flow
  • This occurs even in denervated, isolated, perfused kidneys, but is prevented by drugs which paralyse vascular smooth muscle
  • Produced by a direct contractile response to stretch of smooth muscle of the afferent arteriole, with involvement of NO
  • Angiotensin II - constricts efferent arterioles, therefore maintaining GFR
  • This is why ACE-inhibitors can cause poor renal perfusion

Regional Blood Flow & Oxygen Consumption

  • Renal cortex - filters large volumes of blood through glomeruli therefore has large blood flow with little O2 extraction
  • Cortical blood flow is ~5ml/g of kidney tissue/min, vs. 2.5ml/g/min in outer medullar and 0.6ml/g/minute in inner medulla
  • The medulla needs large amounts of O2 for metabolic work done in the loop of Henle, therefore the medulla becomes vulnerable to hypoxia with reduced renal blood flow
  • NO, prostaglandins and cardiovascular peptides assist in balancing blood flow with metabolic needs