RS 6
Amount and Rate of Blood Flow

  • Rate of renal blood flow under basal conditions, approximately 1200 ml/min
    (400 ml/100 g tissue/min),
  • is very high compared to other tissues.
  • Total renal blood flow is approximately 20% of resting cardiac output,
  • while the two kidneys make <0.5% of total body weight.
  • Range of blood flow to kidney is rather narrow.
  • It allows only a 25% increase (300 ml) over the basal renal blood flow of 1200 ml/min.
  • This reserve is much higher in other organs.
  • Higher blood flow to kidneys is related to
  • Blood flow to the kidneys is directly proportional to the pressure difference between the renal artery and renal vein,
  • and is inversely proportional to the resistance of the renal vasculature.
  • Because the afferent arteriole, the efferent arteriole and the intralobular artery are
    the major resistance vessels in the kidneys, they determine renal vascular resistance.
  • In face of blood pressure changes,
  • During exercise
  • sympathetic tone to renal vessels increases
  • and shunts renal blood flow to the skeletal muscles.
  • the renal blood flow shows
    remarkable constancy due to autoregulation.
  • its excretory function rather
    than its metabolic requirement.

Regulation of Renal Blood Flow

  • The regulatory mechanisms affect the renal blood flow (RBF) and glomerular filtration rate (GFR) by changing the arteriolar resistance.
  • Regulatory mechanisms of renal blood flow include:
  • Autoregulation,
  • Hormonal regulation and
  • Nervous regulation.

Autoregulation of Renal Blood Flow

  • Renal blood flow (RBF) and thus the glomerular filtration rate (GFR) remain constant over a wide range of renal arterial pressures
    (80–200 mmHg).
  • This occurs due to an intrarenal mechanism known as autoregulation.
  • Autoregulation of RBF is accomplished by changing renal vascular resistance.
  • When arterial pressure changes
    (between 80–200 mmHg),
    a proportionate change occurs in the renal vascular resistance which maintains a constant RBF.

Mechanisms of Autoregulation

Two mechanisms are considered responsible for autoregulation of RBF and GFR:

  • one mechanism that responds to changes in arterial pressure,
  • and another that responds to changes in NaCl concentration of tubular fluid.
  1. Myogenic mechanism.
  • It is related to an intrinsic property of vascular
    smooth muscle:
  • the tendency to contract when it is stretched.
  • Thus, when renal arterial pressure is raised, the afferent arterioles are stretched, which contract and increase the vascular resistance.
  • The increased vascular resistance offsets the effect of increased arterial pressure and thereby maintains a constant RBF and GFR.
  1. Tubuloglomerular feedback mechanism.
  • Tubuloglomerular feedback (TGF) mechanism is based on the NaCl concentration of tubular fluid.
  • It involves a feedback loop which operates as:
  • Changes in the GFR cause changes in NaCl concentration of fluid in the loop of Henle.
  • Changes in the NaCl concentration are sensed by the macula densa cells and converted into a signal.
  • The signal from the macula densa cells changes the vascular resistance in afferent arterioles.
  • Signals obtained due to increased concentration of NaCl produce vasoconstriction; conversely signals obtained due to decreased NaCl
    cause vasodilatation of afferent arterioles.
  • The effector mechanism responsible for vasoconstriction and vasodilatation is not exactly known.
  • Perhaps, adenosine triphosphate
    (ATP), which selectively constricts the afferent arterioles and metabolites of arachidonic acid, may contribute to TGF mechanism.