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RS 6 Amount and Rate of Blood Flow - Coggle Diagram
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
its excretory function rather
than its metabolic requirement.
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,
the renal blood flow shows
remarkable constancy due to autoregulation.
During exercise
sympathetic tone to renal vessels increases
and shunts renal blood flow to the skeletal muscles.
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.
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.
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.
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