This document discusses renal blood flow (RBF), including its volume, functions, and regulation. RBF is approximately 1-1.2 L/min, or 20-25% of cardiac output. It is regulated through autoregulation, neural control, and hormonal mechanisms. Autoregulation maintains RBF within a narrow range despite changes in blood pressure via myogenic responses and tubuloglomerular feedback. Neural and hormonal control further regulate RBF through vasoconstriction and vasodilation. RBF can be measured using the clearance of para-aminohippuric acid (PAH), which is excreted by the kidneys without being reabsorbed or metabolized.

1. Renal dynammics-1
(Renal Blood Flow)
Dr. Aamir Magzoub
MBBS, MSc, PhD

2. Objectives
Volume and functions of RBF
Regulation
Measurement

3. Renal Blood Flow (RBF)
• = 1 - 1.2 L/min
• =20-25% of cardiac output
• Directed mainly to the cortex
• (90%) to the cortex, only [10%] to the medulla?
• The low blood flow to the medulla maintains its
high osmolarity. ?

4. Functions of Renal Blood Flow
 Delivers oxygen , nutrient and hormones and
returns CO2 and reabsorbed substances
(fluids and solutes) to the circulation.
 Ensures optimum GFR and hence optimum
substance handling by the renal tubules.
 Participates in the concentration and dilution
of urine

5. Regulation of renal blood flow
Why?
Optimum GFR ; how?
[1] Autoregulation
[2] Neural regulation
[3] Hormonal regulation

6. Manipulation of
the afferent and
efferent arteriolar
resistance

7. [1] Autoregulation
• The ability to maintain RBF & GFR within
narrow limits despite changes in mean arterial
pressure (MAP) - perfusion pressure.
• RBF is maintained constant between (80-180
mm/Hg) of MAP.
• Independent of renal nerves & circulating
hormones.
• Seen in isolated perfused Kidneys
• Autoregulation =stabilization of blood flow

8. 8
Autoregulation of RPF & GFR
“Autoregulatory
range”
1.5
0 0
150
Renal
blood
flow
(l/min)
Glomerular
filtration
rate
(ml/min)
0 80 mmHg 180
Mechanism?
Glomerular blood flow & filtration
are autoregulated
MAP
125
1 L

9. [1] Autoregulation (mechanisms)
Two mechanisms:
A. Myogenic response (reactive contraction
to stretch – afferent arteriole)
B. Tubulo-glomerular feed back (macula
densa)

10. Afferent arteriolar Response to
change in Tension (Transmural Pr)
Stretch-activated
Cation channels
Ca2+ influx –
& VSM Contraction
A. Myogenic response
RPF, GFR
During autoregulation if BP ,
Vascular R also = Constant Flow
Flow = P/R
Afferent Art R
If BP ??

11. B. Tubulo-Glomerular Feedback
1&2 GFR& tubular flow
3. Tubular flow past
Macula Densa (MD)
4.Paracrine factors from MD
(Adenosine)
5. Afferent arteriolar R
GC blood flow
GFR - Back to normal
Normalises flow past macula densa,
completing feedback loop
TALH
1
2
3
4
5
If BP ??

12. (2) Neural regulation of renal
blood flow
Mainly by renal sympathetic nerves.
Stimulation of these nerves constrict both
afferent and efferent arterioles (α1)

13. [3] Hormonal regulation of renal
blood flow
Catecholamines
 RBF by constricting both afferent and
efferent arterioles (afferent more).
Dopamine
 RBF by vasodilatation

14. [3] Hormonal regulation:
• Angiotensin II (at low perfusion pressure)
• by constricting efferent arterioles thus maintain GFR
• ACE inhibitors and Renal failure in patients with poor renal
perfusion
• ADH (Vasopressin)
• In physiological levels decreases blood flow via vasa recta
• High levels decreases renal blood flow
• Prostaglandin (locally produced)
• Vasodilators (PGI2)
• Vasoconstrictors (TXA2)
• Increase the blood flow in the cortex and decrease it in the
medulla ( Chronic use of Aspirin ?)

15. Measurement of RBF
• RBF is measured by:
• Clearance of para aminohippuric acid
(PAH) – why? Criteria.
• Highly secreted (most important)
• Not toxic.
• Not metabolized, stored or produced by the kidney.
• Does not affect RBF.
• Measurable.

16. Concept of clearance
• Clearance is defined as:
– Volume of plasma which is completely cleared
of a particular substance to be excreted in
urine per unit time

17. Clearance formula
• Amount cleared = amount excreted
• As the amount of X =V*Conc.X; therefore:
• Amount cleared=volume of plasma
cleared of substance X*plasma conc. of X
• Amount excreted= urine volume*urine
conc. of substance X
• RCx X Px = UxV
• RCx = UxV/Px (clearance formula)

18. Fick’s Principle for measurement
of blood flow
• The amount of a substance taken up by an
organ in a given time equals the arterio-
venous difference in concentration times
the blood flow to the organ.
• Qx= ([Ax]-[Vx]) x Blood flow
• Renal plasma flow = Qx
Renal[Ax] - [Vx]

19. Measurement of Renal Plasma Flow
using PAH
• Since all quantity infused is taken up by the
kidney and excreted in urine:
QPAH= Excreted PAH = ( UPAH V)
– It is not metabolized, stored or produced by the
kidney.

20. Measurement of Renal Plasma
Flow using PAH
• Since the PAH is highly secreted then its
concentration in renal veins =0
• By applying Fick’s principle:
• Plasma blood flow = Qx = (UPAH V)
[APAH] - [VPAH]
[VPAH]= 0 ; A= PPAH
 RPF= UPAHV/ PPAH (clearance formula)

21. Measurement of Renal Plasma
Flow using PAH
• RCPAH measures the effective renal plasma
flow (ERPF) as the level in renal venous
plasma is not measured
• ERPF = UPAH V/PPAH
• ERPF=90% of the true RPF

22. Measurement of Renal Plasma Flow
using PAH
• Since the ERPF is 90% of the true RPF
• RPF= ERPF ×100/90.
• Renal blood flow includes Hematocrit (PCV)
• Renal blood flow = RPF × 1/ 1- PCV
• Example:
• UPAH =14mg/ml (urine conc.)
• Urine flow rate V=0.9 ml/min
• PPAH =0.02mg/ml .
• PCV = 45% - RBF =?