Renal Blood flow Physiology for students

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 =?