27. Renal Blood Flow

Question 1

Intro

How Much CO to kidney (% + MLS)

Where does it go

Answer 1

The kidney receives

20–25% of cardiac output,
i.e. 500–600 mL/min to each kidney.

More than 90% supplies the cortex
via the renal artery,
and
less than 10% supplies the

renal capsule and renal
adipose tissue.

The cortex is supplied at
500 mL/min/100 g tissue.

Some of this blood passes into the medulla,

with perfusion rates

of 100 mL/min/100 g tissue
to the outer medulla and

20 mL/min/100 g tissue
to the inner medulla.

Question 2

HINT: REMEMBER THE RULE OF 5s

Answer 2

> 1/5 of cardiac output

> 500 mL/min to each kidney

> 500 mL/min/100 g tissue to the cortex

> 1/5 of this (100 mL/min/100 g tissue)
to the outer medulla

> 1/5 of that (20 mL/min/100 g tissue) to the inner medulla

Question 3

Describe the anatomy of the kidney.

Blood supply

Answer 3

The renal artery enters

each kidney at the hilum

and

divides into several branches.

Interlobar arteries give rise 
->
to interlobular arteries 
->
that give rise to afferent arterioles, 
->
which supply the glomerular capillaries 
(site of filtration). 

Glomerular capillaries drain
into efferent arterioles,

which are portal vessels as
they carry blood from
one capillary network to another.

In the outer two-thirds of the cortex,

peritublar capillaries surround
the
proximal and distal convoluted tubules
and collecting tubules.

In the inner one-third of the cortex,
the vasa recta surround
the loops of Henle and
collecting ducts.

Question 4

What are the functions of renal blood flow (RBF)?

Answer 4

> Provision of glucose 
and 
oxygen to meet the metabolic 
demands of renal
tissue.

> Removal of CO2 and other products of metabolism.

> Maintenance of GFR.

> Provision of O2 for active reabsorption of sodium.

Question 5

Describe the autoregulation of renal blood flow.

Answer 5

Autoregulation describes the 
ability to maintain a constant RBF 
over a wide range of 
mean arterial pressures (MAP) or 
tissue perfusion pressures (PP)
from 90–200 mmHg.

> Myogenic theory –

Other factors affecting RBF include:

• Renal sympathetic nerve stimulation
results in vasoconstriction of
afferent arterioles
thereby reducing RBF

• Renal prostaglandins (PG)
attenuate sympathetic-induced
vasoconstriction through
vasodilation, thereby increasing RBF

• Angiotensin 2 vasoconstricts the
efferent arterioles more than the
afferent arterioles,
thus maintaining glomerular filtration rate (GFR)

• Tubuloglomerular feedback mechanism

• Mediators present in blood vessel
walls help to regulate GFR by
vasodilatation (nitric oxide, NO)
or vasoconstriction (endothelin).

Question 6

> Myogenic theory –

Answer 6

This is the most widely accepted explanation
brought about by a

direct contractile response of
the afferent arteriolar
smooth muscle to stretch.

An increase in perfusion pressure
results in smooth muscle contraction

and

an increase in the renal vascular resistance,
so maintaining a constant blood flow.

Question 7

Describe the Tubuloglomerular feedback (TGF).

Answer 7

The rate of flow through the tubules

feeds back (negatively)

to affect glomerular filtration.

The mechanism has three components:

> Sensor:
the macula densa in the
distal tubular epithelium detects fluid
delivery within the tubule

> Transmission of signal to the glomerulus

> Effector:
vascular smooth muscle
in the afferent arteriole adjusts
GFR by vasodilatation or vasoconstriction.

As the fluid load in the
tubule increases,

the afferent arteriole vasoconstricts
and GFR is reduced.

The converse applies.

TGF is mediated by PG,
thromboxane A2,
NO and endothelin,
and plays a role in RBF autoregulation.

Question 8

How can renal blood flow be measured?

Answer 8

RBF can be calculated by plasma clearance
of para-aminohippuric acid (PAH),
as a modification of the Fick principle.

> Fick principle: 
Flow to an organ is equal to the 
uptake/excretion of a substance 
by an organ per unit time 
divided by the arterio-venous (A-V)
concentration difference of 
that substance across that organ (L/min).

> Plasma clearance:

the volume of plasma cleared
of a substance per unit time (mL/min).

PAH, an organic acid is used because:

• it has a high extraction ratio 
(it is almost completely 
removed by the kidneys) 
via filtration and secretion, 
therefore its A-V concentration

difference across the renal vascular bed
is equal to the renal arteriolar concentration.

• it is neither utilised nor excreted
by any other organ,

therefore is peripheral
venous plasma concentration is
identical to its renal arterial
concentration.

> Applying the Fick equation,
PAH uptake by the kidney is given
by the product of urine PAH concentration
and urine flow.

The A-V concentration difference
is substituted by peripheral venous plasma concentration as explained above.

This gives us the equation for clearance.

Clearance of PAH
= Urine [PAH] × urine flow/plasma [PAH]

> Clearance helps us calculate
renal plasma flow (RPF),
since it is plasma,
and not blood which is filtered.

> RBF can then be deduced from RPF if the haematocrit (Hct) is known, by
the equation: RBF = RPF/1-Hct.