lecture 3: renal blood flow and glomerular filtration

Question 1

what is renal/kidney failure?

Answer 1

an abrupt fall in glomerular filtration

Question 2

define glomerular filtration?

Answer 2

passive process with fluid ‘driven’ through the
fenestrated walls of the glomerular capillaries by heart hydrostatic
pressure.

Question 3

what does the passive process refer to?

Answer 3

• no ATP pumping

Question 4

what is the filtration barrier ( endothelium of capillaries) permeable to and impermeable to?

Answer 4

permeable to
- fluids, small solutes

impermeable to

• cells
• proteins
• drugs bound to proteins

Question 5

how does the concentration of the solute change?

Answer 5

• the concentration remains the same

Question 6

what is the primary urine?

Answer 6

A clear fluid (ultrafiltrate), completely free from blood and proteins, is produced containing electrolytes and small solutes

Question 7

In urine?
concentration of important things?
concentration of less important things?

Answer 7

• low concentration as they are reabsorbed

- high as they are actively secreted

Question 8

where does the filtrate move down?

Answer 8

Filtrate moves down fenestra in podocytes foot processes.

Question 9

how to work out the net glomerular filtration pressure

Answer 9

Puf = Pgc- Pt- πgc

Puf= net ultrafiltration pressure

Pgc = osmotic pressure of plasma proteins in glomerular capillaries (πgc)

Pt = hydrostatic pressure of tubule (Pt)

Question 10

how to work at the glomerular filtration rate?

Answer 10

GFR = Puf x Kf

Puf= net ultrafiltration pressure

Kf is an ultrafiltration coefficient

Question 11

what might kidney diseases result in?

Answer 11

Kidney diseases may reduce number of functioning glomeruli = reduced surface area = decrease Kf = decrease GFR

Question 12

what might Dilation of glomerular arterioles by drugs/hormones result in?

Answer 12

more surface area = increase Kf = increase GFR.

Question 13

what is the GFR?

Answer 13

The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min).

it acts as an index of kidney function

Question 14

how to work at the GFR
from

renal blood flow, renal plasma flow and filtration factor ?

Answer 14

Glomerular Filtration Rate (GFR) = RPF x FF

Renal plasma flow
Filtration fraction

Question 15

what are the mechanisms of auto regulation?

Answer 15

Myogenic Mechanism:

Tuboglomerular Feedback:

Question 16

how does the myogenic mechanism work?

Answer 16

Intrinsic vasoconstriction in response to stretch – keeps the GFR constant when BP rises.

Question 17

how does Tuboglomerular Feedback?

Answer 17

NaCl concentration in distal convoluted tubule sensed by macula densa in juxta-glomerular apparatus.

• so the macula densa releases ATP which signals to vasoconstriction

Question 18

what would a severe haemorrhage do to the GFR?

Answer 18

GFR down – lower hydrostatic pressure.

Question 19

what would an Obstruction in nephron tubule do to the GFR?

Answer 19

GFR down – more tubular pressure.

Question 20

what would Reduced plasma protein concentration do to the GFR?

Answer 20

GFR up – less osmotic pressure.

Question 21

what would Small increase in BP do to the GFR?

Answer 21

GFR unchanged – compensated for.

Question 22

what are the main factors which GFR depends on?

Answer 22

• Glomerular capillary pressure Pgc.
  ▪ Plasma oncotic pressure - πgc
  ▪ Tubularpressure–Pt.
  ▪ Glomerular capillary surface area or permeability–Kf.

Question 23

what is renal clearance?

Answer 23

number of litres of plasma that is completely cleared of substance - x per unit time

(Substances are filtered to different degrees,but the extent to which they are removed from the blood is called clearance.)

Question 24

what is the equation for renal clearance?

Answer 24

𝐶 =( 𝑈 𝑥 𝑉 ) / P 𝑚𝑙/𝑚𝑖𝑛.

U = concentration of substance in urine.

P = concentration of substance in plasma.

V = rate of urine production.

Question 25

give examples of substances freely filtered AND not reabsorbed?

Answer 25

• inulin
  Not found in mammals so is transfused if we use this method.
• creatine
  amount that is released is constant
• so measure creatine to look at low clearance or high plasma

Question 26

what is renal plasma flow measured by?

Answer 26

Measured by PAH

• it is essentially filtered and actively secreted in one pass of the kidney
• so it can be used to measure RPF as it is all cleared

Question 27

what is the amount excreted?

Answer 27

amount excreted = amount filtered - amount reabsorbed + amount secreted

Question 28

what does a fall in GFR show us?

Answer 28

• kidney disease
• if the GFR falls excretory products will build up in the plasma and raised plasma concentration of
  creatine is diagnostic of renal disease.

Question 29

what does pharmacogenetics show?

Answer 29

excretion of many drugs is impaired in renal failure so the drug doses need to be monitored in these patients