Physiology - Glomerular Filtration and Renal Blood Flow

Question 1

What are the 3 filtration barriers of the glomerular membrane?

Answer 1

Glomerular capillary endothelium (barrier to RBCs)

Basement membrane (barrier to plasma proteins)

Slit processes of podocytes (barrier to plasma proteins)

Question 2

Why do large plasma proteins avoid the lumen of the Bowman’s capsule?

Answer 2

Negative basement membrane repels negative plasma proteins

Large size of proteins do not make it through filtration

Question 3

Filtration is a ________ process

Answer 3

Filtration is a passive process

Question 4

What is the net filtration pressure?

Answer 4

The pressure that drives filtration of the blood in the kidneys comprised of:

1. Glomerular pressure
2. Bowman’s capsule pressure
3. Capillary oncotic pressure
4. Bowman’s capsule oncontic pressure

Question 5

Why does the glomerular capillary pressure remain constant along the length of the glomerular capillary?

Answer 5

The efferent arteriole is narrower then the afferent

Question 6

What is GFR?

Answer 6

Rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time

Question 7

What is the major determinant of GFR?

Answer 7

Glomerular capillary pressure

Question 8

How can GFR be regulated?

Answer 8

Extrinsic

• Sympathetic control of baroreceptor reflex

Intrinsic

• Myogenic mechanism
• Tubuloglomerular feedback mechanism

Question 9

Why goes GFR remain relatively constant with increasing MABP?

Answer 9

Avoids drastic changes in fluid volumes and salt concentrations

This occurs due to autoregulation

Question 10

Autoregulation can be split into which two categories?

Answer 10

Myogenic

Tubuloglomerular

Question 11

What is myogenic regulation?

Answer 11

Vascular smooth muscle becomes stretched and then contracts in response to this

Question 12

What is tubuloglomerular feedback?

Answer 12

Involves the juxtaglomerular apparatus.

Macula densa cells sense increased NaCl passing through with increased GFR

In response, afferent arterioles are constricted

Question 13

Which pressure is increased in dehydration such as with diarrhoea?

Answer 13

Capillary oncotic pressure

Question 14

Why is GFR reduced in dehydration?

Answer 14

Water is lost yet plasma proteins remain

Water is reabsorbed into the blood reducing net filtration pressure

Question 15

What is plasma clearance?

Answer 15

The volume of plasma completely cleared of a particular substance per minute

Question 16

How can GFR be determined?

Answer 16

The use of inulin

Inulin is not absorbed or secreted

This means clearance = GFR

Question 17

What else can be used instead of inulin, can be used to estimate GFR?

Answer 17

Creatinine

It is produced naturally so no drip is needed

Question 18

Normally glucose is entirely __________

Answer 18

Normally glucose is entirely reabsorbed

(clearance is 0)

Question 19

Which substance is secreted but not reabsorbed?

Answer 19

H+ ions

Question 20

For substances such as H+ ions which are secreted but not reabsorbed, the clearance of this substance will be _____ ______ the GFR

Answer 20

For substances such as H+ ions which are secreted but not reabsorbed, the clearance of this substance will be greater than the GFR

Question 21

If the clearance of a substance is each of the following values, how is it affected by the kidneys?

a) <gfr></gfr>

<p>b) GFR</p>

<p>c) &gt;GFR</p>

<p>d) 0</p>

</gfr>

Answer 21

a) Partly reasborbed, but not secreted (urea)
b) Neither reabsorbed or secreted (inulin, creatinine)
c) Secreted, but not reabsorbed (H+)
d) Completely reabsorbed, not secreted (glucose)

Question 22

Para-amino hippuric acid (PAH) is an exogenous organic ion used to clinically measure renal plasma flow. Clearance PAH is equal to what?

Answer 22

Renal plasma flow (since all is filtered or secreted from peritubular capillaries)

(normal 650ml/min)

Question 23

Which characteristics does a GFR marker require?

Answer 23

Filtered freely - NOT secreted or reabsorbed

Question 24

Which characteristics does a RPF marker require?

Answer 24

Filtered and completely secreted

Question 25

Why is the RPF larger than the GFR?

Answer 25

Only 20% of the plasma is filtered into the Bowman’s capsule hence GFR comprises only 20% of the total RPF