Physiology - Glomerular Filtration and Renal Blood Flow Flashcards
What are the 3 filtration barriers of the glomerular membrane?
Glomerular capillary endothelium (barrier to RBCs)
Basement membrane (barrier to plasma proteins)
Slit processes of podocytes (barrier to plasma proteins)
Why do large plasma proteins avoid the lumen of the Bowman’s capsule?
Negative basement membrane repels negative plasma proteins
Large size of proteins do not make it through filtration
Filtration is a ________ process
Filtration is a passive process
What is the net filtration pressure?
The pressure that drives filtration of the blood in the kidneys comprised of:
- Glomerular pressure
- Bowman’s capsule pressure
- Capillary oncotic pressure
- Bowman’s capsule oncontic pressure
Why does the glomerular capillary pressure remain constant along the length of the glomerular capillary?
The efferent arteriole is narrower then the afferent
What is GFR?
Rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time
What is the major determinant of GFR?
Glomerular capillary pressure
How can GFR be regulated?
Extrinsic
- Sympathetic control of baroreceptor reflex
Intrinsic
- Myogenic mechanism
- Tubuloglomerular feedback mechanism
Why goes GFR remain relatively constant with increasing MABP?
Avoids drastic changes in fluid volumes and salt concentrations
This occurs due to autoregulation
Autoregulation can be split into which two categories?
Myogenic
Tubuloglomerular
What is myogenic regulation?
Vascular smooth muscle becomes stretched and then contracts in response to this
What is tubuloglomerular feedback?
Involves the juxtaglomerular apparatus.
Macula densa cells sense increased NaCl passing through with increased GFR
In response, afferent arterioles are constricted
Which pressure is increased in dehydration such as with diarrhoea?
Capillary oncotic pressure
Why is GFR reduced in dehydration?
Water is lost yet plasma proteins remain
Water is reabsorbed into the blood reducing net filtration pressure
What is plasma clearance?
The volume of plasma completely cleared of a particular substance per minute
How can GFR be determined?
The use of inulin
Inulin is not absorbed or secreted
This means clearance = GFR
What else can be used instead of inulin, can be used to estimate GFR?
Creatinine
It is produced naturally so no drip is needed
Normally glucose is entirely __________
Normally glucose is entirely reabsorbed
(clearance is 0)
Which substance is secreted but not reabsorbed?
H+ ions
For substances such as H+ ions which are secreted but not reabsorbed, the clearance of this substance will be _____ ______ the GFR
For substances such as H+ ions which are secreted but not reabsorbed, the clearance of this substance will be greater than the GFR
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) >GFR</p>
<p>d) 0</p>
</gfr>
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)
Para-amino hippuric acid (PAH) is an exogenous organic ion used to clinically measure renal plasma flow. Clearance PAH is equal to what?
Renal plasma flow (since all is filtered or secreted from peritubular capillaries)
(normal 650ml/min)
Which characteristics does a GFR marker require?
Filtered freely - NOT secreted or reabsorbed
Which characteristics does a RPF marker require?
Filtered and completely secreted
Why is the RPF larger than the GFR?
Only 20% of the plasma is filtered into the Bowman’s capsule hence GFR comprises only 20% of the total RPF
