Physiology 3 renal filtration Flashcards
What are the three “barriers” to filtration within the glomerulus/Bowman’s capsule?
- Capillary endothelium cells (barrier to RBCs)
- Basement membrane (barrier to proteins)
- Epithelial podocytes (inner surface of Bowman’s capsule)
What are the (four) forces that comprise net filtration pressure?
Two main components- fluid pressures (hydrostatic) and colloidal (oncotic) pressure
- glomerular capillary blood pressure (hydrostatic- main force)
- Bowman’s capsule oncotic pressure
- Bowman’s capsule hydrostatic pressure
- capillary oncotic pressure (increases substantially from afferent to efferent end)
Why is Bowman’s capsul oncotic pressure 0mmHg?
No proteins in the Bowman’s filtrate
What is the net glomerular filtration pressure?
About 10mmHg (much higher than we need)
What are the major determinants of glomerular filtration rate?
Kf- the permeability of the membrane X Net filtration pressure
What is the normal GFR?
about 125ml/min
How is GFR regulated
a) extrinisically
b) autoregulation
a) baroreceptor reflex
b) myogenic mechanism; tubuloglomerular feedback mechanism
How does the baroreceptor reflex decrease GFR?
Fall in blood pressure leads to sympathetic activation ;Generalised arteriolar vasoconstriction causes decreased glomerular blood flow and hence reduced GFR
How are RBF and GFR protected from changes in MABP over a wide range of MABP?
By autoregulation:
a) myogenic- increased pressure leads to constriction of arteriole and reduces pressure downstream (in the glomerulus) [AND VICE VERSA]
b) tubuloglomerular feedback- increased GFR leads to increased NaCl in the tubule, causing constriction of afferent arterioles [AND VICE VERSA]
What is the role of the macula densa in the JGA?
Monitor either ?salt or ?fluid flow in the tubular fluid- lead to signalling mechanisms which alter the tone of the afferent arterioles
What might be the result of a kidney stone on GFR?
Increased Bowmans capsule backpressure, reducing GFR
Why does the capillary oncotic pressure decrease in burns victims and how does this affect GFR?
Loss of proteins- causes an increase in GFR
What determines the plasma clearance of a substance X?
rate of excretion divided by plasma concentration in mls/min
In what circumstances does clearance of a substance
a) clearance= GFR
b) clearance = 0
c) clearance < GFR
d) clearance > GFR
a) freely filtered, not reabsorbed, not secreted (e.g. inulin- can be used to calculate GFR.)
b) filtered, all reabsorbed (e.g. glucose) or NOT FILTERED
c) filtered, some reabsorbed
d) filtered and ALSO secreted
What endogenous substance can also be used to calculate GFR?
Creatinine- freely filtered, not reabsorbed and not secreted
