Physiology 3 Flashcards
what is glomerular filtration
the process where 20% of the plasma is filtered through the glomerulus into the tubular fluid
what are the 3 filtration barriers at the glomerulus
glomerular capillary endothelium
basement membrane
podocytes (glomerular epithelium)
how does the glomerular capillary endothelium filter the blood
it its as a barrier to RBCs
gaps/pores in the wall are much bigger than the pores found in other capillaries so allow smaller substances to leak through
what is the basement membrane made out of
collagen and glycoproteins
it is an acellular layer
what are filtration slits
the gaps formed by the extensions of the podocyte slit processes
what happens to substances small enough to fit through the 3 physical barriers in the glomerulus
they bass into the lumen of the renal tubule
what charge is the lumen of the bowman’s capsule (renal tubule)
negative - therefore repels positive plasma proteins making sure they stay in the capillary
what drives fluid filtration into the bowman’s capsule
4 forces that act across the glomerular membrane
- glomerular capillary BP
- Capillary oncotic pressure
- Bowman’s capsule hydrostatic pressure
- Bowman’s capsule oncotic pressure
how does glomerular capillary BP affect filtration
biggest force
increased pressure in the capillary increases filtration
how is glomerular capillary BP maintained
by the afferent capillary being wider than the efferent capillary
this means blood constants sits in the capillary keeping a constant pressure
how does capillary oncotic pressure affect filtration
there are lots of plasma proteins in the capillary but not the bowman’s capsule lumen
this then drags fluid back into the capillary from the bowman’s capsule - against net filtration
how does the bowman’s capsule hydrostatic pressure affect filtration
pressure in the bowman’s capsule push’s things out
increased pressure in the bowman’s capsule = decrease filtration
how does bowman’s capsule oncotic pressure have an effects on filtration
usually doesn’t because there are no plasma proteins in the initial tubular fluid
(affects further down in the tubule)
how do you work out net filtration
take away the forces favouring filtration from those opposing it
what forces favour filtration
capillary BP
bowman’s capsule oncotic pressure
what forces are against filtration
bowman’s capsule hydrostatic fluid pressure
capillary oncotic pressure
what is GFR
the rate at which protein free plasma is filtered from glomeruli into the bowman’s capsule
how do you calculate GFR
filtration coefficient x net filtration pressure
what is the biggest determinant factor of GFR
glomerular capillary fluid (blood) pressure
what extrinsic regulation is used to control renal blood flow
sympathetic control via baroreceptor reflex
what auto regulation of renal blood flow and GFR is there
myogenic mechanism
tubuloglomerular feedback mechanism
how can you change the BP in the glomerular capillary
by changing the diameter of the afferent arteriole
how does the baroreceptor reflex extrinsically control GFR/ glomerular capillary bp
fall in BP leads to decreased arterial bp -
detected by aortic and carotid sinus baroreceptors -
increases sympathetic activity
causes generalised arteriolar vasoconstriction -
construction of afferent arterioles -
decreased BP in glomerular capillary -
decreased GFR -
decreased urine volume
(this helps compensate for fall in blood volume)
why is there the need for intrinsic control as well as extrinsic control of GFR
so that GFR doesn’t decrease every time there is a transient fall/ rise in blood pressure
still need to produce enough urine to get rid of metabolic waste products
how does myogenic intrinsic regulation of GFR work
when vascular smooth muscle is stretched (eg. with a BP increase) stretch receptors cause it to contract in response
this keeps GFR constant when BP rises suddenly and transiently
how does tubuloglomerular feedback work in the intrinsic regulation fo GFR
iIf GFR rises there is an increase of NaCl going through the tubule
this is sensed by MACULA DENSA cells in the juxtaglomerular apparatus causing chemicals to be released which constricts the afferent arterioles
this negative feedback brings back GFR to norm if there is too much NaCl loss
what are some other determinants of GFR
- normal net filtration pressure
- increase in bowman’s capsule pressure (increases with blockage in urinary system)
- increase in capillary oncotic pressure (dehydration)
- decrease in capillary oncotic pressure (burns)
- change in surface area available for filtration (how permeable the membrane is)
what is plasma clearance
the volume of plasma completely cleared of a substance per minute
(every substance has a specific plasma clearance value)
how do you calculate the plasma clearance of a substance
concentration of substance in urine x volume of urine / concentration of substance in plasma
what substance has a plasma clearance than is = to GFR
Inulin
why is inulin not used routinely to check GFR
because it needs to be infused into the patient until there is a constant plasma concentration therefore is not practical
what is creatinine
substance produced by muscles at a constant rate
freely filtered with no tubular reabsorption and little tubular secretion so a good marker of GFR
what percentage of glucose filtered through the glomerulus is reabsorbed in the tubules
100% therefore plasma clearance = 0 as excretion = 0
should be no glucose in the urine
what % of urea is absorbed and what % is excreted
50% absorbed
50% excreted
why is hydrogen plasma clearance > GFR
because hydrogen ions are freely filtered and then more are secreted into the tubules from the capillaries
H+ ions are not reabsorbed back into blood
what 2 substances have a plasma clearance that can be taken as GFR
Inulin
creatinin
what is normal GFR
125 ml/min
what is renal plasma flow
collectively how much plasma is flowing through both kidneys at a time
how do you measure renal plasma flow
para-amino hippuric acid
filtered freely in glomerulus, secreted and not absorbed, completely cleared from plasma
what 2 things do renal plasma flow markers need to do
get freely filtered and completely secreted
what is the filtration fraction
percentage of plasma that enters the glomeruli which is then filtered
how do you calculate the filtration fraction
GFR/Renal plasma flow
what is a normal renal plasma flow
650 ml/min
what is renal plasma flow
collectively how much plasma is flowing through both kidneys at a time
how do you measure renal plasma flow
para-amino hippuric acid
filtered freely in glomerulus, secreted and not absorbed, completely cleared from plasma
what 2 things do renal plasma flow markers need to do
get freely filtered and completely secreted
what is the filtration fraction
percentage of plasma that enters the glomeruli which is then filtered
how do you calculate the filtration fraction
GFR/Renal plasma flow
what is a normal renal plasma flow
650 ml/min
