Physiology 3 Flashcards
What are the filtration barriers in the kidney?
(1) Glomerular Capillary Endothelium (barrier to RBC)
(2) Basement Membrane (basal lamina) (plasma protein barrier)
(3) Slit processes of podocytes (plasma protein barrier) (Glomerular epithelium)
Features of the glomerular capillary
pores/ gaps are 100x times bigger in the glomerular capillaries than in other capillaries
-> leaky barrier
What is the glomerular basement membrane made of?
collagen and glycoprotein
What does the basement membrane do to the negative plasma proteins?
repels negatively charged plasma proteins as the plasma membrane is negative
negative plasma proteins should be retained within the capillaries and shouldn’t pass through the capillary wall through to the tubular flow
What should happen to the RBC in the capillaries?
Shouldn’t move across the glomerular membrane as they are too big to cross the membrane
What is the importance of the podocytes?
Makes the inner layer of the bowmans capsule
Is the glomerular filtration process active or passive?
Passive - doesn’t need energy or transport proteins
What forces control the process of glomerular filtration?
The net filtration pressure as a result of the 4 forces:
2 hydrostatic pressures
2 oncotic pressures
What are the 2 hydrostatic forces?
Pressure of the blood as it passes through the glomerular capillary - 55mm/ Hg
What is the key determinant force in setting up the net filtration pressure?
glonmerular capillary blood pressure
Blood pressure remains relatively same throughout the glomerular cappilary. Why?
as the afferent arteriole is bigger in diameter than the efferent vessel, hence the build up of backstatic of the blood
What is the bowman’s capsule hydrostatic fluid and its
Bownmans capsule hydriostatic opposes the glomerular capillary blood pressure
What does oncotic/ colloid refer to?
Plasma proteins
Where is the plasma protein present in the glomerulus?
in the capillary and not in the capsule
What is the imp of cap oncotic pressure?
against the net filtration pressure
fluid back from the bowmans capsule into the capillary
What is the bowmans capsule oncotic pressure?
o mm / Hg
What is the net filtration pressure?
(Glomerular Capsillary BP + Bowmans capsule oncotic pressure ) - (Bowmans capsule hydrostatic fluid - capillary oncotic pressure)
What are the straling forces?
to describe the balance btw the hydrostatic and oncotic fluid
How is the tubular fluid similar to the blood plasma?
Tubular fluid in the bowmans capsule almost similar to plasma in the fluid except for the large plasma proteins
What does the GFR dependent on?
Net filtration pressure
What is the filtration coefficient in GFR?
How permeable or holey the glomerular membrane is
How does the GFR change on chnaging net filtration pressure?
increasinh the net filtration pressure increases GFR
How does the gfr affect urine output?
decrease gfr -> filtering less -> less urine
What is the key determinant factor for the net filtration pressure?
Blood pressure
How is the gfr regulated?
via two methods:
extrinsic regulation
intrinsic regulation
What is the Extrinsic regulation of GFR
?
(a) Sympathetic control via baroreceptor reflex
What is the Autoregulation of GFR (Intrinsic)?
Myogenic mechanism (b) Tubuloglomerular feedback mechanism
What keep the GFR relatively constant?
the extrinsic and intrinsic factors: Myogenic mechanism
(b) Tubuloglomerular feedback mechanism
Sympathetic control via baroreceptor reflex
What happens if the blood pressure decreases?
net filtration decreses -> filtration by the kidney decreases
How to decrease the GFR by changing the blood flow?
vasoconstriction of the afferent arteriole -> less blood flowing downstream -> decreases the pressure -> decreases net filtration poressure ->decreases GFR
Extrinsic control in the GFR
sympathetic -> vasoconstriction of the arteriole -> decrease in urine production -> deal with loss of ECF volume
very important if someone has lost ECF volume
Intrinsic control of the GFR
on progressively increasing MABP -> not much change in the GFR
What is the myogenic mechanism?
in response to the stretch due to increased blood supply to the area, due to the stretch it contracts the arteriole and keeps the blood flow less
What is the import of tubuloglomerular feedback?
If there is increased GFR –> increased plasma filtrate -> increased salt in the tubular fluid -> this picked up by the macula densa cells in the JGA -> they release vasoactive chemicals which affect the afferent arteriole –> contraction of the afferent arteriole -> decrease the blood flow into the glomerular capillary -0> decrease net filtration pressure -> decreases GFR
What happens if there is increased HPBC (e.g. kidney stone) ?
build up of fluid -> increased bowmans capsule hydrostatic pressure -> opposes net filtration rate -> decreases net filtration rate -> decreased GFR
What happens if there is an increased capillary oncotic pressure?
opposes net filtration rate -> decreases net filtration rate -> decreased GFR
doesnt lose plasma proteins in the capillary, they have a greater osmotic effect -> increases from the normal value of 30
What happens if the capillary oncotic pressure decreases? (burns)
lose plasma proteins from the burn
lose the pressure -> increases net filtration pressure
What happens if you decrease Kf (change in surface area available for filtration)?
decreases the GFR
What is the plasma clearance?
A measure of how effectively the kidneys can ‘clean’ the blood of a substance
the volume of plasma completely cleared of a particular substance per minute
Is the plasma clearance value of each substance same or not?
Specifc not same
What is the equation for plasma clearance?
“Clearance of substance X “= “Rate of excretion of X” /”Plasma concentration of X”
“Clearance of substance X “= ([X]_“urine” ×” “ “V” “urine” )/([X]“plasma” )
What is the unit of clearance?
ml/ min - volume of plasma cleared of that substance from both kidneys in 1 min
how is inulin clearance inportance in GFR?
inulin clearance = GFR
Why is the inulin clearance = GFR?
rate of excretion = rate of filtration
What other substance can be used to find out the GFR instead of inulin?
creatinine clearance
What is the draw back of creatinine clearance?
small amount of creatinine tubular secretion
What is the clearance of glucose ?
0, hence not excreted in the urine
glucose - most of it is reabsorbed in the proximal tubule
What is the clearance of urea?
50% reabsorbed
50% excreted via urine
clearance < GFR
getting reabsorbed
What is the clearance of the H= ions?
H ions get the the glomerular membrane, and are joined by the H+ which are secreted from the peritubular capillaries
hence the clearance >GFR -> are getting secreted
Which value should be taken as the GFR?
Inulin clearance or creatinine clearance
What is the rate of clearance?
rate of excretion/ plasma concentration
If clearance < GFR
substance being reabsorbed bu the nephron
If the clearance = GFR
substance is neither reabsorbed nor secreted
If clearance > GFR
then substance is SECRETED into tubule
What is the importance of calculating the para-amino hippuric acid (PAH)
clnically as a measure of renal plasma flow
no reabsorption, secretion of the PAh from the peritubular plasma into the tubular fluid (blood leaving the kidney doesnt contain any PAH)
so all the pAH that wasnt filtered early on get removed in the loop
What is the renal plasma flow?
650 ml/ min
How can the renal plasma flow(RPF) be measured?
with the help of clearance of the PAH
What are the idea markers for renal plasma flow?
A RPF marker should be filtered and completely secreted (PAH)
What is the ideal marker for a GFR marker?
NOT secreted or reabsorbed
inulin
What is the filtration fraction?
fraction of plasma flowing through the glomeruli that is subsequently filtrered into the tubules
Filtration fraction = GFR/ RPF
=0.19 = 20%
plasma volume
2-3 l
How much fluid do we filter everyday?
we filter our full plasma fluid volume 60x a day –> as the GFR is 125 ml/ min
What is the renal blood flow rate?
1.2 ml/ min
RPF x
(where Hct = Haematocrit) = 650 x 1.85 = ~1200 ml/min*
What is the renal plasma fluid?
how much plasma is flowing collectively into the kidneys every minture
