K3 - Filtration and Renal Blood Flow Flashcards
what bakes up the glomerular membrane?
- glomerular capillary endothelium
- basement membrane
- slit processes of the podocytes
why will plasma proteins not cross the membrane?
basement membrane is acellular and is made up of collagen and glycoproteins and so is negatively charged which repels proteins
what forces favour filtration?
- glomerular capillary blood pressure (55mmHg)
- bowman’s capsule oncotic pressure (0mmHg)
what forces oppose filtration?
- bowman’s capsule hydrostatic pressure (15mmHg)
- capillary oncotic pressure (30mmHg)
what is the Glomerular filtration rate (GFR)?
rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time.
what is the equation for GFR?
GFR = Kf x net filtration pressure
Kf - filtration coefficient
what is the ‘normal’ value of GFR?
125ml/min
what is the major derterminat of GFR?
Glomerular capillary fluid (blood) pressure (BPGC
what is the extrinsic regulation of GFR?
sympathetic control via baroreceptor reflex
what is the intrinsic control of GFR?
(a) Myogenic mechanism
(b) Tubuloglomerular feedback mechanism
What effect will an increase in the arterial blood pressure have of GFR?
increased glomerular capillary blood pressure
increased net filtration pressure
increases GFR
what effect will a decrease in arterial blood pressure have on GFR?
decreased glomerular capillary blood pressure
decreased net filtration pressure
decreased GFR
what effect will vasoconstriction of afferent arteriole have on GFR?
decrease
what effect will vasodilation of arteriole have on GFR?
increase
how does the sympathetic nervous system control alterations in arterial blood pressure?
fall in blood volume
decrease arterial blood pressure
detected by aortic and carotid sinus baroreceptors
increases sympathetic supply
generalised arteriole vasoconstriction
constriction of afferent arterioles
decreases glomerular capillary blood pressure
decreases GFR
decreases urine volume
what is the importance of auto-regulation?
prevents short term changes in systemic arterial pressure affecting GFR
what is the myogenic response of the kidneys?
if vascular smooth muscle is stretched (i.e. arterial pressure is increased), it contracts thus constricting the arteriole
what is the Tubuloglomerular feedback
response of the kidneys?
Involves the juxtaglomerular apparatus where if GFR rises, more NaCl flows through the tubule leading to constriction of afferent arterioles
what cells sense NaCl in the Juxtaglomerular apparatus?
macula densa
describe the negative feedback regulation of salt concentration in the kidneys?
- increase in GFR > increase in tubular fluid
- tubular fluid will have more salt
- macula densa cells will detect this increase and release vasocative chemicals to act on smooth muscle
- causes contraction > narrows diameter of smooth muscles > decreases GFR
what effect will an kidney stone have pn GFR?
increase bowman’s capsule hydrostatic pressure causing GFR to decrease
what effect will an increase in the capillary oncotic pressure have on GFR ?
decrease
what effect will a decrease in Kf have on GFR?
decrease
what is plasma clearance?
the volume of plasma completely cleared of a particular substance per minute
what is the equation of plasma clearance?
Clearance of substance X = [X]urine x Vurine / [X]plasma
what is the units of clearance?
ml/min
why does inulin clearance = GFR?
freely filtered at glomerulus
- neither absorbed nor secreted
- not metabolised by kidney
- not toxic
- easily measured in urine and blood
when does clearance = 0 (glucose)?
For substances which are filtered, completely reabsorbed and not secreted
when is clearance < GFR (urea)?
For substances which are filtered, partly reabsorbed and not secreted
when is clearance > GFR (H+) ?
For substances which are filtered, secreted but not reabsorbed
how is renal plasma flow calculated using para-amino hippuric acid?
PAH is
1) freely filtered at glomerulus,
2) secreted into the tubule (not reabsorbed)
3) completely cleared from the plasma
- all the PAH in the plasma that escapes filtration is secreted from the peritubular capillaries
what is filtration fraction ?
the fraction of plasma flowing through the glomeruli that is filtered into the tubules
what is the equation of filtration fraction?
filtration fraction = GFR/Renal Plasma Flow = 125ml.min/650ml.min
= 0.19 = 20%
- 20% of the plasma that enters the glomeruli is filtered
what properties should a clearance marker have?
(1) Non-toxic
(2) Inert (i.e. not metabolised)
(3) Easy to measure
what property a GFR marker have?
should be filtered freely not secreted or reabsorbed
what property should a RPF marker have?
should be filtered and completely secreted
