Glomerular Filtration Rate and Renal Blood Flow Flashcards
How do you calculate the amount of filtrate excreted in the urine?
amount filtered - amount reabsorbed + amount secreted = amount excreted in urine
Compare and contrast the pressures in the afferent, efferent resistances as well as the pressure in the peritubular capillaries.
afferent arterial resistance is less than arteriolar resistance in the systemic arterioles
efferent arteriolar resistance is in series with the afferent arterioles
pressures in the peritubular capillaries is less than a systemic capillary
What is the primary driving force of glumerular filtration?
the initial high hydrostatic pressure in the glomerular capillaries drives filtration
this pressure is balanced out by the increased oncotic pressures as filtrate moves through
glomerular filtration rate in humans
125 mL/min
represents 20% of renal plasma flow (RPF)
filtration fraction (FF)
GFR/RPF
refers to the value of the GFR in relation to renal plasma flow, not renal blood flow
What are the key differences between glomerular filtration and systemic capillary filtration?
glomerular capillary permeability x surface area is 100x greater than most systemic capillaries
glomerular capillary hydrostatic pressures is 2 tiems greater than in systemic capillaries
Starling forces favoring glomerular filtration
glomerular capillary hydrostatic pressure
Bowman’s space oncotic pressure
Starlin forces impeding glomerular filtration
glomerular capillary oncotic pressure
Bowman’s space hydrostatic pressure
What are some of the forces driving net absorptive pressure along the peritubular capillaries?
17 mmHg of pressure in the beginning of the capillaries
12 mmHg pressure at the end
constant pressure to reabsorb filtrate
What is the equation that describes renal blood flow (RBF)
RBF = (Prenal artery - Prenal vein) / Rrenal vasculature
RBF = 125mL blood/min or 4mL blood/min/gm tissue
Describe the vascularization of the renal cortex, outer medulla, and inner medulla
renal cortex = high vascularization (90% RBF)
outer medulla = low vascularization (8% RBF)
inner medulla = very low vascularization (2% RBF)
anatomical distribution of renal capillary beds
ratio of cortical nephrons to juxtamedullary nephrons is 7:1
glomerular and peritubular capillaries are confined to the cortex
only vasa recta capillaries dive deep into the renal medulla
blood-flow distribution to various regions is not fixed, but can vary
How does oncotic pressure in the capillaries change thorughout the filtration process?
increases in glomerular capillaries
decreases in peritubular capillaries
In terms of pressures and resistances, what is RBF proportional to?
mean deltaP/(Raff+Reff) or 1/(Raff+Reff)
In terms of pressures and resistances, what is GFR proportional to?
deltaPGC/RGC or Reff/Raff
How does constricting the afferent arteriole affect GFR?
decreases due to the fall in both capillary pressure and renal plasma flow
How does constriction of the efferent arteriole affect GFR?
GFR initially increases because of rising capillary pressure
after a point, GFR begins to decrease because of falling RPF
What is the normal range for renal perfusion pressure?
80-180 mmHg
What are the two theories for renal autoregulation?
myogenic theory
tubuloglomerular theory
myogenic theory
smooth muscle of the afferent arteriole contracts or relaxes during changes in transmural pressure
this change in resistance keeps blood flow and GFR relatively constant
tubuloglomerular theory
each individual nephron controls its own filtration rate through a sensor located at the macula densa portion of the distal tubule
arterial pressure induced increases in GFR increases solute delivery rate to the macula densa
signal is generated by the macula densa cells that results in afferent arteriolar constriction
conversely, decreases in solute load in the macula densa leads to increases in GFR
action of adenosine in the kidney
vasoconstrictor
possible signal from the macula densa
intrinsic factors affecting blood flow regulation
nitric oxide
endothelin
prostaglandins
adenosine
kinins
dopamine
extrinsic factors affecting blood flow regulation
sympathetic innervation of afferent and efferent arterioles
blood borne substances (angiotensin II, ANP, ADH, ATP, glucocorticoids)
factors decreasing RBF and GFR
decreased central blood volume (e.g. hemorrhage)
increased circulating renin and angiotensin (e.g. dehydration)
RPF
renal plasma flow = (RBF x (1-Hct))
equations involving load
load = mg/min = concentration x flow
kidney input load = Pax x RPFa
kidney output load = (PVx x RPFv) + (Ux x V)
What are the three ways that filtered substances may be handled?
filtered and secreted
filtered and partially reabsorbed
filtered and reabsorbed
clearance
volume of plasma from which the kidney completely removes (clears) a particular substance per unit time
Cx = [Ux x Vx]/Px
important for determining how efficient they kidneys clear a certain substance per unit time
characteristics of an ideal substance to measure clearance
intert
freely filtered
not secreted
not reabsorbed
not synthesized or broken down by the tubules
ex. inulin
What are the parameters for using clearance to determine net renal handling of substances?
if CQ < GFR, substance Q is filtered and reabsorbed
if CR = GFR, substance R is only filtered
if CS > GFR, substance S is filtered and secreted
Why is GFR so important to know?
single best estimate of functioning renal mass
serial measurements allow the tracking of the course of the disease
GFR measurement is useful for knowing the appropriate dosage of drugs to use to attain therapeutic levels
GFR varies directly with renal function
clearance of creatinine
creatinine is an end-product of skeletal-muscle metabolism
GFR = clearance of creatinine (CCR)
decreased functional glomeruli decreases GFR and CCR
decreased CCR can be estimated by an increased PCR
What is the relationship between PCR and blood-urea-nitrogen (BUN)?
PCR x 10 ~ BUN
PCR is the more accurate estimator of GFR than BUN
