Renal Blood Flow & Glomerular Filtration Flashcards
(26 cards)
Define glomerular filtration
Formation of an ultrafiltrate of plasma in the glomerulus
What is renal failure
Abrupt fail in glomerular filtration
What can abnormalities in renal circulation lead to
Reduced glomerular filtration i.e. renal failure
Explain the process of glomerular filtration
Fluid is driven passively through the semipermeable walls of the glomerular capillaries into the Bowman’s capsule space by the hydrostatic pressure of the heart
Describe the filtration barrier
Fenestrated endothelium of capillaries
Highly permeable to fluids and small solutes
Impermeable to cells, proteins and drugs bound to proteins
What is primary urine
The ultrafiltrate that does not contain any cells of proteins
Contains electrolytes and small solutes
Describe the pathway of movement of filtrate through the glomerulus
Fluid moves through the fenestra of the capillary
Through the basement membrane
Through the filtration slits and capsular space of the podocyte
What determines the net ultrafiltration pressure
Driving force of hydrostatic pressure due to blood (Pgc)
Opposing forces include hydrostatic pressure (Pt) of the tubule and osmotic pressure of proteins in the capillary (πgc)
What is the net ultrafiltration pressure
Using Puf = Pgc - Pt - πgc
net = 10-20mmHg
How do you calculate glomerular filtration rate
GFR = Puf x Kf
(Net ultrafiltration pressure x Ultrafiltration coefficient)
GFR = renal plasma flow x filtration fraction
Give an example of how changes in Kf can change the glomerular filtration rate
Kidney diseases may reduce the no. of nephrons, glomeruli (or loss of a kidney)
Reduced surface area
Reduced Kf
Reduced rate
Membrane permeability reduced by inflammation, drugs and hormones
What is glomerular filtration rate
The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min).
Sum of filtration rate of all functioning nephrons
Index of kidney function
What are the following:
Renal blood flow (RBF)
Renal plasma flow (RPF)
Filtration fraction (FF)
RBF = 1 L/min (1/5 CO)
RPF = 0.6 L/min
FF = 0.2
FF is the ratio between RPF and the amount filtered by the glomerulus (usually 20%)
What does glomerular filtration rate depend on
Glomerular capillary pressure (Pgc)
Plasma oncotic pressure (πgc)
Tubular pressure (Pt)
Glomerular capillary surface area or permeability (Kf).
How is GFR altered in a healthy person
Neural or hormonal input to the afferent/efferent arteriole that results in changes in net ultrafiltration pressure
Describe the myogenic mechanism of auto regulation
Vascular smooth muscle constricts when stretched, keeping GFR constant as the blood pressure rises
e.g. arterial pressure rises, afferent arteriole stretches, arteriole contracts, vessel resistance increases, blood flow reduces
What are some abnormal causes of changes to glomerular filtration rate
Severe haemorrhage (-ve) Obstruction in the nephron tubule (-ve) Reduced plasma protein concentration (+ve)
What is the usual GFR
120ml/min
What is clearance
The extent to which substances are removed from the blood via filtration (and then excreted by the kidney)
Litres of plasma completely cleared per unit of time
How is clearance calculated
C = (U x V) / P
U = concentration of substance in urine P = concentration of substance in plasma V = rate of urine production
How can clearance be used to estimate GFR (but not practically used too much)
Use of inulin (plant polysaccharide that is not reabsorbed or secreted, not toxic and measurable)
But not found in mammals so would have to be transfused
`How is glomerular filtration rate practically measured
Creatinine clearance
waste product from creatine
fairly constant release
low values of clearance - may be renal failure
high plasma values - may be renal failure
How can renal plasma flow be measured
Measured by Para aminohippurate (PAH) clearance (usually 625ml/min)
Filtered and secreted in one pass of the kidney (while creatinine is only filtered)
What can be indicative of renal failure
Raised plasma concentration of creatinine
