US Lecture 3 - Renal Blood Flow and Glomerular filtration Flashcards
What is glomerular filtration?
Formation of an ultrafiltrate of plasma in the glomerulus
What is an abrupt fall in glomerular filtration called?
Renal failure
What can lead to reduced glomerular filtration?
Abnormalities in renal circulation
What does the filtration system look like? FITB
What type of process is glomerular filtration?
Passive process with fluid ‘driven’ through semipermeable wall of glomerular capillaries into Bowman’s Capsule space by hydrostatic pressure of the heart
What is the fenestrated endothelium of capillaries and Bowman’s capsule permeable to?
Fluids, small solutes - freely filtered
What is the fenestrated endothelium of capillaries and Bowman’s capsule impermeable to?
Cells, proteins, drugs carried bound to protein
What is the ultrafiltrate and what does it look like?
A clear fluid, completely free from blood and proteins, containing electrolytes and small solutes - otherwise known as primary urine
What is the basic renal process? FITB
What is the equation for amount excreted?
=Amount filtered + amount secreted - amount absorbed
What is the renal input in the basic renal process?
Renal artery
What is the renal output in the basic renal process?
Renal vein and ureter
What is the driving force of glomerular filtration pressures?
Hydrostatic pressure in glomerular capillaries = Pgc
What are the opposing pressures in the glomerular filtration?
Hydrostatic pressure of tubule (=Pt), osmotic pressure of plasma proteins in glomerular capillaries (= pi(3.14)gc)
How is ultrafiltration worked out?
Puf= Pgc - Pt - [pi]gc Around 10-20mmHg
What is the glomerular filtration rate?
GFR= Puf * Kf
What is Kf and what is it affected by?
Ultrafiltration coefficient - membrane permeability and SA available for filtration
What effect do kidney diseases have on functioning glomeruli?
Decreases, so reduces SA and thus reduces Kf
What occurs to Kf when drugs and hormones dilate glomerular arterioles?
Increase
What is the glomerular filtration rate?
The amount of fluid filtered from the glomeruli into Bowman’s capsule per unit of time (ml/min) Sum of filtration rate from all functioning nephrons
What occurs when nephrons are lost?
SA is lost, Kf decreases so overall GFR decreases
What does the renal blood flow do?
Delivers O2, nutrients and substances for excretion
What is the approx. value of RBF, RPF and FF?
FF= 0.2 RBF = 1/5 of cardiac output - 1L/min RPF= 0.6L/min
How is FF calculated?
Ratio between RPF and amount of filtrate filtered by glomerulus
What does GFR=?
GFR= RPF*FF and is approx X/X* (volume of filtrate formed in 1 min)
What influences GFR?
Glomerular capillary pressure Plasma oncotic pressure Tubular pressure Glomerular capillary SA/permeability
What is GFR?
Not a fixed value but is subject to physiological regulation - achieved by neural/hormonal input to the afferent/efferent arteriole, resulting in changes in Puf
What is the function of autoregulation of the GFR?
Ensures fluid and solute excretion remain reasonably constant (otherwise varying pressure will vary urine production and cause loss of important ions).
What is the difference between decreased GFR and incresaed GFR?
What are the mechanisms of autoregulation?
Myogenic mechanism Tubuloglomerular feedback
What is the myogenic mechanism of autoregulation?
Vascular smooth muscle constricts when stretched, Keeping GFR constant when blood pressure rises. Arterial pressure rises → afferent arteriole stretches → arteriole contracts → (vessel resistance increases)→ blood flow reduces and GFR remains constant:
What is the tubuloglomerular feedback?
NaCl conc in fluid sensd by macula densa in JGA Macula densa signals afferent arteriole and changes its resistance so GFR decreased
What are some event affecting GFR and what effect do they have on GFR?
Severe haemorrhage - decreases (hydrostatic pressure decreases) Obstruction in nephron tubule - decreases (increasing opposing pressure) Reduced plasma protein conc - increases (reduced opposing force) Small increase in BP - no change (myogenic mechanism)
What is the normal maintained GFR?
120ml/min
What is renal clearance?
The extent to which the substances in the blood are filtered from the blood by the kidney Number of litres of plasma that are completely cleared of the substance per unit time
What is the equation for clearance?
C= [U*V]/P ml/min U=conc of substance in urine P=conc of substance in plasma V=rate of urine production
When can we estimate GFR using clearance?
When the molecule is neither reabsorbed nor secreted by the kidneys and is freely filtered - so amount filtered = amount excreted from kidneys, e.g. Inulin
What is inulin?
Plant polysaccharide, freely filtered and neither reabsorbed nor secreted, not toxic and measurable in urine/plasma
What is the normal way of measuring an estimate of GFR in humans?
Using Creatinine clearance - freely filtered, not secreted/reabsorbed, as it is a waste product from creatine in muscle met, amount of creatinine is fairly constant, so renal failure can lead to high plasma [creatinine] and low creatinine clearance
What are the values for renal clearance of Na, K, Ca, Mg, Phosphate, glucose, urea, inulin, PAH? *not necessary to know exact numbers*
Na 1.0ml/min K 11.0ml/min Ca 1.8ml/min Mg 3.6ml/min Phosphate 6.4ml/min Glucose 0.05ml/min Urea 17.0ml/min Inulin 120.0ml/min PAH 625.0ml/min
What is renal plasma flow?
Measured by PAH clearance - filtered and actively secreted in one pass of the kidney so can be used to measure RPF
What is the difference between clearance, reabsorption and secretion?
What does the amount of substance appearing in the urine reflect?
Combined effects of filtration, reabsorption from nephron tubule to blood and secretion from blood into tubular fluid
What is the cardinal feature of renal disease?
A fall in GFR - as excretory products build up in plasma, so raised creatinine is diagnostic of renal disease
What is important to consider when giving drugs to a patient with renal failure?
Excretion of substances will be impaired inc. drugs
What are the integrated functions of the kidney?
Excretion of metabolic products e.g. urea, uric acid, creatinine Excretion of foreign substances (drugs) -PHARMACOKINETICS Homeostasis of body fluids, electrolytes, acid-base balance Regulate blood pressure - CARDIOVASCULAR Secrete hormones (renin, erythropoietin) -ENDOCRINE
What is kidney failure?
A fall in glomerular filtration
What happens to the concentration of the fluid when it passes from the blood to the Bowman’s capsule?
It doesn’t change Only further down, in convoluted tubules/LoH does it change: Reabsorption - decreases concentration Secretion - increases concentration
What does a higher, lower or equal value to GFR of clearance mean?
Equal to GFR= the substance is freely filtered, neither reabsorbed/secreted Higher than GFR= substance is secreted Lower than GFR= substance is reabsorbed
