Lecture 4: RBF + GFR Flashcards
Explain the anatomical or physiological reasons for the following unique hemodynamics and metabolic characteristics of the kidneys:
(A) blood flow per 100g tissue is highest of all organs
(B) O2 consumption/ 100 g tissue is highest of all organs except the heart
(C) (CaO2 - CvO2) different is lowest of all major organs
(A) Higher blood flow because the kidneys have lower vascular resistance
(B) Higher O2 consumption because with more RBF, there is more reabsorption that needs to be done
(C) lowest difference because of preglomerular shunting - so less O2 delivered to tissue itself
What percentage of CO is RBF?
20%
What does the flow through the afferent arterioles equal?
20% of CO
What does flow through the efferent arterioles equal to?
80% of the RPF
How do organs relate in terms of delta P and resistance ?
All organs perfumed by systemic circulation have about the same delta P
Organs have DIFFERENT flows because of differences in vascular resistance
Why is vascular resistance in the kidneys low?
Kidney has multiple resistances in parallel
All capillaries are open
What does low (CaO2 - CvO2) in the kidney mean?
Indicates that VdotO2/RBF ratio is low
Increased blood flow means more demand
Does not indicate that consumption is low!
Differentiate between effects of increased blood flow in kidneys and heart
Heart = inc. CBF protects against ischemia
Kidneys = inc. RBF means inc. risk of ischemia because of more work it has to do
List the approximate PO2 in the renal artery, renal cortex, renal medulla, and renal vein
Explain how preglomerular shunting and countercurrent O2 exchange affect PO2 in the renal cortex and medulla
Cortex = 45 mmHg
Medulla = 30 mmHg
Renal Vein = 70 mmHg
Less O2 delivered into tissue itself - why O2 so low in medulla
What contributes to low PO2 in the medulla?
Preglomerular shunting
Low blood flow to medulla
What does regulation of RBF involves changes in?
RVR
Changes in arterioles….but kidney has 2 arterioles!!! Afferent and Efferent arterioles
What is the distribution of vascular resistance in the kidneys?
50% in afferent arterioles
35% in efferent arterioles
15% in peritubular capillaries
Apply the Starling equation to GFR and compare this equation to the Starling equation for filtration in a typical capillary, specifying the normal pressure values in each equation
typical capillary:
Jv = Kf [(Pc - Pi) - sigma(Pi(p) - Pi(i)]
Glomerular capillary (GFR): GFR = Kf [ P(GC) -P(BS) - Pi(GC) ]
Equation for GFR doesn’t account for proteins filtering into bowmann’s space because the glomerular capillaries are reflective!
Hydraulic pressure in typical: 35 to 17 (arterial to venous end). Oncotic pressure = constant
Hydraulic pressure in GFR = constant. Oncotic pressure: 25 to 35 (afferent to efferent end)
Compare the filtration fraction in the glomerular capillaries to the filtration fraction in typical capillaries, explaining why the former is much higher
Filtration FRACTION is higher in glomerular capillaries
Higher filtration in kidneys because glomerular capillaries have higher filtration coefficient, Kf, from capillaries being highly fenestrated
Define the features of albumin and the filtration barrier that result in a normal filtration fraction for albumin of less than 0.1%, explaining why a low albumin filtration fraction is important
Albumin has a hard time filtering because it is larger than 5000 Da and it is negatively charged
It’s important that albumin has a low filtration fraction, otherwise so much of it would be filtered out and lost - and the liver wouldn’t be able to keep up because it makes only a little bit of albumin daily
Describe the size and charge selectivities of the glomerular capillaries
Easiest to filter: positive charged
Hardest to filter: negative charged
molecules <5000 Da = “freely filtered”
Where is the location of SIZE selectivity in the GFR filtration barrier?
Slits between foot process of podocytes
Layer 3 of filtration barrier
What is the location of the CHARGE selectivity in the filtration barrier?
Surface of capillary endothelium
Basement membrane
Surface of podocytes (epithelial cells of Bowman’s capsule)
= all 3 layers of filtration barrier
What does the surface of the filtration barrier contain to help with charge selectivity?
Glycoproteins to repel large anions
What are podocytes?
Epithelial cells of Bowman’s capsule
Define autoregulation and describe two mechanisms for autoregulation of RBF and GFR
Autoregulation: changes in BP lead to changes in RVR to maintain a relatively constant RBF
Mechanisms:
(1) Myogenic response: increased pressure in lumen stretches the vessel wall and activates cation channels to depolarize and activate LTCC to trigger SM contraction and thereby increase RVR
(2) Tubuloglomerular feedback (TGF): macula densa monitor solutes and will send signals to afferent arterioles to regular RBF and GFR
How can kidneys regulate RBF and GFR?
Via changes to RVR
Given that both RBF and GFR are autoregulated, identify the major site of the changes in RVR
Afferent arteriole
What does TGF do in response to increased NaCl delivery?
1) macula densa will constrict afferent arterioles to increase their resistance, thereby increasing RVR (to decrease RBF) and decreasing P(GC) (to decrease GFR)
2) main mediator triggered by increased absorption of NaCl is Adenosine, which acts as vasoconstrictor in kidneys
