L3 Renal Plasma Flow and Regulation of Blood Flow Flashcards
kidneys receive ___% or more of total blood flow from heart (kidneys constitute ___% of body weight)
kidneys receive 25% or more of total blood flow from heart (kidneys constitute 1% of body weight)
pathway of renal arterial blood flow + venous and lympathic flow
+ blood flows to ___ first then ___!
renal artery enters kidney at hilus → interlobar arteries which pass upward to the cortex / medulla junction → turn horizontally to arcuate arteries → interlobular arteries → afferent arterioles (cortex) → glomerular capillaries (ultrafiltration) → efferent arterioles → peritubular capillaries (superficial nephrons) OR vasa recta (juxtamedullary nephrons)
venous and lympathic flow parallel to arterial blood flow (same terminology) - exception: arcuate veins / lymphatic vessels → superficial veins / lymphatic vessels just below the renal capsule (instead of interlobular arteries → don’t go as deep)
blood flows to cortex first then medulla!
blood flow distribution in the kidneys
91% perfuse the cortex - since most of the absorption happens in the cortex
8% perfuse the medulla
1% perfuse the papilla (tip of inner medulla)
kidneys innervation
- cholinergic (~Ach - vasodilation) + adrenergic (sympathetic) nerve fibers lie adjacent to arteries, afferent arterioles, juxtaglomerular apparatus, and efferent arterioles of juxtamedullary nephrons + innervate them
- little direct innervation of epithelium, but catecholamines bind to specific receptors on proximal tubule + modulate transport
___ and ___ ___ account for the drop in resistance between the renal artery and renal vein
afferent and efferent arterioles account for the drop in resistance between the renal artery and renal vein
how is renal blood flow measured?
in labs:
- by collecting blood flowing out of the renal vein
- with a flow meter (electro-magnetic or ultrasonic) placed around the renal artery
clinically:
- using clearance methods : GFR measured and then RBF is calculated after correcting for the hematocrit
how does blood flow vary in the inner and outer cortex in response to hemorrhage / shock and volume expansion / ACh
- hemorrhage / shock → ↓ total flow → flow decreases most in the OUTER CORTEX
- volume expansion / ACh → ↑ total flow → increases more in the INNER CORTEX
how can blood flow (filtration rate) and glomerular pressure be modulated?
- constriction of the afferent arteriole → ↓ blood flow & ↓ glomerular pressure
- constriction of the efferent arteriole → ↓ blood flow but ↑ glomerular pressure
→ resistance at either end of the glomerulus can fine-tune pressure and filtration rate independently of overall blood flow/volume
intrinsic vs extrinsic control of renal blood flow, and involved hormones with their roles
intrinsic (within the kidney):
- autoregulation: keeps renal blood flow stable across systemic BPs of 80–180 mmHg → maintains glomerular filtration rate (GFR)
! BP < 80 or BP > 180 : the autoregulatory mechanism is no longer capable of varying resistance → flow changes in proportion to pressure
extrinsic:
- sympathetic nervous system (α1 receptors): kicks in during trauma, hemorrhage, exercise, pain → vasoconstriction → ↓ renal blood flow
circulating and locally produced hormones: - vasoconstrictors (norepinephrine, angiotensin, thromboxane, ADH (vasopressin)…) + sympathetic
- vasodilators (acetylcholine, bradykinin, prostaglandin E2, prostacyclin)
what does autoregulation of RBF and GFR depend on? (3)
- an intrinsic response of the vessel walls (myogenic component)
- a baroreceptor-activated, intrarenal renin-angiotensin system that generates angiotensin II (→ vasoconstriction)
- a “tubulo-glomerular” feedback mechanism: macula densa cells sense flow through the distal tubule → ↓ filtration rate of the same nephron (vascular pole of the glomerulus is immediately adjacent to the sensor cells)
how do flow-limited substances work? give an example of a marker and how we calculate its clearance, filtration fraction FF, and Renal Blood Flow
flow-limited substance: completely removed from the plasma in one pass through the kidney (via filtration and secretion), its clearance reflects total Renal Plasma Flow (RPF)
PAH
- 90% is cleared from plasma in one kidney pass → clearance ≈ ERPF
- clearance PAH = Effective Renal Plasma Flo = urine flow rate x ([PAH]u / [PAH]p)
↳ more accurate to measure PAH extraction and correct for the fraction that is not secreted by measuring the arteriovenous difference instead of [PAH]p
FF = GFR/RPF ~ 15–20%
RBF = RPF (only plasma) / (1−Hematocrit)
Practice question
During a dog experiment, a clamp around the renal artery is partially tightened so as to reduce renal arterial pressure from a mean 120 mmHg to 80 mmHg. How much do you predict RBF will change?
a) 33% decrease
b) zero
c) 5 to 10% decrease
d) 33% increase
c) 5 to 10% decrease
Autoregulation prevents the RBF from decreasing in direct proportion to mean arterial pressure, but autoregulation is not 100%
Practice question
The renal clearance of (select the one best answer) :
a) a substance is measured in mg/mL
b) a substance is measured in mg/min
c) sodium is decreased by the injection of aldosterone
d) inulin, at a plasma concentration of 60 mg %, is lower than at a concentration of 120 mg %
e) para-aminohippurate (PAH) clearance at a plasma concentration of 60 mg % is higher than at a plasma concentration of 120 mg %
c) sodium is decreased by the injection of aldosterone
Aldosterone increases the reabsorption of Na+ from the nephron and therefore decreases sodium clearance.
A. B. The correct units are mL/min.
D. Inulin clearance is the same at these plasma concentrations. It is filtered but not secreted or reabsorbed.
E. PAH clearance is less at higher plasma PAH concentrations, where secretion approaches saturation. The conc. range of PAH where its clearance is flow limited and therefore a measure of RPF is 0 – 10 mg %. Thus secretion is nearly saturated at 60 mg % and increasing it to 120 mg % has little effect.
Practice question
The following test results were obtained on specimens from a person over a 2 hr period during infusion of inulin and PAH.
Total urine volume = 0.14 L
Inulin conc. in urine = 100 mg/100 mL
Inulin conc. in plasma = 1 mg/100 mL
Hematocrit = 0.40
PAH conc. in urine = 700 mg/mL
PAH conc. in plasma = 2 mg/mL
What are the clearances of inulin and PAH? What is the effective renal plasma flow? What is the effective renal blood flow? How much PAH is secreted (assuming no PAH reabsorption and complete filterability of PAH)?
