Value, Measurement, Autoregulation, Factors & Conditions affecting--- RENAL BLOOD FLOW Value, Measurement, Autoregulation, Factors & Conditions affecting---

Blood Flow to Kidneys under resting conditions Volume of blood supplied to the two kidneys per minute: 1100ml – 1300ml /min Avg. 22% of Cardiac Output (20-30%)

Renal Blood Flow

The arrangement of blood vessels & capillary net Provide: High hydrostatic press (approx 60mmHg) in glom capill, suitable for Glom filteration & Low press in peritub capill (13mmHg) for absorption--- Aff & Eff arteriolar resistance gets altered to regulate Glom & tubul blood flow -GFR Blood flow to the kidney like other organs is regulated by adjusting vascular resistance in response to change in arterial pressure. The afferent arteriole, efferent arteriole & interlobular artery are the major resitance vessels.

Measurement of Renal Blood Flow Electromagnetic & other Flow meters Doppler Flowmeters / Sonography CT Scanner & Radiographic contrast material,---- Gamma activity tracer-- By Application of Fick’s Principle / PAH Clearance Fick’s Principle‘-The amount of a substance taken up from the circulation per unit of time equals the arterial level of the substance minus the venous level multiplied by the blood flow’.

Applying the Fick’s principle to the kidney Blood Flow = the amt of a sub. taken up per unit of time divided by Arterial –Venous diff. for the sub. Since the kidney filters plasma, The RPF = the amt of a sub. excreted per unit of time divided by the renal A – V diff of the sub. The Sub- measurable, not metabolized / stored, or produced by the kid. & does not affect RBF

Renal Blood flow Measurement (Cont.) RPF = Ux . V (Ax – Vx) Renal Blood Flow = RPF X 1 / 1-Hct (Ux =X conc in urine./ml--, V = urine vol/min Ax = conc in renal artery.-- Vx = renal vein conc. of the sub X)

Renal plasma flow can be measured by infusing p-aminohippuric acid (PAH) & determining its urine and plasma Conc. (PAH Characteristics & what makes it suitable for renal blood flow measurement -?- )

PAH Clearance

Actual Renal Plasma Flow Extraction Ratio- % of Subs removed from blood (Arterial conc. minus venous con divided by arterial conc.x 100)

Regulation of RBF The main determinants of RBF Effects of sympathetic stimulation & Hormones on renal vascular resistance & RBF

Determinants of Renal Blood Flow Q = ΔP / R Varies Directly with (Δ P ) (Renal Artery pressure- Renal vein Pressure) Inversely with: ( R ) (Total Renal Vascular Resistance)

Renal Vascular Resistance Main Sites: 1. Interlobular arteries / arterioles 2. Afferent arterioles 3. Efferent arterioles

↑in the resistance in vascular segments → ↓ RBF A ↓ in the resistance in vascular segments will → ↑ RBF

Autoregulation of Renal Blood Flow & GFR Maintenance of Renal Blood Flow & Glomerular Filtration Rate despite Changes in Arterial Blood pressure

Autoregulation in the Kidney

Autoregulation ( Cont. ) Mechanisms / How Tubuloglomerular Feedback Mechanism 1. At afferent arteriolar level 2. At efferent arteriolar level Myogenic Theory of Autoregulation NO may also be involved

JGA

Autoregulation of Glomerular Pressure & GFR

Myogenic Theory In general most Vessels respond to stretch by contraction of the smooth muscle i.e direct contractile response to stretch of the arteriole Overdistension (↑ of RBF & GFR )→ contraction of smooth muscle of the efferent arteriole → ↓of RBF & GFR

Relationship between Arteriolar Resistance & RBF & GFR

Neuro-humoral & other Factors affecting RBF Neural: Sympathetic Nerves Hormonal Factors: Norepinephrine Angiotensin II Endothelin Prostaglandins Nitric Oxide ( NO ) Bradykinin, Dopamine

NEURAL CONTROL OF RENAL BLOOD FLOW Sympathetic Stim- In healthy individuals at rest has no marked effect. Strong stimulation causes a marked decrease in renal blood flow. Mediated by α-1 adrenergic receptors Sympathetic Nerve Fibers innervate primarily to afferent & Efferent arterioles & Some to interlobular arteries.

Hormonal & autocoid control of RBF ( Cont.) NE, E. Only under extreme conditions e.g. severe haemorrhage → decrease renal blood flow & GFR due to constriction of interlobular arteries & afferent arterioles. Angiotensin II- Constricts efferent arterioles (predominantly)↑ Glom P↓ RBF Endothelin- Vasoconstriction under pathological conditions.

CONTROL OF RENAL BLOOD FLOW (Cont.) Nitric oxide- Endothelial derived vasodilator imp for maintaining vasodilation of Kidneys. Dopamine- vasodilation & natriuresis

CONTROL OF RENAL BLOOD FLOW---(Cont.) Prostaglandins & Bradykinin: Cause vasodilation→ ↑ Renal Blood Flow & ↑ GFR. Not v. imp normally but Of importance in preventing vasoconstrictor effects of sympath.stim & angiotensin II. (nonsteroidal anti-inflammatory drugs may cause ↓GFR by inhibiting PG synthesis).

Conditions affecting RBF Conditions leading to ↓ CO (Hemorrhage, Dehydration--) Hypoalbuminemia High protein intake Uncontrolled diabetes

Blockade of Angiotensin II in a case of renal hypoperfusion can sometimes further reduce GFR & lead to acute renal failure. Explain how ?

Learning Outcomes State average blood flow to the kidneys and compare that with other organs Relate renal blood flow to the glomerular filtration rate State the main determinants of renal blood flow Describe the effects of sympathetic stimulation on renal vascular resistance Explain the relationship of arterial blood pressure with renal blood flow and glomerular filteration rate / phenomenon of autoregulation Explain the mechanisms underlying autoregulation of GFR and renal blood flow Describe the hormonal control of renal blood flow and GFR State effect and underlying mechanism of some of the hormones e.g norepinephrine and epinephrine and autocoids like angiotensin II, prostaglandin and endothelin on renal circulation

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