Renal Microcirculation

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Renal Microcirculation


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Homer W. Smith

From Fish to Phi losopher

, 1961


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The Stuff of Philosophy

Recognizing that we have the kind of internalenvironment we have because we have the kind ofkidneys that we have, we must acknowledge that ourkidneys constitute the major foundation of our

physiological freedom. Only because they work the waythey do has it become possible for us to have bones,muscle, glands, and brains. Superficially it might be saidthat the function of the kidneys is to make urine; but in

a more considered view one can say that the kidneysmake the stuff of philosophy itself.”

Homer W. Smith

From Fish to Philosopher, 1961


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Renal Microcirculation - LOs

• Describe the renal microcirculation, recognize itscharacteristics, and understand how its specific structuredetermines specific renal functions.

• Understand the structural specificity of the renalmicrocirculation in relation to renal function and to othervascular beds

• Recognize the distribution of hydrostatic and oncoticpressures in the renal vasculature

• Correlate pressure values with functional characteristicsof the microcirculatory unit

• State the four basic elements of renal function:glomerular filtration, tubular reabsorption, tubularsecretion, and endocrine function


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The Naked Nephron


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The Nephron Wrapped by Vessels


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Peculiar Facts About The Renal Circulation

• The outer part of the kidney, the renal cortex,receives most of the kidney's blood flow.

• Blood flow in the renal medulla accounts for only 1to 2 per cent of the total renal blood flow.

• Flow to the renal medulla is supplied by aspecialized portion of the peritubular capillarysystem called the vasa recta.

• Vasa Recta descend into the medulla in parallelwith the loops of Henle and then loop back alongwith the loops of Henle and return to the cortex

before emptying into the venous system.


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The Glomerulus: A very high efficiency filter: what

makes the glomerulus such a good filter ?

• Large surface area

• High membrane permeability to water and small solutes

• Elevated hydrostatic pressure leading to high netultrafiltration pressure (overcoming plasma oncoticpressure)

• High plasma flow rate (kidney receives 25% of cardiac

output

• Optimum sieving characteristics prevent loss of plasmaproteins (permselectivity)

* the determinants of glomerular filtration rate


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• Large surface area:

The total length of the capillaries in a

single glomerulus is 0.95 cm, making atotal of 19 km for all 2-millionglomeruli.

The total surface area of allglomerular capillaries is 6,000 cm2.The total filtration surface area is

516.1 cm2

.


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Gl l ill f h


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Glomerular capillary surface area changes

CONTRACTED RELAXED


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Mesangial cell

(MC) contraction

and relaxation

mediates changes in

glomerular size by

varying the

number of opencapillary loops


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• Elevated hydrostatic pressure leading to high netultrafiltration pressure (overcoming plasma oncoticpressure)


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FILTRATION IN SYSTEMIC CAPILLARIES


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The Afferent Arteriole

• Is the first of two resistor sites in the renalmicrocirculation

• Mediates myogenic autoregulation in response tochanges in perfusion pressure

• Is a target for local and systemic vasoconstrictors:e.g. norepinephrin, angiotensin II, thromboxaneA2, endothelins, adenosine, leukotrienes

• Is a target for vasodilators: NO, prostaglandin- I2,lipoxins

• Is the effector arm for tubuloglomerular feedback(TGF)

•

Is affected in diabetes and hypertension


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The Efferent Arteriole

• Is the second of two resistor sites in therenal microcirculation

• Is a target for local and systemic

vasoconstrictors and vasodilators• Is preferentially constricted (more than the

afferent) at low concentrations of

angiotensin II• Gives rise to peritubular capillaries and

vasa recta

ROLE OF THE MICROCIRCULATION IN RENAL AUTOREGULATION


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ROLE OF THE MICROCIRCULATION IN RENAL AUTOREGULATION


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•Elevated hydrostatic pressure leading to high netultrafiltration pressure (overcoming plasma oncoticpressure)

• High plasma flow rate (kidney receives 25% of cardiac

output

• Optimum sieving characteristics prevent loss of plasmaproteins (permselectivity)

* the determinants of glomerular filtration rate


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It is the tight and

clever coupling of

nephron segments to

the renal

microcirculation that

enables the kidney toeffect the three

principal processes of

filtration, re-

absorption, andsecretion which

underlie its clearance

functions.

Renal Microcirculation

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