gus1- k5 - renal blood flow and glomerular filtration

8/11/2019 GUS1- K5 - Renal Blood Flow and Glomerular Filtration

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Renal Blood Flow and Glomerular

Filtration


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Glomerular Filtration and Renal

Blood Flow

more than 1 L/min, or about 20% of thecardiac output

A normal hematocrit is 0.45, and a typical

renal blood flow (RBF) is 1.1 L/min. The renalplasma flow (RPF) = 0.55 x 1.1 L/min = 605

mL/min.

a typical glomerular filtration rate (GFR) isabout 125 mL/min. Thus, of the 605 mL of

plasma that enters the glomeruli via the

afferent arterioles, 125/605, or 20%, filters

into Bowman's space.


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Flow, Resistance, and Pressure in

the Kidneys

The basic equation for blood flowthrough any organ is as follows: Q=

P/R,

Typical glomerular pressures are near60 mm Hg in a normal unstressed

individual, whereas peritubular

pressures are closer to 20 mm Hg. Thehigh glomerular pressure is crucial for

glomerular filtration, whereas the low

peritubular capillary pressure is equallycrucial for the tubular reabsor tion of


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Glomerular Filtration

The glomerular filtrate is nearly protein freeand contains most inorganic ions and low-

molecular-weight organic solutes in

virtually the same concentrations as in theplasma.

On the basis of both molecular sizeand

electrical charge

For molecules with a molecular weight

ranging from 7000 and 70,000 d, the

amount filtered becomes progressively

smaller as the molecule becomes larger.


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Electrical charge is the second variable

determining filterability of macromolecules.

For any given size, negatively charged

macromolecules are filtered to a lesser

extent, and positively charged

macromolecules to a greater extent, thanneutral molecules.

This is because the surfaces of all the

components of the filtration barrier (the cellcoats of the endothelium, the basement

membrane, and the cell coats of the

podocytes) contain fixed polyanions, which


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Direct Determinants of GFR Rate of filtration = hydraulic permeability x surface

area x NFP

Starling forces NFP = (PGC

GC) (PBC

BC)


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Because there is normally little protein in

Bowman's capsule, BCmay be taken as

zero and not considered in our analysis.

GFR = Kf(PGCPBCGC).


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Filtered Load

It is the amount of substance that isfiltered per unit time sodium.

Its normal plasma concentration is 140

mEq/L, or 0.14 mEq/mL. (Note:1 mEqof sodium is 1 mmol.) A normal GFR is

125 mL/min, so the filtered load of

sodium is 0.14 mEq/mL x 125 mL/min =17.5 mEq/min.


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Autoregulation Mechanism

To counteract changes in GFR

Myogenic mechanism

Increased systemic pressure:Autoregulation: afferent arteriole diameter

decreased (constricted) to maintain the GFR

Decreased systemic pressure:

Autoregulation: afferent arteriole diameter

increased (dilated) to maintain the GFR


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In addition to keeping changes in RBFfairly small, autoregulatory processes

also keep changes in GFR fairly small

Again, GFR doesrise with an increasein arterial pressure, just not

substantially.


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tubuloglomerular feedback

As the filtration rate in an individualnephron increases or decreases, the

amount of sodium that escapes

reabsorption in the proximal tubule andloop of Henle also increases or

decreases.

More sodium filtered means moresodium remaining in the lumen of the

nephron and more sodium flowing from

the thick ascending limb into the distal

tubule.


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The macula densa cells sense the amount

of sodium and chloride in the lumen.

One result of changing levels of luminal

sodium chloride is to increase or decrease

the secretion of transmitter agents into the

interstitial space that affect the filtration inthe nearby glomerulus.

High levels of sodium flowing past the

macula densa cause a decrease infiltration rate; low levels of sodium flowing

past allow a higher filtration rate.


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The transmitter agents released by the

salt-sensing macula densa cells produce

vasoconstriction of the afferent arteriole,thereby reducing hydrostatic pressure in

the glomerular capillaries.

These same agents also producecontraction of glomerular mesangial cells,

thereby reducing the effective filtration

coefficient. Both processes reduce thesingle-nephron filtration rate and keep it at

a level appropriate for the rest of the

nephron


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Renal Clearance

renal clearance of a substance is thevolume of plasma that is completely

cleared of the substance by the kidneys

per unit time.


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Qualities of agents to measure GFR

Inulin: (Polysaccharide from Dahalia plant)

Freely filterable at glomerulus

Does not bind to plasma proteins

Biologically inert

Non-toxic, neither synthesized nor

metabolized in kidney

Neither absorbed nor secreted

Does not alter renal function

Can be accurately quantified

Low concentrations are enough (10-20

mg/100 ml plasma)


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Creatinine:End product of muscle creatine

metabolismUsed in clinical setting to measure GFR

but less accurate than inulin methodSmall amount secrete from the tubule

Para-aminohippurate (PAH):An organic anion not present in bodyFreely filtered, secreted but not

reabsorbedby nephronNon-toxic, neither synthesized nor

metabolized in kidneyLow concentrations are enough (10

mg/100 ml plasma)RPF = Clearance

PAH

= UPAH

.V / PPAH


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Solute Clearance:

Rate of removal from the Blood

Figure 19-16: Inulin clearance


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Concept of clearance

Where,Cx = Clearance of substance X (mg/min)

Ux = Urine concentration of X (mg/ml)

Px = Plasma concentration of X (mg/ml)

V = Urine flow rate of X (ml/min)GFR= Cx =

Px

Ux. V

Qx extracted = Qx excreted

Px . Cx = Ux . V

Effective renal plasma flow =GFR

ERBF= Cx =1 - Hct

ERPF

Renal blood flow =RBF=

Extraction ratio

ERBF

Effective renal blood flow =

Extraction ratio (0.9) =A

PAH

APAH

- VPAH

Hct=hematocrit

VPAH = vein plasma PAH

APAH= arterial plasma PAH


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gus1- k5 - renal blood flow and glomerular filtration

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