BIOL 1104 Lecture Notes - Renal Function, Oncotic Pressure, Blood Proteins

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11 Feb 2013
Intro to Renal Physiology & GLomerular Filtration
I. Formation of the Ultrafiltrate
a. Filtration Barrier—(3) fenestrae of endothelial cells, basement membrane, & slit
membranes on podocyte foot processes
i. 3 parameters determine whether solute will cross barrier:
1. size: <5200 A filter freely; > 34000 daltons not filtered *radius &
MW are important!*
2. charge: (-) charge is retarded from crossing
3. shape: rod-shaped pass more easily than globular
ii. concentration ratio—gives measure of how well solute crosses filtration
barrier ([plasma solute]/[urine solute])
b. Physical Factors
i. SNGFR = Kf(ΔP - Δπ) = single nephron GFR
1. ΔP = change in hydraulic pressure between glomerulus (PGC) &
bowman’s capsule (PBC)
a. Normally, PGC = 45mmHg & PBC = 10mmHg
2. Kf = filtration coefficient = (hydraulic permeability of filtration
barrier) x (filtration surface area)
a. Kf for kidney is > 2x any other capillary network in body
3. Δπ = oncotic pressure difference across membrane = colloid
osmotic pressure
a. Rises from 20mmHg to 35 mmHg from afferent to efferent
ends b/c plasma proteins are becoming more concentrated
b. Since π of bowman’s space ~ 0 (no lg – proteins), Δπ = πGC
c. Tends to pull H2O back into capillaries
ii. Because glomerular capillaries lie between 2 arteries, hydraulic pressure is
relatively constant throughout!
iii. Very small net driving force, but large Kf allows for high fluid flow rate
c. Filtration Profiles
i. At any given time, kidney is only using ~ ½ its capillary surface area to
filter => lots of reserve!
ii. Increased renal blood flow => increased length of filtration (b/c its
flowing more rapidly)
1. when blood is flowing slowly, the salts/H2O filtered out very
rapidly ↑ the osmotic pressure (↑[plasma protein])
2. when blood is flowing rapidly, glomerular filtration has less of an
effect on [plasma protein]
II. Auto-regulation
a. Refers to the fact that GFR is maintained ~ constant w/o extrinsic regulation
b. Partially due to the fact that any ↑ in afferent arterial pressure is accompanied by a
↓ in efferent arteriole pressure
c. Juxtaglomerulus Apparatus (JGA)—responsible for tubuloglomerular feedback
d. Regulation of total kidney GFR occurs through control of filtration in each
individual nephron segment
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