GLOMERULAR FILTRATION

GLOMERULAR FILTRATION
- The Filtration Membrane
1. Fenestrated Endothelium (70-90nm)
- highly permeable
2. Basement Membrane (8nm)
- consists of proteoglycan gel
- blood albumin 7nm however it is negatively charged and thus repelled by the membrane
3. Filtration Slits
- located between the pedicels
- around 30nm wide

· Thus, any molecule smaller than 3nm can pass freely through the filtration membrane into the capsular space
à Water, electrolytes, glucose, fatty acids, nitrogenous wastes and vitamins
· Concentration in the glomerular filtrate is similar to that in the blood plasma

- Filtration Pressure
· BHP Blood Hydrostatic Pressure = 60mmHg
à afferent arteriole is substantially larger than the efferent arteriole
à glomerulus has a large inlet and small outlet
· CP Hydrostatic Pressure (capsular space) = 18mmHg
à (slightly negative interstitial pressures elsewhere)
à due to high filtration rate occurring in capsular space and continual accumulation of fluid in the capsule
· COP Colloid Osmotic Pressure = 32mmHg
à Similar to everywhere else
· The glomerular filtrate is almost protein-free and has no significant COP
(this can change markedly in kidney diseases that allow protein to filter into the capsular space)
THUS
o 1 high outward pressure
o 2 lower inward pressures
NFP = 60out – 18in -32in = 10mm Hg out

GLOMERULAR FILTRATION RATE
GFR = NFP x Kf = 10 x 12.5 = 125mL/min

RENAL AUTOREGULATION
- ability of the nephrons to adjust their own blood flow and GFR without external (hormonal or neuronal) control
- Two mechanisms
The Myogenic Mechanism
Tubuloglomerular Feedback