Download Physiology Objectives 24

Physiology Objectives 24
1.
Glomerular ultrafiltration: forces are the same as exchange across a capillary
(Starling equation) except that there is no oncotic pressure in Bowman’s space
because there are no proteins there. Notably, there are many fenestrae in the
endothelial layer, followed by a basement membrane that filters most large
proteins, and finally an epithelium with podocyte foot processes acting as a final
blockade to large proteins. Slits between foot processes are appropriately named
filtration slits.
2.
Size limit of filtered particles: 40 Angstroms
3.
Effects on filtration of:
a.
Charge: negative are repelled while positive are attracted
b.
Shape: more surface area blocks entry, while deformable molecules can
enter readily
4.
Pressures involved in filtration: hydrostatic pressures in both Bowman’s space
and the glomerular capillaries as well as oncotic pressure in the glomerular
capillaries.
5.
Distribution of pressures along glomerular capillaries: along the length of the
capillary, oncotic pressure rises sharply while hydrostatic pressure falls slowly.
Thus, at the afferent end, where there is low oncotic pressure and large
hydrostatic pressure, there is a large ultrafiltration pressure. At the efferent end,
where the oncotic pressure added to the hydrostatic pressure in Bowman’s space
is equal to hydrostatic pressure in the capillaries, there is no ultrafiltration
pressure.
6.
Filtration fraction: fraction of plasma filtered across the glomerular capillary
walls
7.
Essential regulation of pressures in filtration: there are two interrelated
mechanisms: regulation of capillary hydrostatic pressure and regulation of
capillary plasma flow. As capillary hydrostatic pressure increases, filtrate
formation increases and capillary filtration occurs over a longer length of the
capillary (increased plasma flow).
8.
Influence of plasma flow on ultrafiltrate formation: starting with a low
capillary pressure, there is a low capillary surface area for extraction and
therefore, a low GFR. An increase in pressure increases the capillary surface area
and raises the GFR until it reaches a maximum. After maximum GFR has been
reached, increases in pressure do not alter the surface area for exchange, and
while there is an increase in pressure, there is a decrease in filtration fraction and
maximum GFR is maintained.