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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.