GFR is opposed by
(B] (Osmotic pressure in glomerular): (680-Ganong 24th edition, 538- Jain 5th, 317-18; Guyton 10th)Glomerular Filtration RateIn normal resting man the glomerular filtration rate (GFR) averages 125cc/min. The ultrafiltrate is derived from an average total renal plasma flow (RPF) of 600 cc/min. The ratio of glomerular ultrafiltrate to renal plasma flow is referred to as the filtration fraction (FF):FF=GFR/RPFWhere, the FF represents the fraction of plasma which is filtered across the glomerular capillary bed. Alterations in the FF can have an important effect on proximal tubular reabsorption of fluid. The glomerular filtration rate is several fold that of fluid movement in capillary beds of other tissues, principally because the surface area available for diffusion plus the permeability of the glomerular capillary membrane (Kf) is approximately 100 fold that of other capillary beds.Physical Determinants of Glomerular FiltrationThe net filtration across a capillary bed (including the glomerular capillary membrane wall is governed by the algebraic sum of hydrostatic and oncotic pressure across the capillary wall.Schematic depiction of the physical determinants responsible for the formation of an ultra filtrate of plasma in Bowman's space. The principal driving force for the formation of the glomerular ultrafiltrate is the hydraulic pressure within the glomerular capillary (Pgc)- In contrast, the principle forces opposing filtration include the oncotic pressure ( 1/4 GC) within the glomerular capillary and hydrostatic pressure in Bowmans space (PBS). Net ultrafiltration is further modulated by the hydraulic permeability and surface area of the glomerular capillary (Kf).The glomerular filtration rate can be derived using the following mathematical relationshipGFR = Kf x (PGC-PBS)-(pGC-7pBs)= Kf x (AP-Att)= Kf x Pufwhere; Kf is the membrane porosity * surface area;PGC is the hydrostatic pressure in the glomerular capillary network;PBS is the Bowman's space pressure;pGC is the glomerular capillary oncotic pressure;tcBS is the Bowman's space oncotic pressure;AP is the transcapillary hydrostatic pressure (PGC - PBS);Arc is the transcapillary oncotic pressure (rcGC - rcBS) andPUF is the net ultrafiltration pressureSince the glomerular ultrafiltrate is typically protein free, pi BS is equal to zero, and, thus, the oncotic pressure inBowman's space is usually excluded from the above analysis. Therefore, the major direct determinants of GFR include.* PGC* PBS* Kf* rcGCIn addition to the above direct determinants of GFR, RPF also indirectly modulates GFR.PGC - In general, increases in glomerular capillary hydrostatic pressure will increase GFR while decreases result in the opposite effect.p- In general, increases in Kf will tend to increase GFR while decreases will tend to decrease GFR.PBS- Normally Bowman's space pressure remains constant because of unimpeded flow of glomerular filtrate.An obstruction to urinary out flow distal to Bowman's space will result in a fall in delta P (and hence GFR) incident to a rise in Bowman's space pressure.pGC- The glomerular capillary oncotic pressure is the principle factor opposing glomerular filtrationThe pressure in the glomerular capillaries is higher than that in other capillary beds because the afferent arterioles are short, straight branches of the interlobular arteries. Further more the vessels "downstream" from the glomeruli, the efferent arterioles have a relatively high resistance. The capillary hydrostatic pressure is opposed hydrostatic pressure in Bowman's capsule. It is also opposed by the oncotic pressure gradient across the glomerular capillary (pGC - pT) is normally negligible and the gradient is essentially equal to the oncotic pressure of the plasma proteins.Hormones or autocoidsEffect on GFR(i) Norepinephrine, Epinephrine Endothelin,| (ii) Angiotension II -(prevent |)(iii) Endothelial derived nitric oxide prostaglandins (PGE2), Dopamine(iv) High protein intake| (v) Increased blood glucose| Uncontrolled DM|
Osmotic pressure in peritubular
Osmotic pressure in glomerular
Hydrostatic pressure in peritubular
Hydrostatic pressure in glomerular
Kidneys and Body Fluids