Determining GFRStarling forces Figure

- The driving force for glomerular filtration is the net ultrafiltration pressure across the glomerular capillaries.

Glomerular capillary

Pes W Bowman's space

Proximal tubule

Pes W Bowman's space

Proximal tubule

Figure 5-3. Starling forces across the glomerular capillaries. Heavy arrows indicate the driving forces across the glomerular capillary wall. PBS = hydrostatic pressure in Bowman's space; PGC = hydrostatic pressure in the glomerular capillary; ttgc = colloidosmotic pressure in the glomerular capillary.

- Filtration is always favored in glomerular capillaries because the net ultrafiltration pressure always favors the movement of fluid out of the capillary.

- GFR can be expressed by the Starling equation:

a. GFR is filtration across the glomerular capillaries.

b. Kf is the filtration coefficient of the glomerular capillaries.

- The glomerular barrier consists of the capillary endothelium, basement membrane, and filtration slits of the podocytes.

- Normally, anionic glycoproteins line the filtration barrier and restrict the filtration of plasma proteins, which are also negatively charged.

- In glomerular disease, the anionic charges on the barrier may be removed, resulting in proteinuria.

c. Pgc is glomerular capillary hydrostatic pressure, which is constant along the length of the capillary.

- It is increased by dilation of the afferent arteriole or constriction of the efferent arteriole. Increases in x gc cause increases in net ultrafiltration pressure and GFR.

d. PBs is Bowman's space hydrostatic pressure and is analogous to P; in systemic capillaries.

- It is increased by constriction of the ureters. Increases in PBS cause decreases in net ultrafiltration pressure and GFR.

e. irGc is glomerular capillary oncotic pressure. It normally increases along the length of the glomerular capillary because filtration of water increases the protein concentration of glomerular capillary blood.

- It is increased by increases in protein concentration. Increases in ttGc cause decreases in net ultrafiltration pressure and GFR.

f. ttbs is Bowman's space oncotic pressure. It is usually zero because only a small amount of protein is filtered.

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