Final answer:
The main force that pushes water and solutes out of the blood across the filtration membrane is glomerular blood pressure, influenced by net filtration pressure which includes both hydrostatic pressure and osmotic pressure.
Step-by-step explanation:
The main force pushing water and solutes out of the blood across the filtration membrane is glomerular blood pressure. This pressure is a type of hydrostatic pressure, the pressure exerted by a fluid against a surface. Glomerular filtration occurs when the glomerular hydrostatic pressure exceeds the opposing forces such as the osmotic pressure exerted by solutes. The filtration through the glomerulus is facilitated by the fenestrations between endothelial cells, allowing small solutes and water to pass while preventing larger molecules like proteins from filtering through due to size exclusion and the negative charge of the membrane which repels negatively charged substances.
Since large proteins and cells cannot pass through, they remain in the capillary, contributing to a colloid osmotic pressure. However, the osmotic pressure in Bowman's capsule remains nearly zero due to the absence of those proteins, resulting in a net filtration pressure that ensures the movement of fluid into Bowman's space. The GFR or Glomerular Filtration Rate is thus dependent on these pressures and is an essential indicator of kidney function.