How is a high blood pressure built up in a glomerulus?

The glomerulus is supplied blood by the afferent arteriole, then the efferent arteriole takes away blood from the glomerulus. The afferent arteriole has a larger diameter than the efferent arteriole. The efferent arteriole, due to its smaller diameter creates some resistance to blood flow causing a blood back-up which creates a high pressure in the glomerular cavity. Therefore fluid and solute are forced out from the blood and into the glomerular capsule. Most of this filtrate is returned to the blood via peritubular capillary bed surrounding the nephron.