In order to control the amount of glomerular filtration that is occurring the pressure inside the glomerular capillary must be regulated. Blood flows into the glomerular capillaries through the afferent arteriole and flows from the glomerular capillary into the efferent arteriole. We are able to change the diameter of the arterioles. How would this help us to regulate glomerular filtration?

Changing the diameter of the arterioles changes the pressure in the glomerular capillary. Dilating (widening) the afferent arteriole will increase the amount of blood flowing into the capillary. If the efferent arteriole is constricted it will be more difficult for the blood to flow out of the capillary and so capillary pressure will increase. That will increase the amount of filtration that is occurring.