a decrease in afferent arteriole diameter causes a decrease in the gfr (glomerular filtration rate), resulting in a decreased concentration of sodium and chloride ions in the filtrate and/or decreased filtrate flow rate. reduced blood pressure means decreased venous pressure and, hence, a decreased peritubular capillary pressure. this results in a smaller capillary hydrostatic pressure, which causes an increased absorption of sodium ions into the vasa recta at the proximal tubule.