Blood flows at low pressurein peritubular blood capillaries. Why??

 The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries. The peritubular capillaries collect water and nutrients from the filtrate in the tubule through reabsorption. 

The blood pressure is lower in peritubular capillaries because the blood which enter the peritubular capillaries has gone through efferent arteriole which a has lower lumen so it lower the blood pressure entering the peritubular capillaries. Peritubular capillaries are the second set of capillaries so have lower blood pressure  and they are porous also,  both things are good for  allowing substances to diffuse in them during reabsorbtion.