The afferent aerioles contract inn response to stretch-induced by opening of calcium channels. The mechanism is called Myogenic autoregulation of renal blood flow and GFR. Stretch of the vascular wall allows increased movement of calcium ions from the extracellular fluid into the cells, causing them to contract. This contraction prevents excessive stretch of the vessel. This prevents excessive increase of renal blood flow and GFR by raising vascular resistance. This mechanism may be more impoant in protecting the kidney from hypeension-induced injury. Autoregulation has two main mechanisms: Metabolic mechanism: Tissues produce vasodilatory metabolites that regulate flow E.g., Addenosine, prostaglandins, NO, CO2, H+ and K+. Myogenic mechanism. Ref: Guyton and Hall 13th edition Pgno: 206,344