Rennin: It's an enzyme that acts on milk protein casein & changes it into paracasein spontaneously precipitated into insoluble calcium paracaseinate, forming solid curd or coagulated milk. Rennin is present in gastric juice and hence works at highly acidic pH (due to HCl) in the stomach (about 1-3 pH) and absent in adult human beings.

Renin: It is a hormone that is produced by the kidney to control blood sodium levels. Renin is a hormone produced by JG cells of the kidney. the primary function is, therefore, to eventually cause an increase in blood pressure, leading to the restoration of perfusion pressure in the kidney.