Answer. Blood vessels contains muscular walls, when these walls constrict more than usual diameter this is known as vasoconstriction.
Thus, it leads to narrowing of lumen of blood vessels and increase in the blood flow.
Depending upon the medical condition of an individual suppose in case of constant low blood pressure, it is sometimes maintained by giving certain drugs that acts as vasoconstrictor and increases the blood flow and pressure ultimately.
Nephrons are the structural and functional unit of the kidneys. When glomerular filterate decreases than normal inside the tubular part of the nephron then vasopressin is released by the posterior pituitary. This hormone acts as a vasoconstrictor, it increases the blood flow through vasoconstriction and ultimately glomerular filterate increases.