Diameter of arterioles and capillaries decreases under the effect of (A) and causes increase in (B).

Vasopressin, also known as antidiuretic hormone (ADH), has a vasoconstrictor effect on arterioles and capillaries. This decrease in the diameter of blood vessels leads to an increase in peripheral resistance, which consequently causes an increase in blood pressure.

Heparin is an anticoagulant and prevents blood coagulation.

Histamine acts as a vasodilator, increasing the diameter of blood vessels and capillary permeability.

Acetylcholine generally acts as a vasodilator and decreases blood pressure.

Answer: A - vasopressin, B - blood pressure