For each of the physiological responses A–E, select the most appropriate option from the effector mechanisms listed below

A. Option 2 Autoregulatory responses. When the perfusion pressure rises, the stretching of the smooth muscle in the walls of blood vessels causes it to contract and so minimize the increase in blood flow that would otherwise occur.

B. Option 4 Vasoconstrictor nerves. These sympathetic nerves are normally active to exert a steady vasoconstrictor ‘tone’. When their activity is decreased, the smooth muscle in the walls of the blood vessels relaxes to cause an increase in local blood flow.

C. Option 4 Vasoconstrictor nerves. The neurotransmitter for sympathetic vasoconstrictor nerves is noradrenaline

D. Option 4 Vasoconstrictor nerves. When arterial pressure falls, it is countered by a reflex increase in vasoconstrictor tone that raises peripheral resistance.

E. Option 1 Vasodilator metabolites. The increase in local blood flow after circulatory arrest (reactive hyperaemia) is due to the accumulation of products of metabolism causing vasodilation during the arrest period.