55-year-old, Uncle Frank, presents to the Aboriginal Medical Service for

a routine health check (MBS 715). He also would like to receive the flu

vaccination, which has been promoted through the community. Uncle Frank

is a groundsman at the local hospital. He attempts to eat healthily but

wants to learn how to prepare healthy meals. He is currently taking no

medications. His pulse is 80/min, and blood pressure is 165/95 mmHg.

Physical examination shows no other abnormalities.

After taking Uncle Frank’s blood pressure (165/95 mm Hg), which ONE of

the following mechanisms is MOST LIKELY to increase Uncle Frank's blood

pressure further?