The question stem describes a man with presumed orthostatic hypotension. HCTZ is associated with this condition (by causing a relative hypovolemia), and his symptoms of feeling faint when sitting up too quickly also match nicely.

When he stands, gravity causes a redistribution of blood to his legs, causing decreased venous return.

Orthostatic hypotension is defined by an inadequate baroreceptor response, hence decreased baroreceptor activity.

Lastly, we know his cerebral blood flow is decreased if he is feeling dizzy.

Helpful image: http://calgaryguide.ucalgary.ca/wp-content/uploads/2015/10/Orthostatic-Hypotension-Pathogenesis-and-clinical-findings.png

The cardiovascular system must adjust to postural changes to maintain adequate perfusion pressure to the head and upper body.

Baroreceptors in the carotid sinus and aortic arch detect changes in pressure and stimulate the body's response.(INCREASE FIRING)

Inadequate intravascular volume results in symptomatic orthostatic hypotension.