This study tests basic differences in how men and women control brain (cerebral) blood flow
(CBF), at rest and under stress. The stress is low oxygen or high carbon dioxide. The
investigators hypothesize that sex differences per se, plus sex hormone differences, drive
different signals in blood vessels that change the way CBF is regulated. The investigators
will test these mechanisms with medicine infusions during stress, and measure CBF using
state-of-the-art MRI approaches. Research confounding variables like aging and disease will
be mitigated by comparing younger adults (18-40 years old).