Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.
Admitted for 180 days â Canadians admitted in âVisitor for Pleasureâ or B-2 status without a B-2 visa in hand are deemed to be admitted for 180 days under current US Customs and Border Protection (CBP) interpretation of the rules.
A visa is not required for U.S. citizens to visit Canada for up to 180 days. Anyone seeking to enter Canada for any purpose besides a visit (e.g. to work, study or immigrate) must qualify for the appropriate entry status and can see the Canadian immigration website.