Jose wrote:

Ok, so why don't I collapse right away after taking a breath of helium
to talk like Donald Duck?

Because at anything close to sea level, the oxygen that is already in your
bloodstream stays there. At altitude, it comes out of your bloodstream. At
altitude, none of the blood leaving the lungs contains enough oxygen to do you
any good. When you take a breath of helium, there won't be any transfer of
oxygen into the bloodstream from that breath, but there is still oxygen left in
that portion of the blood from the last time it passed through the lungs.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.