If there is a small red heart with the words “organ donor” at the bottom of your driver’s license, that puts you in a group of over 173 million Americans who are in the national organ donor registry.

Organ donation has wide public support, but medical ethicists say there is still confusion about what it looks like and how it affects patients and families.

Any adult can register. In most states, teenagers as young as 15 may also express their intent to donate, though parents can revoke that decision.

Today, around 90 percent of donors join their state registry at their local D.M.V. Others sign up while registering to vote, or through an online form linked to their local organ procurement organizations, known as O.P.O.s, which are nonprofit federal contractors in each state that coordinate transplants.

Donate Life America, an advocacy organization, maintains a nationwide list of online registries.

Donors can sign up or change their status at the D.M.V. or online, though residents of some states have had trouble removing themselves from the registry.

It depends on how you die.

If you are declared legally dead after testing shows no signs of neurological activity — known colloquially as being “brain-dead” — then the organ donor status on your license is legally binding, even if your family disagrees. In these cases, patients are kept on a ventilator until their organs are retrieved.

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