Almost a million Americans a year are affected by blood clots, so it’s worth knowing the signs and symptoms. Blood clots, or thrombosis, can be life-threatening if not treated quickly. Here’s a detailed look at what they feels like, how to identify the symptoms, and when to seek medical help.

1. What is A Blood Clot

A blood clot is a mass or clump of blood that forms in arteries and veins. While clots can be helpful in controlling bleeding after an injury, they can cause serious health issues if they form without an injury to help. “When blood clots form in the deeper system, they can be painful and very dangerous,” says Dr. Luis Navarro, founder of the Vein Treatment Center in New York City.1 These clots, called deep vein thrombosis (DVT), can obstruct blood flow and lead to serious complications.