An adult has five liters of blood. They can lose two before there's no coming back. You have time to act, but not to hesitate.
ACT
Press. Hard. Don't let go.
- Direct pressure on the wound. Palm, fist, both hands if needed. Force matters more than a clean cloth. A dirty piece of clothing that presses beats a sterile pad that waits.
- Don't remove what's soaked. Add more on top. Pulling off the first dressing tears away the clot that was starting to form.
- Lay the person down. Standing, they'll pass out and fall. On the ground, they're already safe and their brain gets more blood.
- Don't let go to fetch supplies. Send someone, or wait. Ten minutes of continuous pressure beat three interrupted ones.
- If an object is stuck in the wound, leave it. It may be plugging what it pierced. Press around it, and wedge it so it can't move.
- If pressure isn't enough on a limb, and only then, put a tourniquet on above the wound, between the wound and the heart. Tighten it until the bleeding stops, no less. Write down the time you put it on. Never loosen it yourself.
ADAPT
You're alone and you're the one bleeding. Press with one hand and get down on the ground against something solid. An improvised tourniquet is tightened with a stick slipped through the band and twisted: that's the windlass, and it works one-handed. A wide band holds better than a thin one, which cuts without compressing.
The person is restless, pale, shivering, thirsty. Those are signs of shock, and they appear before loss of consciousness. Cover them, insulate them from the ground, talk to them. Cold makes bleeding worse by keeping blood from clotting: a casualty who is bleeding and cold is in a spiral, and Hypothermia becomes the second problem to treat.
You have nothing. A belt, a backpack strap, a strip of cloth torn from a sleeve. A pressure dressing can be made from any fabric rolled tight and held with a band.
It isn't arterial. Dark blood that oozes rather than spurts calls for the same thing, with less urgency. Press, raise the limb, let time work.
You're very far from any help. It doesn't change what you do. It only changes what comes next: keep them warm, give them safe water — see Making water safe to drink — if they're conscious and have no abdominal wound, watch their level of consciousness, and decide whether to call for help following Signaling and getting found.
UNDERSTAND
Clotting is a chain reaction that needs two things: time and stillness. Every time you release the pressure to look, you reset the clock. That's why the rule is never to look for several minutes.
It also needs warmth. Below 35 °C (95 °F), the enzymes of the clotting cascade lose their effectiveness. That's why covering a casualty isn't about comfort: it's part of stopping the bleeding.
For a long time the tourniquet was taught as a last resort to avoid, for fear of losing the limb. Twenty years of battlefield medicine reversed that judgment: put on correctly and left in place, it saves lives, with a much lower risk to the tissue than people believed. What remains true is that a loose tourniquet is worse than nothing: it blocks the blood coming back through the veins without blocking the blood arriving through the arteries, and it increases the bleeding.
Finally, a number to keep in mind as a landmark. Blood loss becomes critical at around a third of the body's blood volume. From an uncompressed limb artery, that third goes in a few minutes. From a vein, it takes much longer. You don't tell the two apart by color, you tell them apart by flow.