Here's a summary of how you might perform CPR on a non-responsive adult (There is actually a different procedure used to save infants and young children). To learn all about CPR in detail, so that you could actually practice and perform this life-saving act, you should sign up for training from an organization like the American Red Cross.

A is for Airway
When you pass out, your tongue relaxes, and it can roll back in your mouth and block your windpipe. Before you can start CPR on an unconscious person, you'll probably need move their tongue out of the way. Here's how to clear a blocked airway:

Place the palm of your hand across the victim's forehead and push down gently.
With the other hand, slowly lift the chin forward and slightly up.
Move the chin up until the teeth are almost together, but the mouth is still slightly open.

Tilting the head back and lifting the chin move the tongue out of the airway. At this point, you should check again for breathing. If the victim is choking on something, you may see their chest heave as they try to breathe, but you won't be able to feel or hear air being exhaled. You'll have to take additional measures to clear out what's blocking their windpipe, including:

Compressing the abdomen with forceful thrusts. This creates pressure that forces the object up and out of the windpipe.
Trying to manually dislodge the object with your fingers.

Once this is done, you have to check for signs of breathing again. Just clearing out the windpipe may sometimes be enough to allow the victim to start breathing on their own! If the victim starts breathing and moving around on their own, you can stop CPR. If this doesn't happen, you'll have to help them breath, by providing mouth-to-mouth resuscitation.

B is for Breathing
Your lungs have one main function: remove carbon dioxide and take up oxygen. Normally, the muscles in your chest contract and expand your chest cavity, allowing your lungs to fill up with air. Oxygen and carbon dioxide diffuse across the immense surface area of your lungs. Finally, your chest muscles relax, and you exhale. (To learn more about lungs, see How Your Lungs Work).

Rescue breathing uses your lungs to force air into the victim's lungs at regular intervals. The timing of each breath (about 1.5 to 2 seconds per breath) mimics normal breathing. However, the process is much more like blowing up a balloon than real breathing. You inhale deeply, form a tight seal with your mouth over their mouth, and exhale strongly to push air out of your mouth into theirs. Because you also pinch the victim's nostrils closed, the air has nowhere to go except down into the lungs, which expand as they fill with air.

Mouth-to-mouth breathing is hard work. Normally, when you inhale, the chest muscles drive the process. In artificial respiration, you're working against the victim's relaxed chest muscles. When the chest muscles are relaxed, the chest cavity is small, keeping the lungs in a deflated state. As a rescuer, you have to exhale forcefully into the victim's mouth for 1 to 2 seconds to overcome this resistance. As the lungs fill with air, the victim's chest is pushed up at the same time; you can actually see it rise. When you remove your mouth from the victim's and break the air seal, their chest falls and once again deflates the lungs. As in normal breathing, this results in air being exhaled from the victim's mouth.

Does air exhaled from someone else's mouth really provide enough oxygen to save an unconscious person? Normally, the air you inhale contains about 20 percent oxygen by volume, and your lungs remove about 5 percent of the oxygen in each breath. The air you blow into a victim's mouth thus contains about 15 to 16 percent oxygen, which is more than enough to supply their needs.

After you've given the victim two breaths, you then check to see whether or not they have a pulse and whether they are able to breathe on their own. This will determine what you do next.

If the victim . . .	you should
Is breathing and has a pulse	stop CPR, and stay with them until help arrives.
Is not breathing and has a pulse	continue rescue breathing.
Has no pulse	begin chest compressions, alternating with rescue breathing.

C is for Circulation
If the victim's heart is not beating, all your breathing efforts are for naught; the oxygen that you're getting into their circulation isn't going anywhere! Once again, you have to take over for a failing organ. This time you essentially become a surrogate heart to pump oxygenated blood out to the rest of the victim's body. How can you have any effect on blood flow from outside of the body? All it takes is your hands and some strength. The steps are simple:

Kneeling by the victim, place the heel of your hands one atop the other about .4 to .8 inches (1 to 2 cm) from tip of the breastbone.
Using the weight of your body, push the victim's chest down. You should compress their chest 1 to 2 inches (2.54 to 5.08 cm).
Hold in this position for half a second, then relax for half a second
Repeat steps two and three 29 more times.
Give the victim two rescue breaths as you did before to deliver more oxygen to the blood.
Repeat steps 1 through 5 three more times, then check for a pulse.

In reality, all you are doing is squeezing the heart between the breastbone and the backbone to force blood out. Compressing the chest creates positive pressure inside the chest that pushes oxygenated blood out of the heart through the aorta. From here, it travels to the brain and then on to other parts of the body, delivering oxygen for cellular respiration. When you relax, the pressure inside the victim's chest subsides. Deoxygenated blood moves back into the heart from the veins.

As the chest is compressed, positive pressure is created inside the chest, and oxygenated blood is pushed through the arteries to the brain and other organs. When you release, the pressure drops, and blood flows back into the heart from the veins.