Why do a wound bleed?

OK there are a several good reasons. First - obvious - a vein or artery has been cut. Now why bleeding is a mixed blessing. First, the blood flow often flushes out a wound cleaning it and preventing infection. Secondly, The blood on the outside once exposed to air forms the patch/scab on the wound to seal it, and again, prevent infection and help it stay closed so it can heal. Now the bad side

• deep wounds or large ones generally don't stop bleeding without

help from a doctor or someone with medical knowledge. Apply pressure to large fast flowing wounds and keep clean dressings/rags on them until you can get help from a doctor/medic.