Bleeding, or hemorrhage results from damage to blood vessels either from a cut or blunt force trauma. However, infection and other diseases (e.g. Ebola virus) can also result in the breaking of blood vessels. Although minor bleeding usually resolves itself, severe bleeding can kill a patient in minutes.
Because all cells need the oxygen and nutrients delivered by blood, there are blood vessels throughout the body. At the cellular level, the blood vessels narrow down to capillaries, which reach into every tissue of the body. In most bleeding, it is capillaries which are damaged, as they can be found close to the surface of the skin and most organs.
However, capillaries are fed by arteries and drained by veins. These larger vessels are generally contained deeper in the body, although there are notable exceptions. Piercing or severing a major vein or artery can cause a massive loss of blood particularly quickly, especially one of the major arteries, which are always under pressure (veins are not under pressure unless there is muscular activity). However, even minor bleeding can be fatal if left unattended for long enough.
External bleeding is generally easy to treat. If the wound is too large to repair itself, applying pressure to the wound will stem the bleeding, and applying a bandage or stitches will usually resolve the problem. However, unchecked internal bleeding is often more serious. It is usually not immediately obvious that a patient is bleeding internally unless there are contusions (bruising). Even if a physician suspects internal bleeding, it can often be difficult to determine the source without doing exploratory surgery. In such cases, giving the patient a blood transfusion is usually the only option, but even in such cases blood can fill cavities in the body and encourage infection.
Normally, both internal and external bleeding is stemmed automatically by the body. The blood at the bleeding site begins to clot and platelets are rushed to the site to encourage this process. The body then repairs the damage over time. However, there are some diseases that prevent blood from clotting, the most serious of which is hemophilia, a genetic disorder that is suffered largely by males. Patients who are not clotting on their own can be treated with clotting agents and, in extreme cases, transfused platelets.
A patient who is bleeding is also subject to infection as bacteria that are usually blocked by the skin such as Staphylococcus and gangrene gain access to the blood stream. As such, it is important to treat the wound with an antiseptic and to use sterile dressings.