For: Bloodletting

Bleeding, technically known as hemorrhaging/haemorrhaging (see American and British spelling differences) is the loss of blood from the circulatory system. Bleeding can occur internally, where blood leaks from blood vessels inside the body or externally, either through a natural opening such as the vagina, mouth or anus, or through a break in the skin. The complete loss of blood is referred to as exsanguination, and desanguination is a massive blood loss. Loss of 10-15% of total blood volume can be endured without clinical sequelae in a healthy person, and blood donation typically takes 8-10% of the donor's blood volume.

Causes, prevalence, and risk factors

Hemorrhage generally becomes dangerous, or even fatal, when it causes hypovolemia (low blood volume) or hypotension (low blood pressure). In these scenarios various mechanisms come into play to maintain the body's homeostasis. These include the "retro-stress-relaxation" mechanism of cardiac muscle, the baroreceptor reflex and renal and endocrine responses such as the renin - angiotensin - aldosterone system (RAAS).

Certain diseases or medical conditions, such as haemophilia and low platelet count (thrombocytopenia), may increase the risk of bleeding or may allow otherwise minor bleeds to become health or life threatening. Anticoagulant medications such as warfarin can mimic the effects of haemophilia, preventing clotting and allowing free blood flow.

Death from hemorrhage can generally occur surprisingly quickly. This is because of 'positive feedback'. An example of this is 'cardiac repression', when poor heart contraction depletes blood flow to the heart, causing even poorer heart contraction. This kind of effect causes death to occur more quickly than expected.

Types of bleeding

Hemorrhaging is broken down into 4 classes by the American College of Surgeons' Advanced Trauma Life Support (ATLS).

• Class I Hemorrhage involves up to 15% of blood volume. There is typically no change in vital signs and fluid resuscitation is not usually necessary.
• Class II Hemorrhage involves 15-30% of total blood volume. A patient is often tachycardic (rapid heart beat) with a narrowing of the difference between the systolic and diastolic blood pressures. The body attempts to compensate with peripheral vasoconstriction. Skin may start to look pale and be cool to the touch. The patient might start acting differently. Volume resuscitation with crystalloids (Saline solution or Lactated Ringer's solution) is all that is typically required. Blood transfusion is not typically required.
• Class III Hemorrhage involves loss of 30-40% of circulating blood volume. The patient's blood pressure drops, the heart rate increases, peripheral perfusion, such as capillary refill worsens, and the mental status worsens. Fluid resuscitation with crystalloid and blood transfusion are usually necessary.
• Class IV Hemorrhage involves loss of >40% of circulating blood volume. The limit of the body's compensation is reached and aggressive resuscitation is required to prevent death.