As a type of hematoma, a bruise is always caused by internal bleeding into the extracellular space, usually initiated by blunt trauma, which causes damage through physical compression and deceleration forces. Trauma sufficient to cause bruising can occur from a wide variety of situations including accidents, falls, and surgeries. Disease states such as insufficient or malfunctioning platelets, other coagulation deficiencies, or vascular disorders, such as venous blockage associated with severe allergies  can lead to the formation of bruises in situations in which they would not normally occur and with only minimal trauma. If the trauma is sufficient to break the skin and allow blood to escape the extracellular space, the injury is not a bruise but instead a different variety of hemorrhage called bleeding, although such injuries may be accompanied by bruising elsewhere.

Bruises often induce pain, but small bruises are not normally dangerous alone. Sometimes bruises can be serious, leading to other more life-threatening forms of hematoma, such as when associated with serious injuries, including fractures and more severe internal bleeding. The likelihood and severity of bruising depends on many factors, including type and healthiness of affected tissues. Minor bruises may be easily recognized in people with light skin color by characteristic blue or purple appearance (idiomatically described as "black and blue") in the days following the injury.

Size and shape of bruises

Bruise shapes may correspond directly to the instrument of injury or be modified by additional factors. Bruises often become more prominent as time lapses, resulting in additional size and swelling. Bruising present in a different location than the site of impact is called ectopic bruising and occurs when the tissue at the site of injury is loose, allowing blood to travel under the skin to another location due to gravity or other forces, such as in a black eye.

General factors modifying size and shape of bruises

• Condition and type of tissue: In soft tissues, a larger area is bruised than would be in firmer tissue due to ease of blood to invade tissue.
• Age: elderly skin and other tissues are often thinner and less elastic and thus more prone to bruising.
• Gender: More bruising occurs in females due to increased subcutaneous fat.
• Color of skin: Although the same size, bruises are more prominent in fair complexion.
• Diseases: Coagulation, platelet and blood vessel diseases or deficiencies can increase bruising due to more bleeding.
• Location: More extensive vascularity causes more bleeding.
• Forces: Greater striking forces cause greater bruising.
• Genetics: Despite having completely normal coagulation factors, natural redheads have been shown to bruise more, although this may just be due to greater visibility on commonly-associated fair complexion.

Severity of bruises

Bruises can be scored on a scale from 0-5 to categorize the severity and danger of the injury.

0 Light bruise No damage
1 Less than Moderate bruise Little damage
2 Moderate bruise Some damage
3 Serious bruise Dangerous
4 Extremely serious bruise Dangerous
5 Critical bruise Risk of death

The harm score is determined by the extent and severity of the fractures to the organs and tissues causing the bruising, in turn depending on multiple factors. For example, a contracted muscle will bruise more severely, as will tissues crushed against underlying bone. Capillaries vary in strength, stiffness and toughness, which can also vary by age and medical conditions.

Light bruises

Low levels of damaging forces produce small bruises and generally cause the individual to feel minor pain, either initially or delayed. Repeated impacts worsen bruises, increasing the harm level. Normally, light bruises heal nearly completely within two weeks, although duration is affected by variation in severity and individual healing processes; generally, more severe or deeper bruises take somewhat longer.

Moderate to severe bruises

Severe bruising (harm score 2-3) may be dangerous or cause serious complications. Further bleeding and excess fluid may accumulate causing a hard, fluctuating lump or swelling hematoma. This has the potential to cause compartment syndrome as the swelling cuts off blood flow to the tissues. The trauma that induced the bruise may also have caused other severe and potentially fatal harm to internal organs. For example, impacts to the head can cause traumatic brain injury: bleeding, bruising and massive swelling of the brain with the potential to cause concussion, coma and death. Treatment for brain bruising may involve emergency surgery to relieve the pressure on the brain.

Damage that causes bruising can also cause bones to be broken, tendons or muscles to be sprained, ligaments to be strained, or other tissue to be damaged. The symptoms and signs of these injuries may initially appear to be those of simple bruising. Abdominal bruising or severe injuries that cause difficulty in moving a limb or the feeling of liquid under the skin may indicate life-threatening injury and require the attention of a physician.

Treatment

Treatment for light bruises is minimal and may include RICE (rest, ice, compression, elevation), painkillers (particularly NSAIDs) and, later in recovery, light stretching exercises. Particularly, immediate application of ice while elevating the area may reduce or completely prevent swelling by restricting blood flow to the area and preventing internal bleeding. Rest and preventing re-injury is essential for rapid recovery.

Very gentle massage of the area and application of heat may encourage blood flow and relieve pain according to the gate control theory of pain, although causing additional pain may indicate the massage is exacerbating the injury. As for most injuries, these techniques should not be applied until at least three days following the initial damage to ensure all internal bleeding has stopped; although increasing blood flow will allow more healing factors into the area and encourage drainage, if the injury is still bleeding this will allow more blood to seep out of the wound and cause the bruise to become worse.

As a medical sign

The presence of bruises may be seen in patients with platelet or coagulation disorders. Unexplained bruising may be a warning sign of child abuse, domestic abuse, or serious medical problems such as leukemia or meningoccocal infection. Unexplained bruising can also indicate internal bleeding or certain types of cancer. Long term glucocorticoid therapy can cause easy bruising. Bruising present around the navel (belly button) with severe abdominal pain suggests acute pancreatitis.

During an autopsy, bruises accompanying abrasions indicate the abrasions occurred while the individual was alive, as opposed to damage incurred after death.

Pathophysiology

Nosebleeds are due to the rupture of a blood vessel within the richly perfused nasal mucosa. Rupture may be spontaneous or initiated by trauma. Nosebleeds are reported in up to 60% of the population with peak incidences in those under the age of ten and over the age of 50 and appears to occur in males more than females. An increase in blood pressure (e.g. due to general hypertension) tends to increase the duration of spontaenous epistaxis. Anticoagulant medication and disorders of blood clotting can promote and prolong bleeding. Spontaneous epistaxis is more common in the elderly as the nasal mucosa (lining) becomes dry and thin and blood pressure tends to be higher. The elderly are also more prone to prolonged nose bleeds as their blood vessels are less able to constrict and control the bleeding.

The vast majority of nose bleeds occur in the anterior (front) part of the nose from the nasal septum. This area is richly endowed with blood vessels (Kiesselbach's plexus). This region is also known as Little's area. Bleeding further back in the nose is known as a posterior bleed and is usually due to rupture of the sphenopalatine artery or one of its branches. Posterior bleeds are often prolonged and difficult to control. They can be associated with bleeding from both nostrils and with a greater flow of blood into the mouth.

Treatment

The flow of blood normally stops when the blood clots, which may be encouraged by direct pressure applied by pinching the soft fleshy part of the nose. This applies pressure to Little's area, the source of the majority of nose bleeds and promotes clotting. Pressure should be firm and be applied for at least five minutes and up to 20 minutes; tilting the head forward will help decrease the chance of nausea and airway obstruction. Swallowing excess blood can irritate the stomach and cause vomiting. Local application of an ice pack to the forehead or back of the neck or sucking an ice cube has seen widespread practice, but has been shown to not have any statistically significant effects on nasal mucosal blood flow. There are conflicting opinions in the use of ice or nasal packing in the treatment of nose bleeds. Most suggest there is no detriment to using ice or nasal packing when initial efforts to pinch the nose fail, while others advise against it.

The local application of a vasoconstrictive agent has been shown to reduce the bleeding time in benign cases of epistaxis. The drugs oxymetazoline or phenylephrine are widely available in over-the-counter nasal sprays for the treatment of allergic rhinitis, and may be used for this purpose.

Other products available to promote coagulation include Coalgan (in Europe) or NasalCEASE (in the US). These are a calcium alginate mesh or swabs that is inserted in the nasal cavity to accelerate coagulation. Also there is QuikClot nosebleed available in the U.S. that is a hemostatic OTC formula.

If these simple measures do not work then medical intervention may be needed to stop bleeding, possibly by an otolaryngologist (ENT doctor). In the first instance this can take the form of chemical cautery of any bleeding vessels or packing of the nose with ribbon gauze or an absorbent dressing (called anterior nasal packing). Such procedures are best carried out by a medical professional. Chemical cauterisation is most commonly conducted using local application of silver nitrate compound to any visible bleeding vessel. This is a painful procedure and the nasal mucosa should be anaesthetised first, preferably with the addition of topical adrenaline to further reduce bleeding. If bleeding is still uncontrolled or no focal bleeding point is visible then the nasal cavity should be packed with a sterile dressing, which by applying pressure to the nasal mucosa will tamponade the bleeding point. Ongoing bleeding despite good nasal packing is a surgical emergency and can be treated by endoscopic evaluation of the nasal cavity under general anaesthesia to identify an elusive bleeding point or to directly ligate (tie off) the blood vessels supplying the nose. These blood vessels include the sphenopalatine, anterior and posterior ethmoidal arteries. More rarely the maxillary or a branch of the external cartoid artery can be ligated. The bleeding can also be stopped by intra-arterial embolization using a catheter placed in the groin and threaded up the aorta to the bleeding vessel by an interventional radiologist. Continued bleeding may be an indication of more serious underlying conditions.

Chronic epistaxis resulting from a dry nasal mucosa can be treated by spraying saline in the nose three times per day, lubricating the nose with ointments or creams, such as vasoline, and installing a humidifier in the bedroom.

Application of a topical antibiotic ointment to the nasal mucosa has been shown to be an effective treatment for recurrent epistaxis. One study found it to be as effective as nasal cautery in the prevention of recurrent epistaxis in patients without active bleeding at the time of treatment – both had a success rate of approximately 50 percent.

Nosebleeds are rarely dangerous unless prolonged and heavy. Nevertheless they should not be underestimated by medical staff. Particularly in posterior bleeds a great deal of blood may be swallowed and thus blood loss underestimated. The elderly and those with co-existing morbidities, particularly of blood clotting should be closely monitored for signs of shock.

Recurrent nosebleeds may cause anemia due to iron deficiency.