Bleeding most often occurs due to injury, and depending upon the circumstances, the amount of force required to cause bleeding can be quite variable.

Blunt trauma

Most people understand that falling from a height or being involved in a car accident can inflict great force and trauma upon the body. If blunt force is involved, the outside of the body may not necessarily be damaged, but enough compression may occur to internal organs to cause injury and bleeding.

• Imagine a football player being speared by a helmet to the abdomen. The spleen or liver may be compressed by the force and cause bleeding inside the organ. If the hit is hard enough, the capsule or lining of the organ can be torn, and the bleeding can spill into the peritoneum (the space in the abdominal cavity that contains abdominal organs such as the intestines, liver, and spleen).
• If the injury occurs in the area of the back or flank, where the kidney is located, retroperitoneal bleeding (retro=behind; behind the abdominal cavity) may occur.
• The same mechanism causes bleeding due to crush injuries. For example, when a weight falls on a foot, the weight doesn't give, nor does the ground. The force needs to be absorbed by either the bone or the muscles of the foot. This can cause the bone to break and/or the muscle fibers to tear and bleed.
• Other structures are compressible and may cause internal bleeding. For example, the eye can be compressed in the orbit when it is hit by a fist or a ball. The globe deforms and springs back to its original shape. Intraorbital hemorrhage may occur.

Deceleration trauma

Deceleration may cause organs in the body to be shifted inside the body. This may shear blood vessels away from the organ and cause bleeding to occur. This is often the mechanism for intracranial bleeding such as epidural or subdural hematomas. Force applied to the head causes an acceleration/deceleration injury to the brain, causing the brain to "bounce around" inside the skull. This can tear some of the small veins on the surface of the brain and cause bleeding. Since the brain is encased in the skull, which is a solid structure, even a small amount of blood can increase pressure inside the skull and decrease brain function.

Fractures

Bleeding may occur with broken bones. Bones contain the bone marrow in which blood production occurs. They have rich blood supplies, and significant amounts of blood can be lost with fractures. The break of a long bone such as the femur (thigh bone) can result in the loss of one unit (350-500cc) of blood. Flat bones such as the pelvis require much more force to cause a fracture, and many blood vessels that surround the structure can be torn by the trauma and cause massive bleeding.

Pregnancy

Bleeding in pregnancy is never normal, though not uncommon in the first trimester, and is a sign of a potential miscarriage. Early on, the concern is a potential ectopic or tubal pregnancy, in which the placenta and the fetus implant in the Fallopian tube or another location outside of the uterine cavity. As the placenta grows, it erodes through the tube or other involved organs and may cause fatal bleeding.

Bleeding after 20 weeks of pregnancy may be due to placenta previa or placental abruption, and urgent medical care should be accessed. Placenta previa describes the situation in which the placenta attaches to the uterus close to the opening of the cervix and may cause painless vaginal bleeding. Abruption occurs when the placenta partially separates from the uterine wall and causes significant pain with or without bleeding from the vagina.

Spontaneous bleeding

Internal bleeding may occur spontaneously, especially in those people who take anticoagulation medications or who have inherited bleeding disorders. Routine bumps that occur in daily life may cause significant bleeding issues.

Medication

Internal bleeding may be caused as a side effect of medications (most often from nonsteroidal anti-inflammatory drugs such as ibuprofen and aspirin) and alcohol. These substances can cause inflammation and bleeding of the esophagus, stomach, and duodenum, the first part of the small intestine as it leaves the stomach.

Alcohol abuse

Long-term alcohol abuse can also cause liver damage, which can cause bleeding problems through a variety of mechanisms.

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