A hematoma is more than just a big bruise. Here's when they can be concerning.

Your body's circulatory system is a wondrous thing. It's made up of blood vessels that carry blood to and from your heart and also pumps blood to your lungs so you can breathe. It helps grow and repair cells and delivers nutrients, oxygen and hormones throughout your body. Critical organs like your brain, kidneys, liver and heart, plus all muscle tissue, are dependent on your circulatory system to function normally and survive.

But occasionally, issues within this system arise and certain diseases can affect how well things operate. Diabetes, for instance, can impact your circulatory system by causing fatty deposits to form inside blood vessels, limiting blood flow.

Another manifestation of a usually-mild problem that starts in the circulatory system is bruising. Matters can become more serious, however, when dealing with hematomas.

What is a hematoma?

A hematoma is a collection of blood related to a breakage of blood vessels. "This can occur due to injury or other trauma," says Dr. John Whyte, the Chief Medical Officer of WebMD and the author of "Take Control of Your Heart Disease Risk." He explains that as the blood pools in the surrounding tissues after blood vessels break, it can lead to "swelling and discoloration" – hallmark signs of a hematoma.

Hematomas are also sometimes caused by trauma associated with undergoing an operation.

While there are several instances where hematomas require medical intervention, "they generally can take one to four weeks to resolve on their own, though sometimes longer depending on their location and size," says Dr. Steven Maher, an emergency medicine physician at Mayo Clinic in Arizona.

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How is a hematoma different from a bruise?

Bruises and hematomas are similar in that both can occur as a result of a blow, bump, fall or other injury. But there are some distinctions as well:

  • The first difference is related to size. "While both bruises and hematomas result from bleeding under the skin, a hematoma is usually more pronounced due to a larger accumulation of blood," says Dustin Portela, DO, a board-certified dermatologist and founder of Treasure Valley Dermatology in Boise, Idaho. Hematomas are also often larger because they frequently involve large blood vessels.

  • Location is another factor. While bruises are usually visible just under the surface of the skin, hematomas can occur most anywhere in the body, "including under the skin, in muscles, in organs and spaces within the body," says Whyte.

  • Another key difference is their swelling and firmness. "Hematomas often cause more noticeable swelling and can feel firm or lumpy due to the larger amount of clotted blood they contain," explains Whyte. Because of this swelling, "hematomas are often more painful than a bruise," says Portela.

  • Duration is another distinctive factor. "Hematomas can last longer than bruises because the accumulated blood takes more time to be absorbed back into the body," says Whyte.

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How serious is a hematoma?

Another key difference between bruising and hematomas is that hematomas can sometimes lead to serious medical complications. In cases where a hematoma is located near the brain, for instance, blood can collect between the covering of the brain (called the dura mater) and the surface of the brain. This occurrence is called a subdural hematoma and can become life-threatening if left untreated.

Abdominal hematomas are also serious and often manifest as blood in the urine or stool. A blood clot from a hematoma can also re-enter the bloodstream and block an artery – thereby cutting off circulation in part of the body.

While such occurrences are relatively rare and most hematomas aren't something to become overly concerned about, there are concerning elements one can look out for. "If a hematoma is large or continues to grow, it may indicate ongoing bleeding or a more serious injury that needs medical evaluation," says Whyte. It's also worth having a hematoma checked out if one occurs in one's head, around one's eyes, around one's stomach "or near any vital organs where they can press against tissues and impair function," says Portela.

There are accompanying symptoms that can also be helpful to look out for. "If a hematoma is accompanied by symptoms such as severe pain, numbness, weakness or if it affects the function of a limb or organ, it needs to be evaluated by a healthcare provider," says Whyte.

This article originally appeared on USA TODAY: What is a hematoma? It's more than a big bruise.