*Tennis sensation Serena Williams is said to be recovering after undergoing emergency treatment earlier this week for a hematoma which according to a spokesperson she “suffered as a result of treatment for a more critical situation.”

A hematoma (or sac of blood) is essentially a swelling under the skin. It may eventually dissolve or in some instances it has to be surgically removed.

However, the “more critical situation” which led to the hematoma in the Serena Williams case is called pulmonary embolism. It is a condition which occurs when one or more arteries in the lungs become blocked. In most cases it is caused by blood clots that travel to the lungs from another part of the body – usually the legs.

While a pulmonary embolism can be life threatening, in the vast majority of instances it can be successfully treated with anti-clotting medications. According to authorities at the Mayo Clinic, the condition can occur in people who are otherwise perfectly healthy. Indeed, over 100,000 cases take place in the United States each year.

The chief signs or symptoms vary but generally include an unexplained shortness of breath, chest pain and coughing up blood. The chances of a pulmonary embolism increase with age – especially after age 60. In a release on the condition, the Mayo Clinic experts say, “If left untreated, about one third of patients will die.”