An injury (like a cut) to a blood vessel causes a complex chain of events that will result in a blood clot. This is called the clotting process, which is also known as blood coagulation. Clotting is your body's reaction to bleeding to keep you from losing too much blood. Losing too much blood can be life threatening and can damage your internal organs.
Clotting factors are proteins in the blood that work with platelets -- small blood cell fragments -- to help the blood to clot. When blood vessels are damaged, clotting factors help the platelets stick together to plug cuts and breaks at the site of the injury.
In people with hemophilia, blood does not clot as it should due to missing or low levels of one of these clotting factors. If this occurs, there may be heavy blood loss or injured body tissues and organs, which can result in permanent damage or even death.
Hemophilia A is caused by a defect (mutation) in one of the genes that determines how the body makes certain blood-clotting factors. The gene is located on the X chromosome and is called the F8 gene. The F8 gene makes factor 8 (VIII) protein, which is one type of the blood-clotting factor.
This mutation of the F8 gene prevent clots from forming in response to injury, which leads to excessive bleeding that can be difficult to control.
(Click Causes of Hemophilia to learn more about what causes hemophilia A and how hemophilia A is inherited.)