The circulatory system carries blood throughout the body via an intricate network of arteries and veins. The venous system is the section of the circulatory system that uses veins to return the used, or deoxygenated, blood to the heart and lungs. Occasionally, irregularities in the wall of a vein (especially in areas of slow flow, such as the area surrounding a venous valve) can cause a blood clot, or thrombus, to form. Once formed, additional fibrin and red blood cell deposits can cause the thrombus to grow inside the vein. In addition to causing inflammation of the vein and obstructing blood flow, there is a significant risk that all or part of a venous thrombus may break off and travel through the bloodstream. These mobile thrombi, or emboli, can eventually lodge in the blood vessels of the lungs. The lodged clot, called a pulmonary embolism, may compromise blood flow to the lungs and result in shortness of breath, light-headedness, coughing, chest pain, and even loss of consciousness or death. Development of a pulmonary embolism is a medical emergency that requires prompt medical attention.