A thrombus is simply a clumping of formed components in a blood vessel, usually caused by three triggers: vascular injury, hypercoagulability, and slow blood flow. Platelets in the blood vessel first clump into a meshwork and form a large accumulation of red blood cells within it, which often spreads up the blood vessel, resulting in a blood sausage-like change. Once a thrombus forms in a blood vessel, it will swell in size, so in the case of a superficial thrombus, a corded and enlarged blood vessel can be palpated. The thrombus often causes inflammation in the surrounding area, so redness, swelling, and pain can occur.