Red blood cells are flexible biconcave discs that spend their lives suspended in blood plasma. They are by far the largest component of ordinary blood and consequently play a hugely important role in the way it flows through the body.
In ordinary conditions, red blood cells stack together to form structures called rouleaux like cylindrical packs of coins. These constantly form and break up but they can also become tangled. When this happens, the blood becomes thicker and eventually clots.
