In a nut shell- not much, in the chest, it can be a problem (sorry, couldn't resist the set up line).
is blood in the pleural space, and it can lead to big problems if the blood is not evacuated as completely as possible. It can be caused by trauma or by some medical procedures (e.g. Biopsies). Blood can "harden" around the lung, causing lost lung function and sometimes chronic infxn.