Plate­lets. Very small blood cells with no nucleus con­tai­ning an enzy­me, cal­led throm­bo­ki­na­se, which is essen­ti­al to blood clot­ting. Even minor inju­ries will cau­se the throm­bo­cytes to dis­in­te­gra­te, release the enzy­me, and thus trig­ger blood clot­ting. If the num­ber of throm­bo­cytes in the blood drops to below 30,000 per liter, the­re is a major risk of blee­ding. The throm­bo­cytes are pro­du­ced in the red bone marrow.