Platelets are small fragments of cells, with no nucleus. They are made in bone marrow. Platelets help in the formation of blood clots. When platelets come into contact with a damage tissue, they stick to the edges of the damaged area, and then to each other, forming a plug. Larger wounds however need a larger barrier than this.
Blood plasma contains several substances, which are involved in blood clotting.
There are thirteen of these blood-clotting factors. If any one of them is defective, then blood will not clot. For example, clotting disorder due to a missing factor V111 is referred as Haemophilia. Two of these blood-clotting factors are Prothombin and Fibrinogen, which are soluble proteins dissolved in the blood plasma. If a tissue is damaged, it releases a chemical called Thromboplastin.