For bleeding to occur from a blood vessel, two things need to happen. Firstly, there needs to be damage done to the blood vessel. The second thing that needs to happen is that the pressure in the blood vessel (in the lumen) must be greater then the pressure outside the blood vessel (the pressure in the interstitial fluid). When the blood vessel is damaged, a number of physical and physiological mechanisms are activated that promote hemostasis. Smaller blood vessels are frequently ruptured by minor traumas of everyday life. Hemostasis seals these defects and stops loss of blood so we are not even aware of the damage to these small blood vessels. When there is damage to the blood vessel, the endothelial lining is disrupted and the underlining collagen protein from the subendothelium tissue is exposed. This initiates two separate but overlapping hemostatic mechanisms. These hemostatic mechanisms include vascular spasm, platelet plug formation and blood coagulation (clotting): vascular spasm.