Platelets, which have negatively-charged cell membranes, adh…

Platelets, which have negatively-charged cell membranes, adhere to the positively charged surface of a torn blood vessel.  As they do so, they release substances which attract more platelets to the damaged area and change the charge on their cell membranes to positive.  More platelets adhere to the damaged area.  The cycle repeats until the damaged area is sealed.