There is vasoconstriction of the blood vessels after rapture/ injury. This is brought about by pain, neurogenic substances (substances secreted because of injury) and by smooth muscle. Vasoconstriction as a result of injury to the blood vessel triggers turbulence hence blood flow decreases. It only lasts for few minutes.
After rapture/injury to the blood vessel, the collagen gets exposed and platelets adhere to the exposed collagen. After adherence, platelets get activated and secret substances such as ADP, calcium ions, serotonin and thromboxane A2. ADP and thromboxane A2 attracts more platelets to the site of injury. During this process, platelets equally undergo change in shape (physical metamorphosis) to enhance their aggregation. This whole phenomenon results in in the formation of primary haemostatic plug.