Why platelet not active to clotting blood before we have injury?

Platelets are not activated before an injury occurs because that activation is inhibited while they are circulating and there is no damage to the vacular wall.

Explanation:

Platelets are responsible for forming a haemostatic stopper, at the end of the blood coagulation process. These blood components have the ability to go to the place where the vessel wall is damaged and clump together to prevent blood from leaking out.

Platelet migration and activation occurs when the vascular wall endothelium is injured. When this happens, some substances are released - such as adrenaline, ADP and thromboxane A2, among others - and promote platelet activation.

However, this process should not occur before an injury occurs, as it can promote the formation of thrombi that can circulate in the blood vessels and block them.

An inhibitory mechanism, dependent on nitric oxide and prostaglandins, prevents platelets from being activated and clumping in the absence of injury.