Vascular Spasm & Platelet Plug Formation

Damage to the endothelium allows platelets to adhere to collagen in the subendothelial layer. When this occurs the platelets become activated and release a variety of compounds which attract more platelets that form a 'platelet plug.'

Platelet Activation

Platelets (ovals) contain membranous sacs indicated by small dots. Platelets 'degranulate' when they bind to collagen (curlicues). That is, the sacs fuse with the platelet membrane and they evert, spilling their contents. New phospholipids called platelet factor (yellow complex) appear on the outer surface of the platelet membrane. The 'emptied' platelets are shown as clear ovals.

Released Compounds


When released from platelets this compound diffuses (block arrow) into the surroundings. It causes the smooth muscle in the region to contract as indicated by the solid arrow. This spasm narrows the vessel and decreases blood flow.

Thromboxane A2

This compound plays two roles. Like serotonin, it causes vascular spasm (solid arrow). It also attracts other platelets as indicated by the solid arrow and makes them stick together This platelet plug is illustrated as a group of clear ovals with platelet factors attached.

Aspirin inactivates (dashed arrow) thromboxane A2. This reduces the vascular spasm and dramatically hinders the formation of the platelet plug.

Adenosine Diphosphate

This compound also attracts additional platelets as indicated by the solid arrow to help form the platelet plug.

Last update: 7/19/2005