Introduction

Platelets are deceptively simple cells. Their small size, lack of a nucleus and clear cytoplasm made recognition difficult for early microscopists. As a result, platelets were the last of the cellular elements of circulating blood to be identified. Also, recognition was not helped by the tendency of platelets to remain nondescript. They hide as far from the centre of the flowing column of blood as possible. Other cells are busy carrying oxygen and removing carbon dioxide, supplying nutrients, transporting waste, reacting to foreign invaders or leaving the circulation to participate in inflammatory and immunological responses, but not platelets. This cell desires anonymity and remains as quiet as possible as it rolls along the intact endothelium for its 10 to 12 day lifespan. If it can retire to the spleen without becoming involved in any physiologic activity, the platelet's life can be considered a complete success.

Thus, in this sense the platelet has no function in the circulation, except one: to be there when it is needed to keep blood flowing. It is the sentinel on guard at all times to react immediately at sites of vascular injury as soon as subendothelium is exposed. Within microseconds, platelets undergo dramatic changes in their morphology and biochemistry, fill the site of damage with aggregates to form a hemostatic plug, contract to prevent further loss of blood and restore integrity to the vascular system.