The platelet phase, the second stage of hemostasis, commences around 15-20 seconds after an injury. It follows and overlaps with the vascular phase, during which blood vessels constrict to minimize blood loss.
As the injured blood vessel contracts, endothelial cells undergo contraction, revealing collagen fibers in the basement membrane and underlying connective tissue. Furthermore, the plasma membrane of endothelial cells becomes adhesive, preparing the site for platelet adhesion. Platelets...

Blood clotting or coagulation involves extrinsic and intrinsic pathways, which ultimately merge into the common pathway, forming a fibrin clot.
The Extrinsic Pathway
The extrinsic pathway of coagulation is typically initiated by tissue damage that exposes blood to tissue factor (TF), a protein released by the damaged tissue cells outside the blood vessels—this interaction with TF triggers biochemical reactions involving specific clotting factors. The key player here is Factor VII, which forms a...

The cell fragments known as platelets are disc-shaped, with an average diameter of about 3 μm and a thickness of roughly 1 μm. They play a crucial role in the body's vascular clotting system, which also involves plasma proteins, blood cells, and blood vessel tissues.
Platelets are continually replenished, circulating in the bloodstream for 9-12 days before being removed by phagocytes, primarily in the spleen. A microliter of circulating blood contains between 150,000 and 450,000 platelets, with...