*Introductory Chapter: Molecular Mechanism of Cancer Metastasis DOI: http://dx.doi.org/10.5772/intechopen.109411*

from endothelial cells into the bloodstream. Tumor cells may travel either alone or as emboli (clumps with platelets) within the direction of blood flow.

Once in the vessels, most of the tumor cells are killed by monocytes or natural killer cells. Larger size of the tumor cells at the capillaries encounters a problem hemostatic shear force. Smaller vessels break tumor cells by shear forces due to hydrostatic pressure.

Further, when tumor cells bind to endothelium via E-selectin, the cells are attached/overlapped. In this case, arrested tumor cells can go through apoptosis. This attachment may lead tumor cells to release NO and the process also drives the cells to apoptosis [2, 7].
