Real-time in vivo imaging (600X) allows recording blood flow in blood vessels. Passing through capillaries of a diameter smaller than the RBC size, these red blood cells undergo reversible deformation #1, #2, #3, #4. They return to the original shape of torus (biconcave disc) in larger blood vessels.

TEM image, 3,000X. In osmium fixation, histological sections of blood vessels show that red blood cells in the lumina exhibit a high density of their cytoplasm while the geometrical shape #1, #2, #3 varies. As a rule, these are accidental sections of toroid and, probably, some compressed cells.
There are some platelets in the lumen of this vessel.

SEM image, 2,000X (photo courtesy of V. A. Shakhlamov). A blood drop sample shows a typical shape of biconcave disc #1, #2 in most red blood cells. Discocytes are aging, their diameter and pallor depth (central part of the cytoplasm) diminish #1, #2, #3.
The image also depicts some stomatocytes #1, #2, spherocytes #1, #2, and a lymphocyte.