Human, and pretty much any animal blood is composed of two components: plasma, or fluid, and cells. The cells can be divided into two groups: red blood cells (RBCs) and white blood cells (WBC). RBCs carry oxygen while WBCs perform immune functions. RBCs are the most prevalent cells and that is what you mainly see in the videos. They are like bagels without holes, or discs with central dimples. The videos show moving human fresh blood in microscope, in regular and phase contrast light, at 400x and 600x magnification. Alive cells cannot be stained and the contrast is low. To increase contrast, a phase contrast microscope can be used. It can also produce some artistic look, with rainbows and halos. Fresh blood remains liquid for few minutes but then starts clotting. You can see how some RBC are clumping together. That is how blood clot is formed.
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