Clot retraction wrings out a small amount of fluid. What is that fluid?

1. Contractile proteins within the platelets pull on the fibrin threads and draw the clot edges together. 2. That retraction squeezes a small amount of fluid out of the clot. 3. The clotting factors have been consumed or trapped in forming the clot itself. 4. What is expressed is therefore blood plasma without its clotting factors, which is serum. 5. Plasma with its clotting factors intact is simply plasma, so that option loses the whole distinction. 6. This is exactly why a sample left standing in a glass tube for an hour yields serum: the intrinsic pathway has clotted it and retraction has expressed the serum. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.5 Hemostasis_