try to bleed animal as soon as possible after sacrifice, collect sample into appropriate blood tube (depending on assay that you are planning to run), clot blood for about 10 minutes, spin at 3000RPM for 10 min. Transfer separated serum to a fresh tube asap, this can be stored at -20 freezer for good few months or at fridge for about a week.
you can centrifuge immediately the sample (blood) for 10 min at 3000 rpm and then you can recuperate the serum (antibody) to perform subsequent immunoassays (western blotting ....)
You obtain 10 ml blood from the rtat. Sterylise by 70% alkohol the groin, shave the groin, cut skin 2 cm , open muscles, show arteria femoralis, set tubs with EDTA K2, cut arteria. Centrifuge as recommandetion ubster.