Parabiosis rejuvenation theory

See review https://rlegroup.net/2020/05/31/perelivanie-plazmy-realnoe-sredstvo-omolozhenija/ (in Russian)
https://www.biorxiv.org/content/10.1101/2021.11.11.468258v1
https://conboylab.berkeley.edu/publication/systemic-induction-of-senescence-in-young-mice-after-single-heterochronic-blood-exchange/
https://conboylab.berkeley.edu/publication/erythrocytes-a-new-contributor-to-age-associated-loss-of-blood-brain-barrier-integrity/
https://conboylab.berkeley.edu/publication/the-microfluidic-toolbox-for-analyzing-exosome-biomarkers-of-aging/
https://conboylab.berkeley.edu/publication/rejuvenation-of-three-germ-layers-tissues-by-exchanging-old-blood-plasma-with-saline-albumin/
https://conboylab.berkeley.edu/publication/a-single-heterochronic-blood-exchange-reveals-rapid-inhibition-of-multiple-tissues-by-old-blood/

In application to head transplantation

Old body blood contains inflammatory cytokines and some other bad things. They might accelerate brain aging. Probably there is link between C reactive protein level and chance of getting dementia and with atherosclerosis of brain vessels.

Anyway, it seems to be easier to clean blood from cytokines and other bad things (using dialysis or drugs) than separate head.