Small-animal blood exchange is an emerging approach for systemic aging research.

We describe a small-animal blood exchange approach developed for aging research as an alternative to heterochronic parabiosis or plasma injections. In parabiosis, animals are surgically coupled, which has several disadvantages, including difficulty controlling experimental procedure, the effects of shared organs, environmental enrichment from jointly exploring the housing enclosure, involuntary exercise and an imprecise onset of blood sharing. Likewise, in plasma injections, the added volumes need to be small, and there is little flexibility in changing the relative contributions of ectopic to endogenous blood components. These factors complicate the conclusions and interpretations, including the identification of key mechanisms and molecular or cellular determinants. Our approach, where blood is exchanged between animals without them being surgically coupled, is less invasive than parabiosis. The percentage of exchanged blood or other exchanged fluids is known and precise. The age of plasma and cells can be mixed and matched at all desired relative contributions to the endogenous systemic milieu, and the onset of the effects can be accurately delineated. In this protocol, we describe the preparatory and animal surgery steps required for small-animal blood exchange in mice and compare this process with parabiosis and plasma injections. We also provide the design, hardware and software for the blood exchange device and compare automated and manual exchange methods. Lastly, we report mathematical modeling of the dilution of blood factors. The fluid exchange takes ~30 min when performed by a well-trained biomedical scientist; the entire process takes ~2 h.