That's what I was wondering... is it just the increased growth hormone levels in the young blood or is it the blood cells themselves?

If I'm reading the article right, it was neither, but actually just the plasma:

> Plasma alone had the same effects. “We didn't have to exchange the whole blood”

Did they remove the growth hormones from the plasma? It doesn't say...

From some cursory wiki reading it seems blood plasma is quite the blanket term. Electrolytes, proteins, CO2 removal, it's involved in quite a number of places.