Hormonal

Resistance training performed to volitional failure induces microvascular expansion (increased capillary-to-fiber ratio, capillary contacts, and capillary-to-fiber perimeter exchange index) regardless of whether the load is high (75-90% 1RM) or low (30-50% 1RM).

If you want to improve the blood supply to your muscles, you don't need to lift heavy weights. As long as you perform resistance exercises to the point of failure, using either light weights for many reps or heavy weights for fewer reps, you will get the same microvascular benefits. Focus on reaching failure rather than obsessing over the specific weight on the bar.