with correct deep breathing, the ribs gently massage the spine and internal organs.
Blood circulation throughout the body happens via blood vessels (veins, arteries, and capillaries). There are around 100,000 miles of blood vessels in an average adult body! These lie across or underneath, or are surrounded by muscles. Relaxed muscles that are not stiff and over-contracted won't squeeze and pinch the blood vessels, meaning they transport blood more effectively.
Remain relaxed, even when you are doing exercises that may seem at first to be primarily strength building; consider all the movements to really be a stretch.