Prevent your veins from being attacked by lactic acidosis by distributing heartbeats to the weakest veins.
A human body is always in full motion. The heart pomps bloodcells through the arteries that find their way through the body. The player, as the heart, can allocate to what position these bloodcells should be sent. You have the option between several nodes and the heart can allocate oxygen to one node at a time.
Unfortunately, the bloodcells are not the only moving thing. A body is often under attack by lactic acid, or milk acid. To fight this process of lactic acidosis, the heart should deploy the oxygen at the right moment to the right node. The pink acids that approach the arteries can be fended off by hitting them with the 'blast radius' that exists when oxygen reaches a node.
When the acids touch an artery, the artery becomes weaker. With three hits, the artery disables. Watch the amount of arteries, because if too many arteries are disabled, the heart dies.