The evolution of macroscopic living beings on Earth required the establishment of vascular systems to transport important moieties and entities such as the dissolved gases involved in their respiration, the nutrients and waste involved in their metabolism, and the cells of their immune system. Simple diffusion in an organism does not ensure the exchanges necessary for pluricellular life beyond a millimeter in size. In vertebrates, the cardiovascular system is functional before any other function, to allow the growth of the embryo. Unlike other functions that can withstand a temporary failure, blood flow must be maintained continuously throughout the life of an individual to constantly deliver oxygen from the respiratory organs to the tissues. This function is realized by red blood cells (RBCs), which enclose oxygen-carrying proteins called hemoglobin. Such packaging has numerous physiological advantages: it protects the proteins from the circulatory environment and it allows each RBC to locally tune the hemoglobin’s affinity for oxygen by using other chemical agents, but, most importantly, it is a way of protecting the organism itself from the toxicity of hemoglobin and, in particular, the hemes, whose oxidative properties are essential but deleterious.¹