Red blood cell being dissolved in body fluid) was an important step in the evolution of vertebrates as it allows for less viscous blood, higher concentrations of oxygen, and better diffusion of oxygen from the blood to the tissues. The size of red blood cells varies widely among vertebrate species; red blood cell width is on average about 25% larger than capillary diameter, and it has been hypothesized that this improves the oxygen transfer from red blood cells to tissues. The red blood cells of mammals are typically shaped as biconcave disks: flattened and depressed in the center, with a dumbbell-shaped cross section,
U2 Live at Red Rocks: Under a Blood Red Sky in October 1983 to promote U2's companion live mini-LP "Under a Blood Red Sky", which was released the following month. The mini-LP, whose name was later used as a subtitle for the concert film, features eight live songs from the War Tour, two of which ("Gloria" and "Party Girl") were taken from the Red Rocks concert; the other tracks were taken from performances in Boston and West Germany, as U2 thought they were "more developed performances". "U2 Live at Red Rocks: Under a Blood Red Sky" was U2's first home video release, initially released on VHS and Betamax by MCA