Physiology of marathons amounts of blood to and from the lungs to reach all bodily tissues is dependent on having a high cardiac output along with sufficient levels of total body hemoglobin. Hemoglobin is the oxygen carrying protein within blood cells that allows for the proper transportation of oxygen from the lungs to other bodily tissues via the circulatory system. In order for the transportation of oxygen in blood to be effective during a marathon, the distribution of blood needs to be equally efficient in order for a marathoner's oxygen demands to be met during a race. The mechanism that allows for this
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,