Iron, the element with atomic number 26, is essential for blood because it sits at the center of hemoglobin’s heme groups.
Hemoglobin is the protein in red blood cells that carries oxygen from the lungs to tissues and helps transport some carbon dioxide back. Iron can reversibly bind oxygen, making this delivery system possible. It is also required for myoglobin in muscle and for many enzymes involved in energy production and cellular metabolism.
The body must regulate iron carefully. Too little can cause iron-deficiency anemia, often associated with fatigue, weakness, and reduced oxygen-carrying capacity. Iron comes in heme form from foods such as meat and seafood, and non-heme form from beans, grains, nuts, and leafy vegetables. Vitamin C can improve absorption of non-heme iron.
Atomic number 26 means that a neutral iron atom has 26 protons and 26 electrons. Its symbol, Fe, comes from the Latin ferrum. Cobalt, zinc, and copper are also biologically important metals, but none is the blood oxygen carrier identified by this question.