Hemoglobin increases the solubility of oxygen in blood (made mostly of water). Hemoglobin is a protein that has 4 heme groups. Each heme group has iron at the center (that"s what reversibly binds to oxygen). Each heme group can bind to one oxygen molecule for a total of 4 oxygen molecules can reversibly bind to every hemoglobin molecule. Hemoglobin reversibly binds to oxygen (that"s how it makes oxygen much more soluble in blood). Without hemoglobin, the oxygen-carrying capacity of our blood would be very low. Fig 18-2 gases diffuse down partial pressure gradients. Individual gas components will diffuse from areas of higher partial pressure to areas of lower partial pressure. Oxygen and carbon dioxide partial pressures are different outside from in the alveoli. Partial pressure of oxygen is 100 mmhg in the alveoli and 40 mmhg for carbon dioxide partial pressure in the alveoli.