Each hemoglobin molecule consists of a globin protein made up of four polypeptide chains. Each chain has attached to it the prosthetic group heme. Each heme, in turn, contains a molecule of iron which can combine loosely with oxygen. When all four heme groups have bound with an oxygen molecule, hemoglobin becomes oxyhemoglobin. Hemoglobin's affinity for oxygen is affected by the relative partial pressure of oxygen, by the pH of the blood plasma, and by temperature. When the partial pressure of oxygen is elevated in the blood as it is in the lungs, hemoglobin picks up oxygen readily. When the partial pressure of oxygen is low as it is in the capillary beds of the systemic circulation, hemoglobin releases oxygen. The increased levels of carbon dioxide in the blood as it passes through the capillary beds of the systemic circulation causes an increased acidity in the blood which also causes hemoglobin to release oxygen. The increased temperature of muscles engaged in exercise causes hemoglobin to release oxygen as well, thus, guaranteeing muscles a sufficient supply of oxygen while engaged in strenuous activity.
