Oxygen-Hemoglobin dissociation curve

It is the graph that explains the relation of partial pressure to binding and dissociation of oxygen to heme. Note that gases travel from a high partial pressure area to a low partial pressure area. In the oxygen-hemoglobin saturation curve, a greater number of oxygen molecules bind to heme as the partial pressure of oxygen increases. In contrast, the lower the partial pressure of the oxygen, the less is the number of oxygen molecules bound to heme. Thus, the partial pressure of oxygen determines its binding to heme at the respiratory membrane and its dissociation from heme at the tissues. Temperature, hormones, and pH also regulate oxygen binding and dissociation. High temperatures promote dissociation of oxygen from heme. Thyroid hormones, epinephrine, androgens, and growth hormone stimulate the production of 2-bisphosphoglycerate (BPG). BPG also promotes the dissociation of oxygen from heme. The lower the pH, the greater is the dissociation. The relationship between the pH and affinity of oxygen to hemoglobin is explained by Bohr’s effect. Greater dissociation helps in providing tissues with larger amounts of oxygen.

At the pulmonary capillaries, the above reaction is reversed, giving water and carbon dioxide as end-products. Since most of the bicarbonate is present in erythrocyte, it can exchange negatively charged chloride ions by diffusing along the concentration gradient into the plasma. This is referred to as the chloride shift. In exchange for chloride ions, most of the bicarbonate re-enters the erythrocyte. They combine with hydrogen ions to give carbonic acid, and finally carbon-dioxide. This enters plasma, reaches alveoli, and is exhaled out.

The remaining 20% of carbon dioxide is transported to the lungs in the form of carbaminohemoglobin. Carbon-dioxide binds to amino acids of the globin part of hemoglobin to give carbaminohemoglobin. Like oxygen transport, carbon dioxide binding and dissociation to hemoglobin depend on the partial pressure of carbon dioxide. As lungs release carbon dioxide, the blood that passes from lungs to tissues has a lower partial pressure of carbon dioxide than found in the tissues. Because of high partial pressure, carbon dioxide leaves tissues, enters the blood, and binds to hemoglobin. On the other hand, pulmonary capillaries show high partial pressure when compared to alveoli. Hence, it separates from hemoglobin, diffuses across the respirator membrane, and is sent out.