Figure 4.2 Specific chromosomes relative to human hemoglobin formation.
Oxygen delivery is the principal purpose of the hemoglobin molecule. Additionally, it is a structure capable of pulling CO2 away from the tissues, as well as keeping the blood in a balanced pH. The hemoglobin molecule loads oxygen on a one-to-one basis, one molecule of hemoglobin to one molecule of oxygen in the oxygen-rich environment of the alveoli of the lungs. Hemoglobin becomes saturated with oxygen, oxyhemoglobin, and has a high affinity for oxygen in this pulmonary environment, because the network of capillaries in the lungs makes the diffusion of oxygen a rapid process. As the molecule transits through the circulation, deoxyhemo-globin is able to transport oxygen and unload to the tissues in areas of low oxygen affinity. As hemoglobin goes through the loading and unloading process, changes appear in the molecule. These changes are termed allosteric changes, a term that relates to the way hemoglobin is able to rotate on its axis, determine the action of salt bridges between the globin structures, and dictate the movement of 2,3-DPG. The hemoglobin molecule appears in a tense and a relaxed form.3 When tense, hemoglobin in not oxygenated, 2,3-DPG is at the center of the molecule, and the salt bridges between the globin chains are in place. When oxygenated, the relaxed form is in place; 2,3-DPG is expelled, salt bridges are broken, and the molecule is capable of fully loading oxygen.
The binding and release of oxygen from the hemoglobin molecule are defined by the oxygen dissociation curve (OD curve) (Fig. 4.3). This curve is represented as a sigmoid shape, an "S" shape, not the straight-line shape familiar to most students. The curve is designed to illustrate the unique qualities of oxygen dissociation and to attempt to graphically demonstrate how the hemoglobin molecule and oxygen respond to normal and abnormal physiologies.4 What is essential when considering the hemoglobin molecule is that when the molecule is fully saturated, it has all of the oxygen it needs and a high level of oxygen tension. As it travels from the pulmonary circulation to the venous circulation, it has more of an inclination to give up its oxygen in response to the oxygen needs of the tissue it is serving. Figure 4-3 demonstrates the following5:
1. There is a progressive increase in the percentage of the hemoglobin that is bound with oxygen as the blood PO2 increases.
2. In the lungs in which the blood Po2 is 100 mm
Hg, hemoglobin is 97% saturated with oxygen.
3. In venous circulation, in which the PO2 is
40 mm Hg, 75% of the hemoglobin molecule is saturated with oxygen and 25% of the oxy-
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