0 10 20 30 40 50 60 70 80 90 100 Tissue PO2 (mm Hg)

Figure 4.3 The oxygen dissociation curve. In the normal curve (blue) at 40 mm Hg, 75% of the hemoglobin molecule is saturated with oxygen, leaving 25% capable of being released to tissue. Note the right-shifted curve (red). At 40 mm Hg, hemoglobin is 50% saturated but willing to give up 50% of its oxygen to the tissues. Note the left-shifted curve (black). At 40 mm Hg, hemoglobin is 75% saturated but willing to release only 12% to the tissues.

gen is capable of being released when the hemoglobin level is normal.

Two things are demonstrated by this relationship: depending on the need of tissues for oxygen, the hemoglobin molecule will either hold onto oxygen, oxygen affinity, or it will release more oxygen as physiological circumstances dictate.

• When referring to the OD curve, we speak about it in terms of having a "shift to the right" or a "shift to the left."

• Shifting the curve means that physiological conditions are present in the body that will impact the relationship of oxygen and hemoglobin. In most cases, the hemoglobin molecule will compensate by holding or delivering oxygen depending on tissue need. This compensatory mechanism can be demonstrated through the OD curve.

• When the curve is right shifted, hemoglobin has less attraction for oxygen and is more will ing to release oxygen to the tissues. In a right-shifted OD curve, at 40 mm Hg, hemoglobin is 50% saturated but willing to give up 50% of its oxygen to the tissue because of need.

• When the curve is left shifted, hemoglobin has more of an attraction for oxygen and is less willing to release it to the tissues. In a left-shifted OD curve, at 40 mm Hg, hemoglobin is 75% saturated but willing to release only 12% to the tissues.

Physiological conditions that will shift the curve to the right are a. Anemia b. Decreased pH (acidosis)

c. Increase in 2,3-DPG

Therefore, in the anemic state, even though there are fewer red cells and less hemoglobin, the cells act more efficiently to deliver oxygen to the tissues. In fact, the compensatory mechanism of the OD curve works quite adequately if the hemoglobin level is between 8 and 12 g/dL. It is only when the hemoglobin level drops below 8 g/dL that symptoms start to develop, for the most part. Conditions that shift the OD curve to the left are a. Decrease in 2,3-DPG

b. Higher body temperatures c. Presence of abnormal hemoglobins or high oxygen affinity hemoglobins d. Multiple transfusions of stored blood where 2,3-DPG is depleted by virtue of the storage process e. Increased pH (alkalosis)

Less oxygen is released to tissues under these conditions when 2,3-DPG is lower. Consider this analogy for the OD curve. The OD curve is like a roller coaster. As you start up the incline, you are holding on tight, and then as you roll down the hill, you are more willing to throw your arms up in the air and release or relax your grip. So it is with the right-shifted OD curve.

Abnormal Hemoglobins

Normal hemoglobin is a highly stable protein, which can be converted to cyanmethemoglobin, a colored pigment. This conversion is the basis for most of the colorimetric procedures used to measure hemoglobin, and it depends on a versatile and viable hemoglobin compound. Hemoglobins that are physiologically abnormal have a higher oxygen affinity and produce conditions that are usually toxic to the human body.

Three abnormal hemoglobins are methemoglobin, sulfhemoglobin, and carboxyhemoglobin. Increasing the amounts of any of these abnormal hemoglobins in the bloodstream can be potentially fatal. Often, the production of abnormal hemoglobins results from accidental or purposeful ingestion or absorption of substances, drugs, and so on that are harmful. At times, abnormal hemoglobins are produced as a result of inherited defects. In the abnormal hemoglobin methemoglobin, iron has been oxidized to the Fe3+ state, which is no longer capable of binding oxygen. Methemoglobin builds up in the circulation and if the level is above 10%, individuals appear cyanotic, having a blue color, especially in the lips and fingers.6 Aniline drugs and some antimalarial treatments may induce a methemo-globinemia in individuals who are unable to reduce methemoglobin. Hemoglobin M, an inherited condition arising from an amino acid substitution, may also result in cyanotic conditions. Carboxyhemoglobin levels are increased in smokers and certain industrial workers. As a hemoglobin derivative, carboxyhemoglo-bin has an affinity for carbon monoxide that is 200 times greater than for oxygen; therefore, no oxygen is delivered to the tissues. For this reason, carbon monoxide poisoning, either deliberate or accidental, is efficient and relatively painless. Sulfhemoglobin can be formed on exposure to agents such as sulfonamides or sulfa-containing drugs. The affinity of sulfhemoglobin for oxygen is 100 times lower than that of normal hemoglobin. It may be toxic at a very low level (Table 4.1).

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