USMLE Step 1 Respiratory Physiology Review 60 18 Left shifted curve

USMLE Step 1 Respiratory Physiology Review 60 18 Left shifted curve

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Play USMLE Audio MP3 60 18 Left shifted curve Below

USMLE Step 1 60 18 Left shifted curve Audio

Begin 60 18 Left shifted curve Transcription

The oxygen dissociation curve will be shifted to the left by the opposite of the four factors that caused a right shift.

What are the four changes that will shift the curve left rather than right? Just list them please.

  • Decreased partial pressure of carbon dioxide, decreased temperature, decreased 2,3 DPG, and increased pH.

What are two other things that will cause a shift to the left of the curve?

  • Fetal hemoglobin, that is hemoglobin F, and carbon monoxide poisoning.

If the curve is shifted to the left, for any level of oxygen in the blood, what happens to the percent saturation of hemoglobin?

  • The percent saturation of the hemoglobin is increased.

How does fetal hemoglobin result in a shift to the left?

  • The affinity of fetal hemoglobin for oxygen is increased.

Why is fetal hemoglobin’s oxygen affinity increased?

  • Fetal hemoglobin doesn’t bind 2,3 DPG as strongly as adult hemoglobin does.

What toxic gas, when present, competes for oxygen binding sites on hemoglobin?

  • Carbon monoxide

Does hemoglobin have a higher affinity for oxygen or for carbon monoxide?

  • Hemoglobin’s affinity for carbon monoxide is much higher than its affinity for oxygen.

Explain what happens to hemoglobin when carbon monoxide is present in inspired air?

  • The hemoglobin loads up with carbon monoxide rather than with oxygen.

Why, then, does the presence of carbon monoxide shift the oxygen dissociation curve to the left?

  • When there is carbon monoxide bound to the hemoglobin, there is an impaired ability to unload oxygen into the tissues.

What happens to the total oxygen content of the blood in the presence of carbon monoxide?

  • Oxygen content of the blood is reduced.

Student doctor, please pause the tape and summarize what has been discussed about a left shifted oxygen dissociation curve.

  • The oxygen dissociation curve will be shifted to the left by the opposite of the four factors that caused a right shift. That is, a left shift is caused by decreased partial pressure of carbon dioxide, decreased temperature, decreased 2,3 DPG, and increased pH. In addition to these factors, the curve will also be shifted left by fetal hemoglobin, that is, hemoglobin F, and carbon monoxide poisoning. If the curve is shifted to the left, for any level of oxygen in the blood, the percent saturation of the hemoglobin is increased. Fetal hemoglobin doesn’t bind 2,3 DPG as strongly as adult hemoglobin does. Therefore, the affinity of fetal hemoglobin for oxygen is increased which results in a shift to the left of the curve. When present, carbon monoxide competes for oxygen binding sites on hemoglobin and hemoglobin’s affinity for carbon monoxide is much higher than its affinity for oxygen. This causes the hemoglobin to load up with carbon monoxide rather than with oxygen. When there is carbon monoxide bound to the hemoglobin there is an impaired ability to unload oxygen into the tissues and total oxygen content of the blood is reduced.


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