An acceleration in oxygen dissociation from hemoglobin, and thus oxygen delivery to the tissues, is caused by:
A. A decreasing oxygen pressure in the blood.
B. An increasing carbon dioxide pressure in the blood.
C. A decreasing oxygen pressure and/or an increasing carbon dioxide pressure in the blood.
D. An increasing oxygen pressure and/or a decreasing carbon dioxide pressure in the blood.
Correct Answer: C. A decreasing oxygen pressure and/or an increasing carbon dioxide pressure in the blood.
The lower the PO2 and the higher the PCO2, the more rapidly oxygen dissociated from the oxyhemoglobin molecule. Factors that contribute to a right-shift in the oxygen dissociation curve and favor the unloading of oxygen correlate with exertion. These include increased body temperature, decreased pH (due to increased production of CO2), and increased 2,3-BPG. (Figure) This right shift of the oxyhemoglobin curve can be viewed as an adaptation for physical exertion.
Option A: In the setting of hypoxia or low blood oxygen levels, irreversible tissue damage can rapidly occur. Hypoxia can be the result of an impaired oxygen-carrying capacity of the blood (e.g., anemia), impaired unloading of oxygen from hemoglobin in target tissues (e.g., carbon monoxide toxicity), or from a restriction of blood supply.
Option B: Hemoglobin (Hgb or Hb) is the primary carrier of oxygen in humans. Approximately 98% of total oxygen transported in the blood is bound to hemoglobin, while only 2% is dissolved directly in plasma. Hemoglobin is a metalloprotein with four subunits, each composed of an iron-containing heme group attached to a globin polypeptide chain. One molecule of oxygen can bind to the iron atom of a heme group, giving each hemoglobin the ability to transport four molecules of oxygen.
Option D: The body maintains adequate oxygenation of tissues in the setting of decreased PO or increased demand for oxygen. These changes often express shifts in the oxygen dissociation curve, which represents the percentage of hemoglobin saturated with oxygen at varying levels of PO.
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