Question

Explain the relationship between a patient’a O2 content and O2 saturation of hemoglobin?

Answer 1

The oxygen concentration (usually termed “oxygen content”) of systemic arterial blood depends on several factors, including the partial pressure of inspired oxygen, the adequacy of ventilation and gas exchange, the concentration of haemoglobin and the affinity of the haemo­globin molecule for oxygen. Of the oxygen transported by the blood, a very small proportion is dissolved in simple solution, with the great majority chemically bound to the haemoglobin molecule in red blood cells, a process which is reversible.

The content (or concentration) of oxygen in arterial blood (CaO2) is expressed in mL of oxygen per 100 mL or per L of blood, while the arterial oxygen saturation (SaO2) is expressed as a percentage which represents the overall percentage of binding sites on haemoglobin which are occupied by oxygen.

In healthy individuals breathing room air at sea level, SaO2 is between 96% and 98%.The maximum volume of oxygen which the blood can carry when fully saturated is termed the oxygen carrying capacity, which, with a normal haemoglobin concentration, is approximately 20 mL oxygen per 100 mL blood.

Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it. A hemoglobin molecule can bind up to four oxygen molecules in a reversible way. The oxygen content of the blood does not change significantly even with large increases in the oxygen partial pressure. To get more oxygen to the tissue would require blood transfusions to increase the hemoglobin count (and hence the oxygen-carrying capacity), or supplemental oxygen that would increase the oxygen dissolved in plasma.