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The level of oxygenation of peripheral venous blood, however, will vary depending on local metabolism and oxygen consumption. Following circulation through the tissues, the average oxygen saturation in the venous blood returning to the right side of the heart (mixed venous blood) is typically about 75% in healthy individuals at rest, a figure which implies a considerable “reserve” in the oxygen delivery system. Hence oxygen delivery can be compromised as much by a low haemoglobin concentration or low cardiac output as by a fall in the S aO 2. Oxygen delivery to the tissues each minute is the product of arterial oxygen content and cardiac output. In healthy individuals breathing room air at sea level, S aO 2 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. The content (or concentration) of oxygen in arterial blood ( C aO 2) is expressed in mL of oxygen per 100 mL or per L of blood, while the arterial oxygen saturation ( S aO 2) is expressed as a percentage which represents the overall percentage of binding sites on haemoglobin which are occupied by 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 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. Oxygen delivery is dependent on oxygen availability, the ability of arterial blood to transport oxygen and tissue perfusion.
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The main function of the circulating blood is to deliver oxygen and other nutrients to the tissues and to remove the products of metabolism including carbon dioxide. The clinical role of venous blood gases however remains less well defined. While arterial sampling remains the gold-standard method of assessing ventilation and oxygenation, in those patients in whom blood gas analysis is indicated, arterialised capillary samples also have a valuable role in patient care. The use of pulse oximetry reduces the need for arterial blood gas analysis ( S aO 2) as many patients who are not at risk of hypercapnic respiratory failure or metabolic acidosis and have acceptable S pO 2 do not necessarily require blood gas analysis. Oxygen saturation by pulse oximetry ( S pO 2) is nowadays the standard clinical method for assessing arterial oxygen saturation, providing a convenient, pain-free means of continuously assessing oxygenation, provided the interpreting clinician is aware of important limitations. In a study of 3524 clinical specimens, we found that this equation estimated the S O 2 in blood from patients with normal pH and S O 2 >70% with remarkable accuracy and, to our knowledge, this is the first large-scale validation of this equation using clinical samples. Historically this curve was derived from very limited data based on blood samples from small numbers of healthy subjects which were manipulated in vitro and ultimately determined by equations such as those described by Severinghaus in 1979.
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The haemoglobin–oxygen dissociation curve, a graphical representation of the relationship between oxygen saturation and oxygen partial pressure helps us to understand some of the principles underpinning this process. The delivery of oxygen by arterial blood to the tissues of the body has a number of critical determinants including blood oxygen concentration (content), saturation ( S O 2) and partial pressure, haemoglobin concentration and cardiac output, including its distribution.