. In a normal person, the hemoglobin level is about 15 g per 100 ml. The
capacity of 1 g of hemoglobin to combine with O, is 1.34 ml. Therefore, arterial blood carries about
20 ml of 0./100 ml of blood. Under normal conditions, the O, level falls to about 14.4 ml/100 mi in the
venules. It indicates that under normal conditions, approximately 5 ml oxygen is transported by blood.
Under strenous conditions or during exercise, the o, level falls to about 4.4. ml/100 ml i.e.,
approximately 15 ml of O, is transported by Hb during exercise.
hinal of relationshin between po and percentage explain this
Answers
Answer:
In clinical practice, the level of arterial oxygenation can be measured either directly by blood gas sampling to measure partial pressure (PaO2) and percentage saturation (SaO2) or indirectly by pulse oximetry (SpO2).
This review addresses the strengths and weaknesses of each of these tests and gives advice on their clinical use.
The haemoglobin–oxygen dissociation curve describing the relationship between oxygen partial pressure and saturation can be modelled mathematically and routinely obtained clinical data support the accuracy of a historical equation used to describe this relationship.
Educational Aims
To understand how oxygen is delivered to the tissues.
To understand the relationships between oxygen saturation, partial pressure, content and tissue delivery.
The clinical relevance of the haemoglobin–oxygen dissociation curve will be reviewed and we will show how a mathematical model of the curve, derived in the 1960s from limited laboratory data, accurately describes the relationship between oxygen saturation and partial pressure in a large number of routinely obtained clinical samples.
To understand the role of pulse oximetry in clinical practice.
To understand the differences between arterial, capillary and venous blood gas samples and the role of their measurement in clinical practice.
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Oxygen carriage in the blood
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. Oxygen delivery is dependent on oxygen availability, the ability of arterial blood to transport oxygen and tissue perfusion [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.
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Oxygen delivery to the tissues
Oxygen delivery to the tissues each minute is the product of arterial oxygen content and cardiac output. Hence oxygen delivery can be compromised as much by a low haemoglobin concentration or low cardiac output as by a fall in the SaO2. 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. The level of oxygenation of peripheral venous blood, however, will vary depending on local metabolism and oxygen consumption. The reserve in the system is called upon, for example, during exercise when the contracting muscles extract more oxygen such that the saturation of venous blood falls. Relatively greater extraction of oxygen by vital organs also occurs if cardiac output is impaired resulting again in reduction in mixed venous saturation. The complex regulatory mechanisms involved are reviewed in detail in the physiology section of the British Thoracic Society emergency oxygen guideline [2].