Oxygen content of blood
The theoretical maximum oxygen carrying capacity is 1.39 ml O2/g Hb, but direct measurement gives a capacity of 1.34 ml O2/g Hb.
1.34 is also known as Hüfner’s constant.
The oxygen content of blood is the volume of oxygen carried in each 100 ml blood.
It is calculated by: (O2 carried by Hb) + (O2 in solution) = (1.34 x Hb x SpO2 x 0.01) + (0.023 x PaO2)
SO2 = percentage saturation of Hb with oxygen
Hb = haemoglobin concentration in grams pre 100 ml blood
PO2 = partial pressure of oxygen (0.0225 = ml of O2 dissolved per 100 ml plasma per kPa, or 0.003 ml per mmHg)
For a normal adult male the oxygen content of arterial blood can be calculated:
Given arterial oxygen saturation (SpO2) = 100%, Hb = 15 g/100 ml and arterial partial pressure of oxygen (PaO2) = 13.3 kPa, then the oxygen content of arterial blood (CaO2) is:
CaO2 = 20.1 +0.3 = 20.4 ml/100 ml
Similarly the oxygen content of mixed venous blood can be calculated. Given normal values of mixed venous oxygen saturation (SvO2) = 75% and venous partial pressure of oxygen (PvO2) = 6 kPa, so:
CvO2 = 15.2 + 0.1 = 15.2 ml/100 ml
Oxygen delivery (DO2) and oxygen uptake (VO2)
Oxygen delivery is the amount of oxygen delivered to the peripheral tissue, and is obtained by multiplying the arterial oxygen content (CaO2) by the cardiac output (Q). For CaO2 = 20.1 ml/100 ml and Q = 5 l/min:
Oxygen delivery (DO2) = 1005 ml/min
The oxygen returned is given by the product of the mixed venous oxygen content (CvO2) and the cardiac output. For CvO2 = 15.2 ml/100 ml and Q = 5.0 l/min:
Oxygen return = 760 ml/min
Oxygen uptake is the amount of oxygen taken up by the tissues that can be calculated from the difference between oxygen delivery and the oxygen returned to the lungs in the mixed venous blood.
Oxygen uptake (VO2) = (oxygen delivery) – (oxygen return) = 1005 – 760 = 245 ml/min
The primary goal of the cardio respiratory system is to deliver adequate oxygen to the tissues to meet their metabolic requirements, a balance between VO2 and DO2.
The balance between oxygen uptake by the body tissues and oxygen delivery to them is assessed by:
The oxygen content of mixed venous blood CvO2, which is normally about 15 ml/100 ml
The extraction ratio, which is the ratio of VO2 to DO2 expressed as a percentage. Normally the extraction ratio is about 25% but can double to 50% if tissue demand increases
Both of the above indices are dependant on mixed venous saturation (SvO2), and cardiac output.
The figure shown below illustrates that if the level of haemoglobin is halved, the oxygen content of arterial blood will be halved.
Figure 1: Oxygen dissociation curve (ODC)
Carbon monoxide (CO) interferes with the O2 transport function of blood by combining with Hb to form carboxyhaemoglobin (COHb). CO has about 240 times the affinity of O2 for Hb. For this reason, small amounts of CO can tie up a large proportion of the Hb in the blood, thus making it unavailable for O2 carriage. If this happens, the Hb concentration and PO2 of blood may be normal, but its O2 concentration is grossly reduced. The presence of COHb also shifts the O2 dissociation curve to the left, thus interfering with the unloading of O2. This is an additional feature of the toxicity of CO.
The sigmoid shape of the oxygen dissociation curve is a result of the cooperative binding of oxygen to the four polypeptide chains. Cooperative binding is the characteristic of a haemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen. Thus, haemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.
Factors that Influence Oxygen Binding
Temperature- Increasing the temperature denatures the bond between oxygen and haemoglobin, which increases the amount of oxygen and haemoglobin and decreases the concentration of oxyhaemoglobin (Schmidt-Nielsen, 1997). The ODC shifts to the right.
pH- A decrease in pH by addition of carbon dioxide or other acids causes a Bohr Shift. A Bohr shift is characterized by causing more oxygen to be given up as oxygen pressure increases. The ODC shifts to the right.
Organic Phosphates:2,3-diphosphoglycerate (2,3-DPG) is a substance made in the red blood cells. It controls the movement of oxygen from red blood cells to body tissues. Haemoglobin uses 2,3-DPG to control how much oxygen is released once the blood gets out into the tissues. The more 2,3-DPG in the cell, the more oxygen is delivered to body tissues. 2,3 DPG binds to haemoglobin which rearranges the haemoglobin into the T-state, thus decreasing the affinity of oxygen for haemoglobin (T and R State). The ODC shifts to the right.
Hyperbaric oxygen therapy (HBOT)
This is oxygen therapy at greater than atmospheric pressure, usually 2-3 atmospheres, HBOT increases the amount of dissolved O2 in the blood according to Henry’s law. In 100 ml blood, 0.3 ml O2 dissolves at PO2 of 13.3 kPa (100mmHg). Thus for 100% O2 at 3 atmospheres, dissolved O2 = 5.7 ml. HBOT may be used in the treatment of carbon monoxide poisoning.