Carriage Of Oxygen
At rest, a man consumes about 250 mL oxygen/min, which may rise to more than 4000 mL/min in exercise if he is very fit Oxygen diffuses from alveolus to blood until equilibrium is reached when pulmonary capillary Po2 equals alveolar Po2. The solubility of oxygen in blood is low - 0.000225 mL oxygen per mL of blood per kPa (0.00003 mL/mL/mmHg) - so that at a normal arterial Po2 of 13.3 kPa (100 mmHg) there is only 3 mL dissolved in each litre of blood. The main function of the red blood cell pigment, haemoglobin, the key features of whose structure is shown in Fig. 8a, is to carry the large quantities of oxygen needed by the tissues.
Each gram of haemoglobin combines with up to 1.34 mL oxygen, so with a haemoglobin concentration, [Hb], of 150 g/L, blood contains a maximum of 200 mL/L oxygen bound to haemoglobin. This is known as the oxygen capacity, which varies with [Hb]. The actual amount of oxygen bound also depends on the Po2. The percentage of the available binding sites bound to oxygen is known as the oxygen saturation.
Amount of oxygen bound to haemoglobin (mL/L)
Oxygen capacity (mL/L) × 100%
The oxygen content of the blood (mL/L) equals the sum of haemoglobin-bound oxygen and the small amount of dissolved oxy- gen. The rate of rise of oxygen content with increasing partial pressure depends on the number of free haemoglobin-binding sites remaining and their affinity for oxygen. As each oxygen molecule binds in turn to the four haem groups, the quaternary structure alters and the affinity of the remaining binding sites for oxygen increases. This cooperative binding increases the steepness of the oxygen-haemoglobin dissociation curve in the middle (Fig. 8b), but the curve flatten again at partial pressures above about 8 kPa (60 mmHg) because there are few unfille binding sites remaining. In arterial blood, Po2 is normally about 13 kPa (100 mmHg), oxygen saturation about 97%, and, with a normal [Hb], an oxygen content of about 200 mL/L. Rises or modest falls in Po2 from 13 kPa (100 mmHg), for example during hyperventilation or mild hypoventilation, cause little change in the arterial oxygen content, as the dissociation curve is fla in this region. More severe reductions in Po2, to levels in the steep region (<8 kPa, 60 mmHg), are associated with significan reductions in oxygen saturation and content. Consequently, breathing oxygen-enriched air may significantl raise arterial oxygen content and hence exercise capacity at high altitude and in patients with chronic hypoxic respiratory disease, but has little effect on a healthy person at sea level.
Low Po2 in tissue capillaries causes oxygen release from haemoglobin, whereas the high Po2 in pulmonary capillaries causes oxygen binding. The affinity of haemoglobin for oxygen, and hence the position of the dissociation curve, varies with local conditions. A reduced oxygen affinity, shown by a right shift in the curve, is caused by a fall in pH, a rise in Pco2 (the Bohr effect) or increased temperature (Fig. 8b). These changes occur in metabolically active tissues such as exercising muscle and encourage oxygen release. In the lungs, oxygen uptake is aided by the increasing affinity of haemoglobin for oxygen, caused by falling Pco2 and temperature and increased pH and reflecte by a left shift of the curve. The Po2 at which the haemoglobin is 50% saturated is known as the P50. Under normal arterial conditions (pH = 7.4, Pco2 = 5.3 kPa or 40 mmHg, temperature = 37◦C) P50 = 3.5 kPa (26.3 mmHg); right shifts raise the P50 and left shifts lower it. A rise in the concentration of 2,3-di(or bi)phosphoglycerate (2,3-DPG), which is a by-product of glycolysis in red cells, also causes a right shift. A rise in 2,3-DPG occurs in anaemia, causing a modest increase in P50. Blood bank storage causes progressive depletion of 2,3-DPG and an undesirable left shift, but this can be minimized by storing the blood with citrate-phosphate-dextrose.
Anaemia and carbon monoxide poisoning In anaemia, at any given Po2, the oxygen content is reduced because of the reduced concentration of binding sites. Figure 8c shows the dissociation curve for normal blood and for blood with [Hb] = 75g/L. Alveolar and arterial Po2 is normal in anaemia and therefore arterial O2 content is 100 mL/L. At rest, the tissues need to remove about 50 mL/L of oxygen from the blood passing through them. To achieve this mixed venous content, Po2 will need to fall to about 5.3 kPa (40 mmHg) (A in Fig. 8c) when [Hb] = 150 g/L and about 3.6 kPa (27 mmHg) (B) when [Hb] 75 g/L. The reduced venous and hence capillary Po2 reduces the partial pressure gradient driving diffusion of oxygen to the tissues, which is adequate at rest but which may become inadequate in exercise when oxygen consumption increases.
Figure 8c also shows the dissociation curve for blood that has 50% of oxygen-binding sites occupied by carbon monoxide (CO, dashed line). Arterial oxygen content is 100 mL/L, but there is also an altered shape and leftward shift of the dissociation curve, because CO binding increases the affnity of the remaining (CO-free) sites for oxygen. This impairs oxygen release in the tissues. Mixed venous Po2 will now have to fall to 2 kPa (15 mmHg) (point C) to release the 50 mL/L required, and this will greatly reduce the pressure gradient for diffusion. At about 50-60% carboxyhaemoglobin, symptoms of impaired cerebral oxygenation (headache, convulsions, coma and death) are severe, whereas anaemic patients with the same arterial oxygen content are typically asymptomatic at rest. Haemoglobin has a high aff nity for CO ( 240 times that for oxygen), so breathing even at low concentrations causes a progressive increase in the cherry-red carboxyhaemoglobin. A cherry- red complexion is sometimes a feature of CO poisoning, although pallor and cyanosis (discussed in Chapter 23) are more common.
Other Respiratory Pigments
Fetal haemoglobin, HbF, differs from adult haemoglobin, HbA, in that there are two γ -chains instead of two β-chains. The HbF dissociation curve lies to the left of that for HbA, reflectin its higher O2 affinit . This difference is enhanced by the double Bohr shift: in the placenta Pco2 moves from the fetal to maternal blood, shifting the maternal curve further right and the fetal curve further left. The high affinit of HbF relative to HbA helps transfer oxygen from mother to fetus, and even though blood returning from the placenta to the fetus in the umbilical vein has a Po2 of only about 4 kPa (30 mmHg), its saturation is 70%. Oxygen transport in the fetus is also helped by a high [Hb] of about 170-180 g/L.
Myoglobin, the respiratory pigment found in muscle, is composed of a single haem group attached to a single globin chain. With no cooperative binding, its dissociation curve is hyperbolic. It is also far to the left of HbA and its high affinity means that its oxygen store is only released when local Po2 is severely reduced, for example in heavy exercise.