The oxygendissociation curve

Oxygen carriage in the blood relies on haemoglobin - the concentration, its carrying capacity and saturation. The percentage of Hb saturation with oxygen at different partial

Hb + O2 HbO2

Oxygen tenion (kPa)

Figure 2.3 The oxygen-dissociation curve. Sigmoid shape is due to "positive cooperativity". Acidosis, raised 2,3DPG and raised temperature all cause the curve to shift to the right. Alkalosis, myoglobin and fetal Hb cause shift to the left. The shift caused by pH is called the Bohr effect pressures of oxygen in blood is described by the oxygen-dissociation curve (Figure 2.3). It is sigmoid shaped because oxygen molecules demonstrate "positive cooperativity" when associating with Hb. Each Hb molecule carries four oxygen molecules. The first oxygen molecule helps the others to associate as well. The reason this curve is so important and clever is that large amounts of oxygen can be released from the blood for only a small drop in partial pressure in the capillaries (because of the steep lower part of the curve). Other local factors affect the curve, or the affinity of Hb to carry oxygen, like acidosis, which shifts it to the right. This means more oxygen is released in acidotic tissue, which needs it. The oxygen-dissociation curve flattens after 94% saturation (usually equivalent to a PaO2 of 8 3 kPa or 64 mmHg).

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