Transport of Oxygen

Oxygen is transported in human blood by two parallel routes, though these two routes are very far from equal in how much oxygen they actually carry. A genuinely small fraction of the total oxygen content of blood -- only about 3% -- is simply dissolved directly in the blood plasma, in accordance with the gas's own, fairly limited, solubility in an aqueous fluid at body temperature; on its own, this dissolved fraction would fall very far short of meeting the body's actual oxygen demand. The overwhelming majority of oxygen carried in blood, therefore, is instead bound reversibly to haemoglobin, the iron-containing respiratory pigment packed in very large quantities within red blood cells, each molecule of haemoglobin capable of binding up to four molecules of oxygen (one per haem group) to form oxyhaemoglobin.

The binding of oxygen to haemoglobin is not a simple, one-step, all-or-nothing reaction; instead, haemoglobin's four haem groups bind oxygen cooperatively, meaning that the binding of one oxygen molecule to one haem group makes it progressively easier for the next oxygen molecule to bind to a neighbouring haem group on the very same haemoglobin molecule. This cooperative behaviour is the direct chemical reason why a graph of haemoglobin's percentage oxygen saturation against the partial pressure of oxygen (pO2) it is exposed to -- the oxyhaemoglobin dissociation curve -- takes on its characteristic sigmoid (S-shaped), rather than a straight or simply curved, shape: saturation rises comparatively slowly at very low pO2, then rises very steeply across an intermediate range of pO2, and finally levels off, approaching full saturation, at high pO2.

This sigmoid shape is precisely what makes haemoglobin such an effective oxygen carrier under the real physiological conditions the body actually presents it with. At the high pO2 found in alveolar air (approximately 95 to 104 mm Hg), haemoglobin sits on the flat, upper portion of the curve and becomes almost fully saturated (typically 97 to 98%) -- so the lungs load haemoglobin with very nearly the maximum oxygen it is capable of carrying. At the considerably lower pO2 typical of actively respiring tissue (approximately 40 mm Hg, and lower still in more actively metabolising tissue), however, haemoglobin sits instead on the steep, middle portion of the curve, where its saturation falls off sharply -- to roughly 60 to 70% at rest, or considerably lower in tissue that is metabolising particularly actively -- so that a substantial fraction of the oxygen haemoglobin was carrying is unloaded exactly where, and because, it is most needed. …