Every minute at rest the human body consumes about 200 mL of oxygen (O2) and produces about 200 mL of carbon dioxide (CO2).
Oxygen
Forms of oxygen carriage and haemoglobin binding
O2 is poorly soluble and so only 1.5% is transported dissolved in plasma. The remaining 98.5% is bound to haemoglobin (Hb) inside erythrocytes. Each erythrocyte contains 250–640 million Hb molecules (and there are 4.5–5.1 million erythrocytes per 1 mm3 of blood). Hb is a protein composed of four polypeptide chain subunits. Each subunit is bound to an iron-containing porphyrin haem group. Because ferrous (Fe2+) ions serve as O2 binding sites, each Hb can rapidly and reversibly bind four O2 molecules (Figure 25.1). A Hb molecule bound with O2 is called oxyhaemoglobin (HbO2), whereas a Hb molecule that has released its O2 is called deoxyhaemoglobin. As O2 binds it changes the optical absorption spectrum of the Hb and hence the colour of blood from cyan to red. Thus with an increase in the relative proportion of deoxyhaemoglobin to oxyhaemoglobin, a bluish discolouration of the skin and mucous membranes can be observed, which is referred to as cyanosis.
When the first O2 molecule binds the first iron molecule, the Hb changes shape, and the affinity for the other three O2 molecules progressively increases. Similarly, when the first O2 is unloaded, the affinity for O2 is decreased and it becomes progressively easier for the other three O2 molecules to dissociate from the Hb. Hb is fully saturated when all four haem groups are bound to O2. If fewer than all four haem groups are bound, the Hb is said to be partially saturated. Binding of the second is easier than the first, binding of the third is easier than the second, and binding of the fourth is easier than the third. The fourth O2 molecule to bind has 300 times the affinity of the first. Thus, the affinity of Hb for O2 changes depending on how much O2 is bound to the Hb. The more saturated Hb is, then the easier it is to bind more O2, and the harder it is to unbind O2 from that Hb. So, binding gets more efficient as each O2 binds, and release gets easier as each O2 is released.
Oxyhaemoglobin dissociation curve
The association between O2 and Hb is represented by a sigmoid-shaped (S-shaped) curve (Figure 25.2), because changes in affinity of Hb depend on O2 saturation. The curve has a steep slope between 1 and 6 kPa PO2 and a plateau between 9.3 and 13.3 kPa PO2. The upper linear portion of the curve reflects where both O2 loading in the lungs (from venous to arterial) and unloading in the tissues (from arterial to venous) occurs. Hb saturation (%; ‘O2 sats’) depends on PO2, not normally on Hb content. Near 100% Hb saturation implies healthy gas exchange, but could still occur with anaemia (reduced Hb content). O2 content depends on both PO2 and Hb content. So, low Hb content can cause tissue hypoxia, even if both PO2 and Hb saturation are normal.
O2 tension refers to the PO2 difference between two sites, determining the direction and flow of O2. Changes in O2 tension do not correspond to equal changes in Hb saturation. As blood moves from arteries through tissues to veins there is a large decrease in PO2 (13.3 to 6 kPa), associated with a much smaller decrease in Hb saturation (98% to 75%). PO2 falls by more than 50%, whereas O2 sats fall by less than 25%. This occurs because it is hardest to unload the first O2 molecule and thus initially PO2 falls without much decrease in O2 sats. Hb is almost completely saturated at any PO2 above 9.3 kPa, enabling normal O2 delivery to tissues. When PO2 decreases to below 9.3 kPa, Hb more readily gives up O2. So, when blood arrives at tissues with low PO2 it readily unloads O2. A significant amount of O2 is still available in venous blood and can be unloaded from blood to tissues if required (e.g. during vigorous exercise). Since Hb in arterial blood is 98% saturated, heavy breathing during exercise increases PO2 above the normal 13.3 kPa, but causes almost no change in % saturation. Thus more Hb or increased cardiac output is needed to increase the transport of O2.
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