Why does oxygen move into the blood and carbon dioxide move out? You learn about partial pressure, the thin membrane of the alveolus, the part that haemoglobin plays in carrying oxygen, and how to read the oxygen dissociation curve.
3 lessons, about 1 h 25 min3 simulations11 problems3 practice questionsPython in every lesson
Created by StemPy LabGases move across the walls of the alveoli and the tissues by simple diffusion. The direction depends on the partial pressures, and the rate on the gradient, the solubility of the gas and the thickness of the membrane. Carbon dioxide is 20 to 25 times more soluble than oxygen, and the three-layered membrane of the alveolus is much thinner than a millimetre.
Blood is the medium that carries the gases. About 97 per cent of the oxygen travels in the red blood cells, bound to haemoglobin, and about 3 per cent is dissolved in plasma. One haemoglobin molecule carries at most four oxygen molecules. Every 100 mL of oxygenated blood gives about 5 mL of oxygen to the tissues.
The oxygen dissociation curve is an S-shaped graph of the per cent saturation of haemoglobin against pO2. It is nearly flat at the high pO2 of the lungs and steep at the pO2 of the tissues. A rise in pCO2, in hydrogen ions or in temperature shifts it to the right, so oxygen is released more easily.
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