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their walls are too thick. The concentration of oxygen in the blood arriving at the capillary bed is therefore the same as blood leaving the lungs. Blood arriving at the tissues has been cleansed of carbon dioxide and saturated with oxygen during its passage through the lungs. Therefore, it has a higher concentration of oxygen and a lower concentration of carbon dioxide than the tissues. This creates concentration gradients between the blood and the tissues, and gaseous exchange occurs.
Oxygen diffuses from the bloodstream through the capillary wall into the tissues. Carbon dioxide diffuses from the cells into the extracellular fluid, then into the bloodstream towards the venous end of the capillary.
Lung volume and capacity Only about 10 percent of the total respiratory capacity is used during each breath in quiet breathing. An average breath during ordinary quiet breathing, known as the tidal volume ('A' in the diagram below), is about half a litre, although the total amount of air which can be inhaled may be as much as 4.5 to 5 litres. This is called the vital capacity ('B' in the diagram). There is always a reserve volume above and below the tidal volume ('C' and 'D' in the diagram, respectively), so that inhalation as well as exhalation may be increased. There is also a certain volume of air which always remains in the lungs, no matter how completely one exhales; this is called the residual volume ('E' in the diagram) and it is just a bit more than one litre.
So, although the vital capacity may be 5 litres, the total lung volume would be about 6 litres. Most of the tidal volume goes to lung areas that exchange oxygen and carbon dioxide with blood, but about 150 ml is dead space from passages that cannot contribute to gas exchange. Dead space volume is relatively constant whereas tidal volume varies greatly with physical exercise, breathing pattern, and other factors. Thus, larger tidal volumes have a smaller proportion of dead space. Dead space can increase significantly with lung disorders.
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