Question 3

Compare and contrast the body’s bicarbonate, phosphate and protein buffer systems.

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College Answer

This question was generally well answered. Successful candidates illustrated their answer through the use of a table. For a good pass candidates were expected to include a definition of a buffer; mention the buffering capabilities of the differing buffering systems (bicarbonate, phosphate and protein buffers) in relation to the pKa; the area in the body where they are most effective and whether it was an open or closed system.

The protein buffer system was the least well conveyed by candidates, despite approximately 60 to 70 per cent of the total chemical buffering of the body fluids being inside the cells, and most of this from the intracellular proteins. For a good answer it was expected that candidates also mention that, except for the red blood cells, the slowness with which H+ and HCO3 - move through the cell membranes often delays for several hours the maximum ability of the intracellular proteins to buffer extracellular acid-base abnormalities. In addition to the high concentration of proteins in the cells, another factor that contributes to their buffering power is the fact that the pKas of many of these protein systems are fairly close to 7.4.