Hi Josh R.!
H2PO4- and HPO42- make up a conjugate acid-base pair, and therefor constitute a buffered solution. The function of any buffered solution is as a guard against large changes in pH when acid or base is added to the system. In particular, the pKa of H2PO4- is 7.21, which is just around physiological pH.
Within the context of the human body, phosphate buffer can be found in the interstitial fluid, blood, intercellular fluid and urine.
However, the concentration of phosphate buffer is unlikely to be sufficiently high to play a major role in the blood or the interstitial fluid. Instead, the bicarbonate system is a major player in the interstitial fluid to buffer against metabolic acids, while both the bicarbonate system and hemoglobin are the major players in the blood. The bicarbonate system again guards against metabolic acids, while the hemoglobin buffers carbon dioxide.
Nevertheless, the phosphate system has a sufficiently high concentration to exert major effects buffering intracellularly and the urine. As mentioned above, at the pH values prevailing in the human body, H2PO4- and HPO42- are the two phosphate species present owing to the pKa values for the different phosphate species and the pH found in many compartments of the human body.
For reference, the pKa values are as follows:
H3PO4 <----> H2PO4- pKa1 = 2.12
H2PO4- <----> HPO42- pKa2 = 7.21
HPO42- <----> PO43- pKa3 = 12.44
Hope this helps!
Cheers
