Answer: HCO3- + H3O --> H2CO3 + H2O (If an a strong acid is present)
H2CO3 + OH- --> HCO3- + H2O (If a strong base is present)
Step-by-step explanation: Thankfully, we have buffers in our blood, without this buffer, one cup of lemon juice contains enough Hydronium ions (H3O) to kill us. This brings us to our first reaction. In this H2CO3/HCO3- buffer system, HCO3- acts as a weak base. If some strong acid that 100% dissociates into H3O+ happens to enter your blood, instead of the pH decreasing significantly it will only decrease slightly as HCO3- eliminates the H3O+ maintaining a relatively equal pH.
On the other hand, we have the H2CO3 reaction, if, for example, you decided to inject yourself with NaOH (Don't do it) it will dissociate 100% into OH. Since H2CO3 is a weak acid it will react with a strong base eliminating the OH- thus maintaining a stable pH.
Hope this helps. and if you need an explanation of why pH goes up slightly please comment. Good luck with Acid/Base/Equilibrium!!