def find_exchange_reactions(model):
    """ Finds the exchange reaction in given FBA model.

    Currently the exchange rids are found via 
    prefix EX_.
    
    :param model: SBML model
    :returns dictonary of ex_rids: sids
    """
    ex_rids = {}
    for reactions in model.getListOfReactions():
        rid = reactions.getId()
        if rid.startswith('EX_'):
            r = model.getReaction(rid)
            sid = r.getReactant(0).getSpecies()
            ex_rids[rid] = sid
    return ex_rids
