1. A system for determining cardiovascular information for a patient, the system comprising: at least one computer system configured to: receive patient-specific data regarding a geometry of an anatomical structure of the patient, the anatomical structure including at least a portion of a patient's coronary arteries; create a three-dimensional model of at least an area of interest within the portion of the patient's coronary arteries using the received patient-specific data; generate a reduced-order model of a second area of the portion of the patient's coronary arteries that is adjacent to the area of interest and coupled to the three-dimensional model of at least the area of interest; solve equations of blood flow through the portion of the patient's coronary arteries using the three-dimensional model and the reduced-order model; and calculate a value of a blood flow characteristic within at least the area of interest using one or more solutions to the equations of blood flow.