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 plurality of coronary arteries emanating from the patient's aorta; create a three-dimensional model representing the anatomical structure using the patient-specific data; estimate a mass or volume of myocardial tissue of the patient using one or both of the received patient-specific data and the created three-dimensional model; create a blood flow model defining blood flow through the anatomical structure of the patient and modify the created blood flow model using, at least in part, a coronary blood flow associated with the estimated mass or volume of myocardial tissue of the patient; and determine at least one blood flow characteristic within the anatomical structure using the three-dimensional model and the blood flow model.