BACKGROUND: Although a rapid rise in left atrial pressure during exertion is considered pathognomonic of heart failure with preserved ejection fraction (HFpEF), the fundamental circulatory determinants of this response are not clear, impacting upon the development of more effective therapies. We aimed to comprehensively describe the circulatory mechanics of patients with HFpEF at rest and during exercise in comparison with controls. METHODS AND RESULTS: We performed simultaneous right-heart catheterization and echocardiography at rest and during exercise in 22 healthy control volunteers and 60 patients with confirmed HFpEF. Using detailed individual patient-level hemodynamic and left ventricular ejection fraction data we performed computer simulations to evaluate the circulatory parameters including the estimated stressed blood volumethat contribute to the resting and exercise pulmonary capillary pressure. At rest and during exercise, left ventricular stiffness (V₃₀, the end-diastolic pressure–volume relationship at a filling pressure of 30 mm Hg), left ventricular elastance, and arterial elastance were all significantly greater in HFpEF than in controls. Stressed blood volume was significantly greater in HFpEF (26.9±5.4 versus 20.2±4.7 mL/kg, P<0.001), becoming even more pronounced during exercise (40.9±3.7 versus 27.5±7.0 mL per 70 kg, P<0.001). During exercise, the magnitude of the change in stressed blood volume (r=0.67, P<0.001) and left ventricular stiffness (r=−0.44, P<0.001) were key determinants of the rise in pulmonary capillary wedge pressure. Further detailed modeling studies showed that the hemodynamic response to exercise results from a complex non-linear interaction between circulatory parameters. CONCLUSIONS: The circulatory determinants of HFpEF physiology are complex. We identified stressed blood volume at rest and during exercise is a novel, key factor, therebyrepresenting an important potential therapeutic target.