Haemostasis depends on an intricate relation among plasma coagulation and fibrinolytic factors, blood cells, vessel walls, extracellular matrix, blood viscosity, and blood flow. Hormonal systems, such as the renin-angiotensin system (RAS) act as a protector against acute fluid volume loss by inducing both vasoconstriction to maintain blood pressure and the retention of salt and water to restore normal fluid volume. Studies have confirmed that this cascade of events is brought into action by the RAS in coordination and sequentially. Inappropriate activation of the RAS may occur, however, in patients with left ventricular dysfunction and heart failure. Angiotensin-converting enzyme inhibitors are indicated to counteract the vasoconstriction and salt and water retention associated with activation of the RAS.