<qa>  What is the main factor in understanding the pathophysiology of resistant hypertension?Pathophysiology of resistant hypertension: The mechanisms involved in the pathophysiology of RH are the vascular smooth muscle tone and increased blood volume, intensification of the activity of the sympathetic system and hyperactivity of the renin-angiotensin-aldosterone system RAAS. Increased sensitivity to sodium appears to be the main factor in understanding the pathophysiology of this syndrome, not only as it incorporates the above mechanisms, but also as it explains, in part, the variability of therapeutic response of patients with RH. The RAAS is vital to the regulatory system that controls total body sodium as are atrial natriuretic peptide factors and pressure receptors in the atria and kidney. Sodium and water retention can lead to resistance against antihypertensive drugs. Under the physiological point of view, BP is maintained by continuous regulation of cardiac output and peripheral vascular resistance exerted at three anatomic sites: arterioles, postcapillary venules capacitance vessels and the heart. A fourth anatomical site of control, the kidney, contributes to the maintenance of BP by regulating intravascular volume. 