The heart and vasculature are regulated, in part, by neural (autonomic) and humoral (circulating or hormonal) factors. Neural mechanisms primarily involve sympathetic adrenergic and parasympathetic cholinergic branches of the autonomic nervous system. In general, the sympathetic system stimulates the heart and constricts blood vessels resulting in a rise in arterial pressure. The parasympathetic system depresses cardiac function and dilates selected vascular bed. There are several very important humoral mechanisms including circulating catecholamines, the renin-angiotensin system, vasopressin (antidiuretic hormone), atrial natriuretic peptide, and endothelin. Each of these humoral systems directly or indirectly alter cardiac function, vascular function, and arterial pressure.