Spaceflight studies have demonstrated that adaptation to gravitational stress after prolonged microgravity includes sympathetic activation, water retention, and arterial pressure increase, i.e. a process very similar to development of essential hypertension, which from this perspective looks like an advanced stage of that adaptation with sympathetic hyperactivity, vasoconstriction, volume overload, and arterial hypertension in case of some further increase in gravitational stress. This book contains theoretical analysis of human cardiovascular function in real conditions of Earth gravitational field leading to the scientifically sound hypothesis of essential hypertension as advanced stage of natural adaptation of the human cardiovascular system to abnormally increased gravitational stress associated with modern 'sitting' lifestyle. Also clinical and experimental data are presented supporting this hypothesis according to comprehensive literature search. In upright position gravitation shifts blood downwards emptying upper body and reducing blood supply to the brain. Passively filling heart cannot pump blood out of the lower into the upper body. Compensation is provided through lower body vasoconstriction and circulating volume increase. The gravitational stress (GS) in the cardiovascular system in an upright position may be defined as amount of work necessary to return the blood upward and maintain adequate upper-body circulation calculated as the product of the gravitational potential (Ugr=g*h) and the mass of blood shifted by gravitation: GS=U*Mshift. In a complex vascular network, this blood shift is actually a function of time, estimated in a first approximation as Mshift(t)=(Ugr*t)/Rd (Rd: the resistance to downward blood flow) so that gravitational stress is proportional to the time spent upright GS=U*Mshift(t)=(Ugr2*t)/Rd=(g2*h2*t)/Rd. From this analysis typical for modern life regular, prolonged sitting should cause a significant increase in gravitational stress in the cardiovascular system, requiring advanced antigravitational response with sympathetic hyperactivity, vasoconstriction, volume overload, and arterial hypertension. The hypertensive effect of prolonged sitting has been directly demonstrated in several clinical studies. Numerous population studies demonstrated higher prevalence of hypertension for occupations with predominantly sitting posture during worktime. Thus, essential hypertension in scientifically sound way is explained as adaptation to increased gravitational stress resulted from modern «sitting» lifestyle. This gravitational hypothesis of essential hypertension fully integrates two existed major concepts of sympathetic hyperactivity and abnormal sodium reabsorption as complementary mechanisms of antigravitational response but contrary to them offers a way to complete healing of the disease through elimination of the primary factor of abnormal gravitational stress.