Blood pressure regulation in the human body is predominantly orchestrated by the adrenal gland. The adrenal gland secretes hormones such as adrenaline and aldosterone, which play crucial roles in controlling blood vessel constriction and fluid balance. Adrenaline increases heart rate and constricts blood vessels, raising blood pressure during moments of stress or excitement. Meanwhile, aldosterone regulates sodium and potassium levels in the blood, impacting fluid retention and blood volume. Together, these mechanisms finely tune blood pressure to maintain proper circulation and organ function. Thus, the correct option is (a) Adrenal gland. The thyroid gland, thymus, and gall bladder do not directly regulate blood pressure.