The relationship between the heart and the kidney is mainly signified by inter-dependence between heart-yin/heart-yang and kidney-yin/kidney-yang. The heart pertains to fire in the five elements and is located in the upper energizer; the kidney pertains to water in the five elements and is located in the lower energizer. Heart-fire (heart-yang) has to descend to the kidney to warm kidney-yang and prevent abnormal flow of water; kidney-water (kidney-yin) has to ascend to nourish heart-yin and prevent hyperactivity of heart-yang.
Such a coordination between the upper and the lower is called "coordination between the heart and the kidney" or "coordination between water and fire". If kidney-yin fails to nourish heart-yin due to insufficiency, it will lead to deficiency of heart-yin and hyperactivity of heart-yang, bringing on dysphoria, insomnia, tinnitus and weakness of the waist and knees. If heart-fire fails to descend, it will cause heat in the upper and cold in the lower, leading to dysphoria, insomnia and cold sensation below the waist and knees. Such pathological cases are called "disharmony between the heart and the kidney".
Besides, the heart governs blood and the kidney stores essence. Blood and essence can transform into each other. So insufficiency of heart-blood will cause deficiency of kidney-essence and deficiency of kidney-essence, in turn, will result in insufficiency of heart-blood, consequently leading to disorder of both blood and essence with the symptoms of pale complexion, palpitation, tinnitus and weakness of the waist and knees.