As you know, the English word 'heart' has several meanings. 1. a hollow, pumplike organ of blood circulation, composed mainly of rhythmically contractile smooth muscle, located in the chest between the lungs and slightly to the left and consisting of four chambers. -> 심장 in Korean. 2. the center of the total personality, especially with reference to intuition, feeling, or emotion. the center of emotion, especially as contrasted to the head as the center of the intellect -> 마음 in Korean. 3. the innermost or central part of anything. -> 핵심 in Korean 4. the breast or bosom -> 가슴 in Korean. Thus, you must use appropriate word according to the context.

hi. basically it's different. 심장 is heart as part of your body. =>my heart is strong. 마음 is mind.=> you have a warm mind. thank you.