Heart has 2 known aspects, one of it has link to the physical aspect of human body including veins, blood and others physical links to our body parts. The other aspect is the emotions which can be love, depression, happiness, anger, ego, compassion etc... Apart from these 2 know aspect let me also help you know a different aspect of Heart. This is about the link that we make with something apart from us (physically). If we try to see synonyms of the word Heart, we can see the word "Spirit" in that list. Spirit is something which is not visible but yet very important. What I am trying to highlight in this post is the relation of Heart and Spirit.
My son, give me your heart and let your eyes observe and delight in my ways,
Physical aspect of Heart is in control very easily, but the spiritual aspect of Heart has to be learned to be controlled. When our spirit is linked with someone or something then doesn't matter where it is or will be we will be linked to it always. Some people have their heart (spirit) linked to their job, so what we see for them is that doesn't matter where they will be they will be talking and thinking about their job and its related activities and planning. Unconsciously we will be linked to where our heart (spirit) will be attached.