The illustration above from Paul Tripp illustrates the reality of verses like these:

“Out of the abundance of the heart the mouth speaks.” (Matt. 12:34; Luke 6:45)

“Keep your heart with all vigilance, for from it flow the springs of life.” (Prov. 4:23)

For a book-length treatment of the heart from a biblical perspective, see Craig Troxel, With All Your Heart: Orienting Your Mind, Desires, and Will toward Christ.