Arrows indicate actions (or responses) that are triggered or movement of certain elements. Most importantly: dark blue arrows indicate movement, whereas green and red arrows indicate a respectively simulatory action (positive response) or a inhibitory action (negative/feedback response). To indicate that (for example) a drug changes such an action, the line becomes dashed.
Other symbols are more indicative to ease the readability (e.g. the 'description pointers' or the 'zoombox'), and generally speak for themselves.