One is conscious of things like pain, yellow afterimages, and the urge to breathe (when holding one's breath), but one is seldom conscious of the actions of the stomach or of one's pupils. Like motor programs and low-level perceptual processing, the latter are unconscious. Science is beginning to understand the nature and function of conscious awareness like never before. For example, we now know why we are seldom aware of the complicated actions of the digestive tract or the pupils, which dilate or constrict depending on light conditions. These actions are very visible in someone with light-colored eyes. In contrast, we are almost always aware of the urge to breathe when holding our breath under water, or our urge to drop a hot dish that we are carrying from the kitchen to the dinner table.
One difference between the action tendencies that we are aware of (e.g., to breathe when holding one's breath underwater) and the ones that we're never aware of (e.g. the 'urge' [sic] to dilate or constrict the pupil) depends on the kind of muscle that the brain is trying to control: If it is skeletal muscle--the kind of muscle that is attached to our bones--we have conscious urges; if it's another kind of muscle, like the muscle controlling the pupillary reflex, there is no conscious awareness about the action.
Figuratively speaking, the skeletal muscle system is like a big steering wheel that different parts of the brain are trying to control at the same time. When these different systems are in conflict, as when one voluntarily holds one breath--and part of us wants to inhale and another part of us does not want to inhale--we have a strong conscious experience. Consciousness is the way the different regions communicate with each other. Without consciousness, there would be no cross-talk between the different systems and one would inhale reflexively. One doesn't need consciousness to withdraw from a painful stimulus, but one does need it in order to keep touching it. For current media coverage of this research, click here; for a more advanced review, see Three Kinds of Bindings on this website.
Another way to think about it is as follows. There are many quasi-independent computers in the brain, and each can do complicated things and influence overt action, which we can think of as influencing the actions of a printer. Each computer can influence the printer, but in order for two computers to interact and then influence the printer, you need a wifi system. Consistent with the integration consensus (stating that consciousness integrates information/processes that would be independent otherwise), in this way consciousness functions as a wifi system to integrate the different processes in the brain and yield adaptive action.
Click here to see a video of Dr. Morsella explaining his theory about consciousness and the brain.
To learn more about Ezequiel Morsella's research and books, please visit his lab's website, the Action and Consciousness Lab.