In the digestive track carbohydrates are converted to monosaccharides, mostly glucose and fructose. Proteins are broken down into amino acids and fats are broken down into fatty acids. Glucose and amino acids are absorbed directly into the blood from the intestines. Once the fuel is in your body it has to be used immediately or stored. You are constantly using your fuel to support the functions of the body's various organs.
Different organs have different fuel preferences.
Your brain accounts for a large percentage of your resting energy consumption. In a well fed body, it uses glucose as fuel almost exclusively. The heart is a scavenger. Although it prefers fatty acids, it will use any fuel available to it. This makes sense, since it is one of the most ancient organs from an evolutionary standpoint and if it stops pumping, everything else stops soon after.