Your digestive and circulatory systems are among the large organ systems in your body. Digestive organs pass food through your system, breaking it down and absorbing nutrients so you'll get the benefits, while your circulatory system -- made up of your heart and blood vessels -- transports oxygen and other compounds throughout your body. Your circulatory and digestive systems do not directly convert food into energy, but they process and circulate nutrients so your cells can use them for fuel.
Food and Energy
The foods you eat supply carbohydrates, proteins or fats -- and many foods contain a mix of all three. These three macronutrients serve as your main energy source. Carbs and proteins each contain 4 calories per gram, while fat is higher in calories, at 9 per gram. Each type of macronutrient undergoes different processing in your body and has unique pathways your body uses to turn them into usable energy.
Carbs: The Preferred Source of Energy
Your body -- especially your muscles -- prefers to run on carbs, and sugar is the sole source of energy for your brain. After a meal, your digestive tract breaks carbohydrates down into simple sugars, like glucose. From there, the glucose gets absorbed into your bloodstream, and your circulatory system distributes it throughout your body, allowing your tissues to take up the sugar to use for energy.
Fats: A Concentrated Energy Source
Fat is the most efficient way to store excess energy since it has a higher energy density than proteins or carbs. Most of the fat in food is found in the form of triglycerides, which contain three fatty acids bound together with a glycerol backbone. Your digestive tract breaks triglycerides down into individual fatty acids for absorption.
Your circulatory system carries fats throughout your body so it's available to your cells, which can absorb the fat and convert it to energy. The fat in your bloodstream is especially important for long-lasting energy during exercise because it's used in combination with carbohydrates to power your muscles.
Protein: The Less Preferred Source
Protein's key role is maintaining healthy cells and tissues, not producing energy. Your digestive system breaks protein down into its individual components, called amino acids, and your cells and tissues use amino acids to make new cellular proteins. If you're not getting enough calories, carbohydrates and fats, however, your body can break down protein and convert it into sugar to use for energy. On the other hand, if you're getting too many calories and too much protein, you'll store the excess energy as fat.