As in the tilte, does human body produce it, (and if how?) or does it get from other source?
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Your body produces energy by reacting various biological molecules with oxygen. Since biological molecules are almost always based on a carbon backbone, this usually ends up reacting carbon with oxygen, which produces carbon dioxide.
The textbook example is [glucose](https://en.wikipedia.org/wiki/Glucose) (C6H12O6), which is “burned” by reacting it with oxygen:
C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O
but the same basic idea applies to all sorts of things your body can burn for energy.
The carbon ultimately comes from the food you eat. That food is either plants themselves, or animals that ate plants, so we can trace it further to plants, which capture carbon from the atmosphere as part of photosynthesis. Ultimately, carbon goes around in a loop from being captured by plants, plants being eaten by animals, and animals burning that carbon for fuel. This is (part of) the [carbon cycle](https://en.wikipedia.org/wiki/Carbon_cycle); any given carbon atom in the atmosphere is recycled by this process roughly every five or ten years on average.
The carbon in exhaled CO2 comes from the carbon contained within the foods we eat. Carbon is the building block of life, so much so that organic chemistry just means “the study of carbon”. Fats, carbohydrates, and proteins are all made of carbon (with a few other elements mixed in). When you metabolize these molecules, you are making them undergo chemical reactions very similar to combustion (which is why we say burning calories). These reactions break down the energetic carbon-carbon bonds in the food molecules, and capture that energy to make ATP. Then, the carbon is released at CO2 and water (just like the byproducts of a combustion reaction!)
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