Protein is an important nutrient for humans because it provides us with the building blocks of our bodies. Without protein, we would not have muscles, bones, skin, hair, nails, organs, or any bodily function. Protein also helps regulate body temperature and keeps our blood flowing properly throughout the body. When a person eats food containing protein, their stomach breaks down the food and digests it into small particles called peptides. These peptides then travel through the bloodstream until they reach the cells where they are combined together to make protein.