In this process of utilization of the food substances by the cells to obtain muscular energy with its resulting heat, there are certain waste products. These are carried by the blood to the various points in the body where they are thrown off. Some of these waste products are the nitrogenous compounds spoken of in the paragraph above. These compounds are the natural result of cell activity and if protein is not provided in the food it will be taken from the cell itself, which causes the wasting away of the tissue. These products are carried by the blood to the kidneys where they are excreted. A second waste product is carbon dioxide, which is carried by the venous blood back to the lungs and is exhaled. A third waste product is water. This can be given off from the lungs in the form of water vapor, or excreted from the kidneys in urine, or from the skin as perspiration. "Insensible perspiration" by a person at rest may amount to 60 per cent of the water eliminated through the kidneys. It may be many times as much in the case of a person at hard labor. The last waste product is the mineral matter from the food in excess of body requirements. Some of this is returned to the digestive tract and eliminated through the large intestine. The elements eliminated in this way are phosphorus, calcium, and iron, while others, such as potassium and sodium, are found in the urine.

Changes Which Food Undergoes In Passing Through The Body