The most important digestive processes take place in the small intestine and involve three types of secretions. The pancreas secretes pancreatic juice, which contains many enzymes that split proteins, starches, fats, and other food components as well as bicarbonate that neutralizes the gastric acid in the chyme as it comes from the stomach. The liver secretes bile, which is concentrated and stored in the gallbladder; the bile helps to digest fats in the small intestine by emulsifying them and making them more accessible to digestive enzymes. And finally the cells of the intestinal lining itself produce intestinal juice, which contains a number of enzymes, most for processing carbohydrates, two for protein, and one for fats.
While this is going on, the villi – small, fingerlike projections that line the wall of the small intestine – go to work to absorb and transport nutrients to the bloodstream. Peristaltic and rhythmic actions mix the food and carry it through the small intestine, causing it to brush against the villi, which are constantly in motion and are able to “pick out” molecules of nutrients with such efficiency that by the time the chyme has passed through the small intestine, nearly 95 percent of some nutrients will have been absorbed.
The process of absorption itself operates in a number of physical and chemical ways, both active and passive. Vitamins, minerals, and the end products of carbohydrate and protein digestion are absorbed mainly into the blood of the capillaries, and the end products of fat digestion first mainly enter ducts known as lymphatic ducts and are then transported to the bloodsteam. Only at this point are the nutrients from the food we have eaten ready to be transported to the cells where they will be utilized for energy; for building and repairing tissue or for regulating body functions; converted to storage forms for future use; or transported elsewhere in the body for elimination. Any material present in the small intestine that has not been absorbed for use in the cells is passed into the large intestine.
The large intestine, so-called because it is wider than the small intestine, is the final reservoir of the digestive tract. Its main job is to actively reabsorb excess fluid and dissolve mineral salts from the digested food mass to help maintain the body’s fluid balance. Water not used in other processes is excreted by being flushed (along with waste products) through the kidneys. The large intestine also helps form the body’s solid waste, the feces. Fecal matter is approximately two-thirds water and one-third cellulose (fiber that must be present in adequate amounts for intestinal muscle tone), and large amounts of bacteria, as well as sloughed cells from the small intestine lining, which replaces itself every 48 hours.
The intestinal flora – a population of microbes found mainly in the large intestine – are also important to nutrition. These flora synthesize vitamins K, biotin, and pantothenic acid, thereby preventing serious deficiency of any of these three vitamins.