When you eat, your digestive system is responsible for turning food into the energy needed to survive. Digestion involves a series of physical and chemical processes in which food is broken down into nutrients and absorbed from the intestines into the blood stream. The digestive system extends from the mouth to the anus. Each section of the digestive system has a specific role in nutrient absorption.
Mouth
Digestion begins in the mouth. Chewing breaks the food into pieces that are more easily digested, while the digestive enzymes found in saliva mix with the food to start breaking it down into a form your body can absorb and use.
Esophagus
The esophagus, a hollow muscular tube, transports food from the mouth to the stomach through a series of muscular contractions called peristalsis.
Stomach
The stomach is an expandable, sack-like organ that holds food while it is being mixed with enzymes and strong acid that continues the process of breaking down food into a soft, semi-solid form. When the contents of the stomach are adequately processed, they are released into the small intestine.
Small Intestine
The small intestine is made up of three segments, the duodenum, the jejunum, and the ileum. A new born baby has between 6 to 8 feet (200 and 250 cm) of small bowel at birth. Most adult people have between 12 and 20 feet (365 to 600cm) of small intestine. Peristalsis moves food through the small intestine and mixes it with digestive secretions from the pancreas and liver. The duodenum is largely responsible for continuing the breakdown process, while the jejunum and ileum are mainly responsible for the absorption of nutrients into the bloodstream.
Enzymes released in the ileum help regulate the emptying of food from the stomach and the movement of food through the intestines. The last portion of the ileum is also responsible for the absorption of Vitamin B12 and unused bile salts. Once the majority of nutrients have been absorbed from the food; the leftover liquid (bile, enzymes, mucus, water, electrolytes, undigested fibers and waste) moves into the large intestine or colon.
Pancreas
The pancreas secretes digestive enzymes required for the digestion and absorption of carbohydrates, protein and fat. These secretions are released into the duodenum when food is present and helps break the food down into components which can be absorbed into the blood stream.
Liver
The main functions of the liver within the digestive system are to process nutrients absorbed from the blood stream and to produce and excrete bile salts. Once the liver has removed impurities from the blood, the nutrient-rich blood is delivered to the cells of the body. Bile salts are produced by the liver, stored in the gallbladder and secreted into the small intestine where they aid in the digestion and absorption of fats.
Colon (Large Intestine)
The colon, which is 2.5 feet at birth and 5-foot-long in adults, is a muscular tube that connects the small intestine to the rectum. It is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon, and the sigmoid colon, which connects to the rectum. Undigested material moves from the small intestine into the colon, where water and electrolytes, such as sodium and potassium, are absorbed. The rectum functions as a temporary storage site for the waste before it is passed from the body through the anus. Bacteria within the colon perform several useful functions such as making various vitamins, processing waste products and food particles, fermenting fibers into additional sources of energy, and protecting the body against harmful bacteria.