The digestive system is a wonderfully integrated and efficient bodily process that handles the intake of food, from the mouth onward to the elimination stage. How long it takes an individual to digest an entire meal is dependent on metabolism, age, health conditions, etc. It can take anywhere from twenty-four to seventy-two hours to completely digest food to where it is finally excreted from the body.
The primary function of the digestive system, also known as the gastrointestinal tract (GI tract), is to break down foods into smaller particles so the body can utilize them to build, to repair, to nourish cells, and provide energy. This is done through seven different processes: ingestion, propulsion, secretion, mechanical and chemical digestion, absorption, and defecation.
Ingestion involves the process of eating, while propulsion is the GI tract's ability to move food along the tract through a movement called peristalsis (wave contractions). While secretions such as digestive enzymes liquefy and chemically break down foods, the GI tract uses both mechanical and chemical digestion to make food into even smaller molecules, to be absorbed into the blood stream and lymphatic vessels. The defecation stage involves getting rid of materials that cannot be digested, where they are eliminated through the anus.
The major parts of the digestive system include the salivary glands, pharynx, esophagus, stomach, small and large intestine, and rectum. Accessory digestive organs like the liver, gallbladder, and pancreas aid in the digestive process as well. The body is able to digest food through a long hollow tube which begins in the mouth, extends and twists all the way from the top of the torso to the rectal end. This tube is lined with mucosa (mucous membrane) which contain juices to help digest food. This mucosa is found in the mouth, stomach and small intestine.
The liver and pancreas also produce digestive juices that access the intestine through several small tubes. Other organs such as the nerves and blood vessels also play a major role in functions of the digestive system.
Digestion begins in the mouth where the teeth, jaw, and tongue work in conjunction to bite and chew food into smaller bits. Salivary glands in the mouth contain enzymes that further help break down the food.
When food particles are small enough to swallow, it goes from the mouth into the esophagus, a tube which is connected to the stomach. As previously mentioned, this food is pushed down by wave contractions called peristalsis. At the bottom of the esophagus lies the lower esophageal sphincter (LES), which prevents food from going back into the esophagus from the stomach.
Should the sphincter not be able to do its job efficiently, acid can re-enter the esophagus and cause heartburn, or chronically: acid reflux or GERD. At the same time, should a person consume something the stomach doesn't like, the stomach muscles contract and force this food up the LES, hurling contents from the stomach through vomiting.
There are several functions that the stomach performs. It can store food for a long period of time. It uses its digestive juices and enzymes to dissolve food into tiny particles so they can enter the blood stream. The stomach also uses these acidic juices to mix, churn, and squeeze the food into liquid form. Then, this liquefied material is slowly released into the small intestine for further processing.
As liquefied foods are drained from the small intestine, the liver is able to produce simple sugars by processing fat and proteins from this mixture. The liver also detoxifies the body by taking poisons out of the blood and changing them into a safer form, to be excreted out of the body.
The liver also makes use of calcium to reduce the amount of acid in body wastes. By doing this, it allows a person to defecate without too much discomfort or damage. As well, the liver secretes bile that contains a fatty material, to help absorb fatty foods and further aid in digestion.
This organ, shaped like a little pouch, is located near the liver. It has the function of taking bile from the liver and storing it. During digestion, the gallbladder releases the stored bile into the small intestine so it can help dissolve fats.
The pancreas produces and sends digestive juices to the upper part of the small intestine, through a tube called the pancreatic duct.
The small intestine is 20 ft. long and divided into three sections called the duodenum, jejunum, and ileum. In the duodenum, carbohydrates are broken down further into sugars or glucose; proteins into smaller particles called amino acids; fats into fatty acids. Other nutrients like vitamins/minerals/electrolytes are also made smaller before entering the second section, the jejunum.
The jejunum contains a specialized lining containing villi that enable the small intestine to absorb proteins and carbohydrates into the blood stream, and to provide fuel to various body parts so they can do their jobs.
The ileum is responsible for absorbing bile salts and fats. Because pores in the ileum are larger than those in the jejunum, they are able to allow vitamin B12, fat soluble vitamins like A,D, and E, bile salts, electrolytes and water to enter the blood stream. The ileum is joined to the large intestine by a valve called the ileocecal valve, which prevents materials from re-entering the small intestine.
The large intestine is 5 ft. in length, and deals with foods that are not digested. The large intestine is also called the colon, and like the small intestine, is divided into 3 parts: ascending, transverse, and sigmoid. As wastes are passed from the small intestine into the colon, it picks up water. By the time it reaches the sigmoid colon, this material is quite firm. The sigmoid colon is like a catch basin that slows down the passage of waste until it is ready to be excreted.
The sigmoid is the part of the large intestine that gets the most grief in issues such as constipation, development of pockets in the intestine called diverticula, risks of cancer/tumors, polyp formation, and the accumulation of toxins in the colon. The rectum is at the end of the digestive process and serves to keep wastes in, until the body is ready to dispose of it. A specialized muscle called the anal sphincter prevents wastes from escaping until the appropriate time.