Digestion

Each compartment and accessory organ serves a specific function. At each stage, the food is transformed into a slightly different form that allows it to be passed along to the next compartment. The coordination of functions is done through the nervous and endocrine systems. Through the process of digestion, food is transformed from large complex particles into basic elements.

The process of digestion begins in the mouth. Within the mouth lie the teeth, tongue and jaws. Through a chewing motion, the food is mechanically broken down between the teeth and mixed with saliva, which aids in chemical digestion. Upon stimulation, saliva is produced in the salivary glands and brought into the mouth. It contains salivary amylase, an enzyme that digests starch. Once the digestion in the mouth is completed, the first phase of swallowing is initiated. This stage is voluntary and is characterized by contraction of the muscles of the floor of the mouth and tongue that propel the food bolus into the pharynx.

The role of the pharynx is to facilitate the passage of the food bolus into the esophagus. The pharynx is designed to direct the food bolus in this direction. It is here where the second phase of swallowing takes place. After the moistened food bolus is moved to the back of the mouth by the tongue, an involuntary swallowing reflex is triggered which prevents food from entering the respiratory tract. The tongue closes off the mouth, the soft palate blocks the nose, and the larynx rises such that the epiglottis closes off the trachea. Food then moves from the pharynx into the esophagus.

The esophagus is the first part of the digestive tract proper. It is also where the third phase of swallowing occurs. Approximately 10 inches in length, it consists of three tissue layers consistent with the rest of the gut. Once in the proximal portion of theesophagus, the muscles of peristalsis begin propelling the food bolus through the esophagus into the stomach.

No further digestion takes place in this compartment. The pharynx and esophagus serve only as conduits for digestion.

The stomach is a C-shaped pouch that receives the food bolus from the esophagus. It aids both in mechanical and chemical digestion. Acting like a churn, the stomach mixes the food with gastric acid and breaks down the food into a milky substance known as chyme. The acid reduces the pH of the stomach, in the process allowing activation of an enzyme called pepsin. This starts the chemical digestive process.

The majority of digestion occurs in the small intestine. This compartment has three distinct portions, each of which is highly specialized for different digestive functions. The first third is the duodenum. This part is responsible for signalling the distal digestive tract that food is arriving and that the stomach should stop sending food. Chemical digestion is very active at this stage, and food is broken down into basic proteins, carbohydrates, and fats. Digestive enzymes are released from the pancreas to enable the degradation process. Bile is released from the gall bladder and mixes with the chyme to aid in fat digestion and absorption.

The second portion of the small intestine is the jejunum. It is composed of many folds that increase the surface area for absorption, known as the brush border membrane. Chemical degradation continues at this point and absorption begins. Once the chyme is broken down into its basic constituents, the process of absorption occurs. Nutrients are moved across the membrane via active and passive diffusion. In some instances, carrier molecules made by the liver, known as micelles, help carry fat across the border into the blood stream.

As the digestive products move through the small intestine, different parts of the brush border membrane selectively allow the absorption of different nutrients. The third portion of the small intestine is the ileum. It is responsible for the very selective absorption of some significant nutrients, including vitamin B-12 and vitamin C. By the end of the small intestine, the majority of the nutrients has been absorbed. The material is now mostly waste products and water, and is referred to as liquid stool.

The large intestine (the colon) has several major components. The cecum is the large boggy entrance to the right side of the colon. There is a circular muscle, or sphincter, that separates the small intestine from the large intestine. The sphincter protects the small intestine from the concentrated bacteria of the large intestine. Following the cecum, the ascending colon rises up and leads into the transverse colon. From there, the descending colon dives back down on the left side of the abdomen into the sigmoid colon and rectum. Waste is finally eliminated through the anus.

There are three pairs oF salivary glands: the parotid, submandibular, and sublingual glands. Each gland releases salivary amylase that begins digestion by degrading starch in the mouth. The glands are composed of specialized cells that make enzymes. They are connected to the mouth by a duct. These ducts are lined with epithelium, which serves as a protective barrier from autodigestion. They also have some muscle in them that allows the ducts to contract. This enables the one-way flow of enzymes from the glands into the mouth.

The pancreas has both an endocrine and an exocrine function. The endocrine function allows the production of insulin, which aids in the regulation of sugar but is not involved in digestion. The exocrine function, on the other hand, is responsible for digestion. It is in this portion of the pancreas where enzymes of digestion are made. A neutral buffer, bicarbonate , is also produced here. Bicarbonate keeps the intestinal environment at a neutral pH. The enzymes and the bicarbonate are then released into the lumen of the intestine through the pancreatic duct.

The biliary tract is composed of the liver, bile ducts, and the gall bladder. Bile is made in the liver. It is then carried to the gall bladder via the bile ducts. The gall bladder stores the bile until it is ready to be released into the duodenum. Through molecular signalling from the small intestine, the gall bladder will contract and empty its contents into a larger bile duct that will end in the lumen of the small intestine.