The digestive system is responsible for processing food, breaking it down into useable proteins, carbohydrates, minerals, fats, and other substances, and introducting these newly produced metabolites into the bloodstream, where they will be used by the body in a vitual plethora of functions. The digestive or alimentary tract, begins at the mouth, where the teeth and tongue begin the breakdown of food, aided by saliva secreted from the salivary glands. The chewed food, combined with saliva, is swallowed, carrying it in peristaltic (contractile) waves down the esophagus to the stomach. In the stomach, the food is combined with hydrochloric acid to complete the process of 'breaking down' or 'digestion' of solid food matter. When the food is thoroughly digested, the fluid remaining called chyme, is passed through the pyloric sphincter to the small intestine and large intestines. Within the long convoluted intestinal canals, the nutrients are absorbed from the chyme into the bloodstream, leaving the usuable residue. This residue passes through the colon (where most of the H20 is absorbed into the bloodstream) and into the rectum where it is stored prior to excretion. This solid waste, called feces, is compacted together and upon excretion,passes through the anal canal and the anus. Along the way through the digestive tract, the pancreas, spleen, liver, and gallbladder secrete enzymes which aid in the digestive process.
The mouth is a very versatile area of the human anatomy. Responsible for articulation in speech, and tasting, chewing, and swallowing food, The mouth cavity is located just below the nasal cavity and is formed by the palantine processes of the maxilla on top and by the mandible on bottom, At the opening of the mouth cavity are the lips - muscular structures which are covered with thin, membranous skin. The lips occlude the opening of the mouth during chewing and swallowing to keep food and liquid within, facilitate articulation in speech, and can even give a friendly kiss - although presumably not at the same time. Within the mouth cavity, the teeth extend down from their maxillary sockets and up from their mandibular sockets to form the dental arcade.
When food is brought into the mouth, the lips close while the salivary glands produce saliva. The saliva lubricates the mouth and moistens the food. The inner surface of the lips, the tongue, and the cheeks, manipulate the food so that it is brought between the teeth as the teeth clamp down on the food. In a combined action of these motions, with a semi-circular, grinding motion of the teeth, the food is chewed into a paste with the saliva. Enzymes within the saliva begin to break down the food and the tongue moves a portion of this food paste to the back of the mouth cavity by pressing it up and back along the hard palate. The soft palate, meanwhile, raises to seal off the nasal cavity. The ball of food paste, called a bolus, is passed into the pharynx. The epiglottis lowers to cover the airway so that the food does not enter the larynx. From the pharynx, wave like contractions, called peristalsis waves, push the bolus down into and through the esophagus and into the stomach, where it is further digested.
The salivary glands are responsible for secreting saliva, a clear, alkaline, semi-viscous liquid which helps in the digestion of food. The salivary glands include the large parotid gland and the smaller submaxillary and sublingual glands. So vital is the role of saliva in the initial processes of digestion that the glands have taken on an autonomic response which is quite irrelevant to the actual presence and/or taste of food. Combining with the psychology of societal norms which place place pleasurable feelings and events around the family table, it takes a sight, an aroma, even a fleeting thought of favorite foods to activate the production of saliva.
Saliva also performs a cleaning function, serving to keep exfoliated epithelial cells, some bacteria, and food particles away from the teeth. Saliva keeps the mouth lubricated for articulation and speech and also helps to moisten food to assist in swallowing. Enzymes in saliva begin digestive breakdown of the food even before it reaches the stomach.
The esophagus is a long, flexible tube which leads from the pharynx in the upper throat to the stomach. The average adult esophagus is about 10 inches long and its walls are made of muscle fibers which contract in waves called peristalsis to push the food bolus down to the stomach. The common ailment of heartburn occurs when stomach acid washes back up into the esophagus. Since the esophagus has no protective mucosal layer, as does the stomach, this acid causes pain which generates just behind the sternum and appears to come from the heart. This is where we get the term "heartburn".
The liver is the largest of the body's glands and is responsible for several important functions. Weighing in at approximately 3 pounds, this reddish brown organ features a very high degree of vascularity which is responsible for its dark color. Located primarily on the right side of the abdominal cavity, just above the duodenum, the liver aids in the digestion of fats by secreting bile into the duodenum. The liver also destroys red blood cells, forms urea for the excretion of nitrogenous wastes, forms fibrinogen, used in blood coagulation, stores glycogen, helps in the metabolism and the storage of vitamins, and produces protective and antitoxin substances.
The gallbladder serves the function of concentrating and storing bile, produced in dilute form by the liver, and secreting the bile through the cystic ducts into the duodenum, providing for the next step in the digestive process. this step is, primarily, the breakdown of and redistribution of fats and their metabolic by-products where it can help in digestion. The gallbladder itself is a blueish organ approximately 3 inches long located on the underside of the liver. Bile is composed of cholesterol, bile salts, and bile pigment. The gallbladder is not critical to the survial of man (or woman:)) and may be removed without severe, or even partticularly adverse effects. The crystallization of the bile salts in the gallbladder may give rise to gallstones, which often require surgical correction.
The large intestine is a broad, corrugated tube which accepts the by products of digestion from the small intestine and passes it along to be excreted, continuing to process the material on the way. Any unabsorbed food materials are stored in the large intestine until the body can partially reabsorb water from it, then pass the remains along to the anus for elimination. The over absorption of water from the waste material may lead to hard relatively dry feces which can become impacted, making elimination difficult. This condition is also known as constipation. Conversely, reabsorption of water from the large intestine may become diminished to the point of being insufficient. Certain endocrine disorders may interfere with an absorbefacient processes as can infectious processes which are localized as versus systemic, in particular yeast and fungal infections. The unabsorbed fluid passes from the large intestine to the anus, also known as diarrhea. The large intestine is divided into 8 sections: the cecum, appendix, ascending colon, transverse colon, the descending colon, the sigmoid colon, rectum, and the anus.
The pancreas is a long, lobe like gland which is responsible for secreting the hormone insulin and an alkaline fluid which aids in digestion. Insulin is important in the body's utilization of blood sugar and lack of this hormone leads to diabetes mellitus. The pancreas also secretes a highly alkaline substance which contains the enzymes tripsin, amylase and lipase. These enzymes are tilized in the metabolism of proteins, carbohydrates and fats. Interestingly, the hormone that activates the pancreatic enzymes, secretin, is produced in the small intestine.
The appendix is a small, attachment at the apex of the cecum. Due to its shape, it is often called the vermiform (worm shaped) appendix. Evidence has shown that the appendix once may have taken a part in the digestion of durable matter such as tree bark, but now is apparently unnecessary in the modern anatomy. The inflammation of the appendix is called appendicitis, and its removal is one of the most comonly performed surgical procedures.
The anus is the sphincter muscle which regulates the lower orifice of the digestive tract. This sphincter muscle keeps the anus closd, opening it during excretion to allow feces to pass through.
This is the digestive system. To sum it up simply:
IT BEGINS WITH MASTICATION!
IT ENDS WITH DEFECATION!