DIGESTION Basic Ideas
Digestion is achieved by the GastroIntestinal Tract (GIT). The food supply must be matched to the capacity of each GIT to process food. Dysfunction and disease arise when there is a mismatch. The GIT is not a passive processor of food, but actively manages and responds to food. When there is trouble in the GIT, attention is directed to the abdomen by a variety of discomforts - pain, nausea, noise, distention, and abnormal bowel movements.
Digestion of food is accomplished by secretions from the GIT's surface and accessory glands. The liver and pancreas are the major secretory organs that contribute digestive factors. After food molecules are absorbed, they are carried in the venous blood (portal veins) from the GIT to the liver.
The liver is a complex chemical factory whose function is to control the food supply for the rest of the body. The liver's main task is to take in newly absorbed food molecules for further chemical processing, storage, and slow release.
Food molecules leaving the liver are distributed throughout the body in the bloodstream. The liver also acts as immune-filter, removing some of the "wrong-stuff" admitted through the GIT wall. If the efficiency of the liver filter is reduced, delayed pattern food allergy symptoms are increased.
Inflammation of various parts of the GIT is described by terms using the suffix 'itis' attached to the name of the GIT part; thus, we have descriptive names such as gastritis (stomach), colitis (colon), ileitis (ileum or small intestine), hepatitis (liver) and cholecystitis (gallbladder).
Immune sensors along the surface of the GIT trigger immune responses causing symptoms such as nausea, vomiting, burning, pain, itching and swelling at the anal exit. The GIT knows when the wrong stuff has entered its space, and reacts defensively to get rid of it. Vomiting and diarrhea are defensive responses designed expel unwanted material.
The GIT's lymphatic system is a fluid transport system that absorbs some nutrients and directs cells and absorbed molecules to flow through lymphatic filters (lymph nodes) scattered around the GIT. Eventually, lymph fluid returns to the blood via the superior vena cava. All food molecules are absorbed by the GIT, mostly from the small intestine (duodenum, jejunum and ileum).
Digestive Tract Motility
The GIT is muscular and contracts rhythmically to conduct food through its length (peristalsis). The timing, strength, and coordination of muscular contraction is influenced by the composition of food and by state-specific body-brain conditions. If the contractions are too vigorous, crampy pain is experienced. If the contractions are too frequent, diarrhea results. Constipation occurs when contractions are sluggish and slow, or if sustained contractions act to obstruct the tube.
Disorders of peristalsis are referred to as "Motility Disorders." Medical diagnoses sometimes refer to motility disorders as if they were the cause of symptoms. With a few exceptions, motility problems are not causes but they are effects of food-GIT interaction and GIT-brain interactions.
The surface of the digestion tract changes as you proceed from the mouth to the anus. Specialized surfaces secrete different enzymes, antibodies and signaling molecules. The entire surface is exposed to outside materials and is at risk of infection and physical damage. The entire surface regenerates rapidly and has several mechanism of defense. Picket suggested: The cell lining of the gastrointestinal tract is continually damaged by mechanical stresses and scratching by partially digested food as it traverses the gut… the digestive tract has evolved two defense mechanisms: the formation of membrane patches that plaster over holes in the membrane, and the secretion of a lubricating mucus that cushions the membrane against further abrasions.” Miyake et al. studied repair of injured rat intestine and reported that mucus secretion quickly sealed holes in the lining.
Digestion of food is accomplished by secretions from the bowel surface and glands accessory to the GIT. The liver and pancreas are the major secretory organs that contribute digestive factors to the GIT. The first digestive secretions are produced by the salivary glands that secrete saliva into the mouth. Saliva is a lubricant watery fluid, containing amylase, an enzyme that reduces starch into its component sugars. As you chew starchy food, you experience increasing sweetness as amylase liberates individual sugar molecules from the starch polymer.
Saliva is a prototype of all the digestive juices secreted by the GIT. Secretion of saliva is controlled by the autonomic nervous system and varies with smells, sight, sounds and emotions. Activity in both sympathetic and parasympathetic nerves increases salivary secretion; drugs that block this activity will produce a dry mouth (e.g. antidepressant drugs). Saliva contains antibodies, especially IgA, as the first line of immune defense against ingested noxious substances.
Digestion continues in the stomach, which mixes food by rhythmic muscular contractions with strong Hydrochloric acid (HCl), produced by the parietal cells, lining the stomach surface. The stomach secretes more enzymes: Renin coagulates milk; pepsin digests proteins to polypeptides chains; lipase begins fat digestion.
The stomach mucosa is protected from its own acid by surface factors including mucous, and prostaglandins. If the surface defense breaks down, hydrochloric acid may erode the surface of the stomach or duodenum, producing a "peptic" ulcer. Partly digested food is squeezed out of the stomach through the outlet valve, the pylorus, into the small intestine.
The small intestine is a coiled muscular tube that reaches twenty feet in length, and does most of the chemical work of digesting and absorbing food. The liver produces bile as its contribution to digestion. Bile flows into the duodenum via a tubular system with a storage sac, the gall bladder, connected by a T-junction. Beyond the duodenum is the jejunum where nutrients are derived from food and absorbed. Absorption is completed in the third section of intestine, the ileum. The ileum delivers watery slurry of leftover food waste to the colon.