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.
The task of the GIT is to take apart complex food molecules and to absorb
simpler, safer molecules into the blood stream, both directly and indirectly via
the lymphatic drainage of the bowel. After food molecules are absorbed, they are
carried in the venous blood (portal veins) from the GIT to the liver. The liver
removes newly absorbed food molecules from the blood for further chemical
processing, storage and later release. The liver is a chemical factory whose
main function is to regulate the nutrient supply for the rest of the body. The
buffering, filtering, regulating functions of the liver compensate for the
vagaries in the food supply and mistakes made by the GIT.
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
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
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
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