Overview of the Digestive System
The digestive system is by
far the largest and most complex of the internal organ systems. The interplay
of its multiple organs, its intrinsic hormonal and neural systems, and its
intricate and interacting physiologic functions are among the most fascinating
aspects of human physiology. Although the primary function of each organ is to
interact effectively with other organs to provide nutrition to the rest of the
body, several organs also have distinct metabolic functions that are of vital
importance.
Appreciating the role of each organ begins with asking how the four
essential functions of each are regulated and how immune and other defense
mechanisms are protecting that organ. The wall of each luminal organ is
composed of three layers of distinctly functioning muscle groups responsible
for moving nutrients and fluids from the mouth until they are discharged from
the anus. The electromechanical coupling mechanisms responsible for motility by
which this occurs are surprisingly distinct for each organ. An electrical syncytium regulates these contractions with rhythmic depolarizations called slow
waves and contractioninducing depolarizations resulting in action
potentials. Action potentials are similar throughout the luminal organs,
but slow wave activities in the stomach, duodenum, and colon vary in
frequency.
All luminal and solid gastrointestinal organs are involved with
secretions that facilitate digestion and mucosal protection, leading to
nutrient absorption. In contrast, the esophagus has the least secretion and no
absorption and the liver and pancreas are involved with secretion only and not
motility or absorption.
The liver is the most important and largest metabolic organ. Metabolic
functions are also provided by proteins synthesized by the small bowel and by
hormones secreted by the stomach, pancreas, and small bowel.
Regulation of each organ is achieved with a complex interaction between
the extrinsic autonomic nervous system, intrinsic or enteric nervous system,
and hormones secreted both within and outside the digestive system. The
hormones of the digestive system were the first endocrine substances to be
discovered. The small intestine is clearly the largest of all endocrine organs.
Each of the neurotransmitters identified in the enteric nervous system of the
digestive system is also found in the brain. Again, many were first discovered
in the gut. The lumen of each of the digestive organs is filled with
potentially lethal chemicals and microorganisms. Distinct, organ-specific,
highly effective defense mechanisms exist in each organ to prevent disease. The
gut’s microbiome develops shortly after birth and grows to a point of
containing 10 trillion organisms, or nearly 10 times as many cells as the rest
of the body! Other important defense mechanisms with unique actions in each organ include motility, intrinsic
secretion, lubrication with fluid and mucus, and frequent cell turnover.
When these protective systems break down or become impaired, disease
begins. Disorders of the digestive tract are the second most common reason,
after upper respiratory tract disorders, that patients seek help from a primary
care physician or are absent from work or school. In a typical year,
approximately 60% of individuals experience some digestive system dysfunction,
whether acute or chronic. Common digestive disorders affecting 15% to 20% of
the U.S. population include functional bowel disturbances, gastroesophageal
reflux disease, peptic ulcer disease, hepatitis, gallbladder stones, and
infectious diseases of the stomach and intestines. These and other
gastrointestinal disorders account for 25% of all hospitalizations. Of all
primary neoplasms leading to death, one third originate in digestive system
organs. Cancers of the digestive system continue to be the most common cause of
all cancer deaths. Lung cancer is overall the most common type of cancer, but
cancers of the colon, pancreas, liver, stomach, and esophagus are among the 10
commonest cancers.
One of the most fascinating aspects of the pathophysiology of the digestive
system is the marked difference in the prevalence of disorders in men and
women. Eosinophilic esophagitis, esophageal adenocarcinoma, and hepatocellular
carcinoma are much more common in men, and irritable bowel syndrome and
gallstone disease are much more
common in women.
