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Digestion
The whole digestive system is around 9 meters long. In a healthy human
adult this process can take between 24 and 72 hours. Food digestion
physiology varies between individuals and upon other factors such as the
characteristics of the food and size of the meal.
Food enters the mouth, being chewed by teeth, with chemical
processing beginning with chemicals in the saliva from the salivary
glands. This is called mastication. Then it travels down the esophagus
into the stomach, where hydrochloric acid kills most contaminating
microorganisms and begins mechanical break down of some food (e.g.,
denaturation of protein), and chemical alteration of some. The
hydrochloric acid also has a low pH, which allows enzymes to work more
efficiently. After some time (typically an hour or two in humans, 4–6
hours in dogs, somewhat shorter duration in house cats, ...), the
resulting thick liquid is called chyme. Chyme will go through the small
intestine, where 95% of absorption of nutrients occurs, through the
large intestine, and are eliminated during defecation.
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Human Gastro Intestinal Tract
The human gastrointestinal tract refers to the stomach and
intestine, and sometimes to all the structures from the mouth to the
anus. (The "digestive system" is a broader term that includes other
structures, including the accessory organs of digestion).
In an adult male human, the gastrointestinal (GI) tract
is 5 metres (20 ft) long in a live subject, or up to 9 metres (30 ft)
without the effect of muscle tone, and consists of the upper and lower
GI tracts. The tract may also be divided into foregut, midgut, and
hindgut, reflecting the embryological origin of each segment of the
tract.
Transit Time
The time taken for food or other ingested objects to transit
through the gastrointestinal tract varies depending on many factors,
but roughly, it takes 2.5 to 3 hours after meal for 50% of stomach
contents to empty into the intestines and total emptying of the stomach
takes 4 to 5 hours. Subsequently, 50% emptying of the small intestine
takes 2.5 to 3 hours. Finally, transit through the colon takes
approximately 30 hours.
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Immune System
The gastrointestinal tract is a prominent part of the immune
system. The surface area of the digestive tract is estimated to be the
surface area of a football field. With such a large exposure, the immune
system must work hard to prevent pathogens from entering into blood and
lymph.
The low pH (ranging from 1 to 4) of the stomach is fatal for
many microorganisms that enter it. Similarly, mucus (containing IgA
antibodies) neutralizes many of these microorganisms. Other factors in
the GI tract help with immune function as well, including enzymes in
saliva and bile. Enzymes such as Cyp3A4, along with the antiporter
activities, also are instrumental in the intestine's role of
detoxification of antigens and xenobiotics, such as drugs, involved in
first pass metabolism.
Health-enhancing intestinal bacteria serve to prevent the
overgrowth of potentially harmful bacteria in the gut. These two types
of bacteria compete for space and "food," as there are limited resources
within the intestinal tract. A ratio of 80-85% beneficial to 15-20%
potentially harmful bacteria generally is considered normal within the
intestines. Microorganisms also are kept at bay by an extensive immune
system comprising the gut-associated lymphoid tissue.
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How Digestion Works.
