Download Digestive System Summary

Digestive System Summary
Digestion starts with the mouth, which begins the mechanical
breakdown of food by the grinding and chomping action of the teeth.
Chemical digestion (breakdown) of starches, such as crackers, into glucose
because of enzymes such as amylase that are secreted into the mouth from
the salivary glands. The tongue mashes the food into a soft ball called a
bolus, now moistened from saliva. Saliva contains water, enzymes and
mucus. The softened ball of food is swallowed where it travels down the
esophagus (a tube that connects the mouth to the stomach) because of
wavelike involuntary muscular contractions called peristalsis. No digestion
occurs in this structure.
Food next enters the stomach through a valve called the cardiac
valve. The stomach is responsible for storing food while its thick, muscular
walls contract and relax, churning and mixing food with gastric juices
including hydrochloric acid, mucus, enzymes and hormones. Mucus lines
the stomach walls, protecting the lining from acid it produces. The lining is
also replaced every 3 days to replace damaged tissues. The churning action
of the stomach continues mechanical digestion, but some chemical
digestion also happens. Proteins are clumped together by HCl. Enzymes
then continue chemical digestion by breaking them into smaller molecules
called amino acids. The food is now a liquid called chyme as it leaves the
stomach through another valve called the pyloric valve and enters the small
intestine. If something harmful, such as bacteria, is consumed and causes
irritation (an internal stimulus) then the response of the stomach will likely
be vomiting which removes harmful or disease-causing organisms from the
body.
Most nutrients are digested and absorbed in the small
intestine. The small intestine is responsible for most of the chemical
digestion in the digestive system. It digests fats, sugars, starches and
proteins. Most digestion in the small intestine occurs in the first 10 inches.
The remainder of the small intestine absorbs nutrients. Villi, tiny finger-like
projections, increase the surface area making absorption more efficient, but
the microvilli add even more surface area, allowing the small intestine to
absorb even larger amounts of nutrients.
There are 3 main accessory organs that assist the small intestine
to chemically digest food. Food never enters these organs. Instead, the
liver, gall bladder and pancreas, make and secrete fluids into the small
intestine. The liver produces bile, which helps break down fats in the small
intestine. The gall bladder stores the bile until it is needed. The pancreas
produces hormones and enzymes, such as insulin, that are released into the
small intestine to further break down sugars and other nutrients.
From the small intestine the remaining material, mostly plant
fiber and water, moves into the large intestine. Only about 10% of the
original material and secretions makes it to this structure. The primary
function of this organ is to reabsorb water from undigested
food. This is very important to prevent dehydration. The large intestine
prepares the indigested materials, now called feces, for removal from the
body. Foods that are high in fiber such as beans are necessary for a healthy
colon, the final part of the large intestine because they allow the feces to
move through the colon efficiently. The large intestine does not have any
digestive enzymes to get energy and instead have to depend on bacteria.
Feces spend 12 to 24 hours in the large intestine being absorbed by bacteria.
These bacteria create large amounts of intestinal gas as a waste product as
they digest fruit and vegetable fiber. These bacteria are necessary for
survival because in addition to digesting fiber, they produce vitamins B and
K. If bacteria are killed through illness or antibiotics they cannot produce
vitamins or allow the absorption of water, when illness strikes and the large
intestine is unable to absorb the excess water, diarrhea occurs which can lead
to dehydration. Eating yogurt, which is full of healthy bacteria, can help
replace the bacteria lost.
From here, feces waits in the final section of the large intestine, the
rectum, before being expelled from the body through the final valve, or
opening, called the anus.