The gastrointestinal tract (GI tract) is 9m (30') and is composed of the
mouth pharynx, esophagus, stomach, small intestine, and large intestine.
Various accessory structures also function in the processing of food:
teeth, tongue, salivary glands, liver, gall bladder, and pancreas.
The mouth possesses cheeks and lips help keep food between the upper and
lower teeth. The tongue has muscles can move it in and out and from side
to side for swallowing and chewing. Intrinsic tongue muscles alter the
size and shape of the tongue for chewing and speech. Projections are known
as papillae; some have taste buds. A number of glands around the oral
cavity produce saliva which moistens the food and prepares it to be swallowed.
The movement of food from the mouth to pharynx is a voluntary activity;
after swallowing all muscle movement is involuntary. In swallowing, the
soft palate moves upwards to close opening to nasopharynx; larynx moves
upward so that epiglottis covers the opening to the trachea so that food
does not enter lungs.
The esophagus is a 10" long tube connecting the mouth and the stomach.
It secretes mucus to facilitate the passage of the bolus of food--a dry
bolus can't pass because of friction (dry white bread absorbs available
liquids; hard to eat without a drink). A wave of involuntary muscle movements
called peristalsis (which also occur in other areas of the GI tract) moves
the bolus down the esophagus and through the lower esophageal sphincter.
The muscular lining of the stomach has 3 layers: an outer longitudinal
layer, a middle circular layer, and an inner oblique; these layers allow
the stomach to contract in a number of ways. Most chemical digestion (using
enzymes to divide large molecules such as proteins and polysaccharides
into smaller molecules such as amino acids and monosaccharides) occurs
in the small intestine rather than the stomach. The stomach is primarily
responsible for mechanical digestion. As the food is divided into smaller
and smaller particles, this increases the ability of the digestive organs
to act on food. Peristaltic movements pass over stomach every 15-25 seconds
called mixing waves and the food is broken down and mixed with gastric
secretions to form chyme.
The pyloric sphincter separates the stomach from the small intestine and
is very narrow. Each mixing wave pushes a little gastric juice through.
As food is pushed towards the narrow sphincter, it meets resistance and
returns to the body of the stomach, allowing for constant mixing.
Two pancreatic ducts connect the pancreas to the small intestine. The
pancreas makes a variety of enzymes which aid in the digestion of carbohydrates,
lipids, and proteins.
The liver weighs about 3 pounds and after the skin, it is the body's 2nd
largest organ. Over 200 functions have been assigned to the liver, including
carbohydrate and lipid metabolism, the production of urea, synthesis of
blood proteins, the altering of drugs, storage of glycogen, and the synthesis
of bile. When bile is released into the small intestine, it emulsifies
the lipids in food (breaks them into smaller droplets ) so that digestive
enzymes can act on them.
F) GALL BLADDER
The gall bladder stores and concentrates bile (made in the liver) until
it is needed in the small intestine. If the bile salts are too concentrated,
they can crystallize forming gallstones. Bile ducts connect the liver,
gallbladder, and small intestine,
G) SMALL INTESTINE
Most chemical digestion and absorption occur in the small intestine and
the small intestine secretes many digestive enzymes. From the pyloric
sphincter to the ileocecal sphincter, it measures an average of 21 ft.
long and 1 inch wide. It has a large surface area for absorption and digestion
because its surface area is increased by:
a) individual cells have microvilli that form the brush border
b) mucosa forms projections called villi
c) circular folds which create permanent ridges
If the small intestine had smooth walls, the absorptive area would be
3.6 sq. feet as opposed to the actual 2200 sq. feet.
H) LARGE INTESTINE
The large intestine collects indigestible material and absorbs water.
Chyme may remain here for 3 to 10 hours.
Although we have carbohydrates, lipids, proteins, and nucleic acids in
our diets, these do not become the carbohydrates, lipids, proteins, and
nucleic acids in our bodies. Our digestive system breaks down polysaccharides
to monosaccharides, lipids to glycerol and fatty acids, proteins to amino
acids, and nucleic acids to their components. These building blocks enter
the blood and can be absorbed by our body's cells (fats reach the blood
after passing through the lymphatic vessels first). Our body cells may
perform catabolic reactions to obtain energy from these molecules or anabolic
reactions in which the body makes large human molecules from these building
blocks. Not only does the digestive system depend on the circulatory and
lymphatic systems for transport, it requires the respiratory system as
well. Oxygen is required for the reactions which produce energy from the
metabolism of food. During this process, carbon dioxide is produced as
a) hemorrhoids: enlarged and inflamed rectal veins (perhaps due to constipation
or straining during defecation) causes blood to ooze, creating itching,
swelling, and pain (especially when clots form)
b) molecules such as cellulose can't be digested and are referred to as
fiber; this improves the functioning of the digestive system and may reduce
vulnerability to a number of conditions from obesity to colon cancer
c) peptic ulcer disease (PUD) 5-10% population; may occur in duodenum
(most) or stomach
--hypersecretion of gastric acid or hyposecretion of protective mucus
& bicarbonate exposes mucosa to acid
--stress, smoking, and a bacteria may be involved
d) appendicitis--blockage of the lumen of appendix leads to inflammation
and possible rupture
e) gallstones are caused by crystallization of cholesterol molecules;
these may block flow from the gall bladder as they grow
f) essential amino acids
The human body is capable of synthesizing 10 of the 20 amino acids. Eight
amino acids cannot be made in the body and 2 amino acids cannot be made
in sufficient quantities (especially in children who need more protein
for growth); these are the essential amino acids. All animal protein is
complete protein in that all amino acids are present. Plant proteins are
usually incomplete in that certain amino acids may be absent; vegetarians
cannot survive on only beans or only corn but are fine when they eat both