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.




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,



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.

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 waste.

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 .