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Digestion
and Nutrition

YOUR BODY
How It Works

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DIGESTION AND NUTRITION

has several important functions in the body. It regulates

carbohydrate, protein, and lipid metabolism and detoxifies

body wastes and drugs that have entered the body. In addition,

the liver eliminates bilirubin, a waste product of dead red
blood cells, by incorporating it into bile. This fluid, which is

stored in the gallbladder, helps digestion by emulsifying fats

into smaller molecules for absorption. Bile is composed of

bile salts, cholesterol, and phospholipids, as well as other
substances. The bile salts and lipids work to emulsify fats.

When needed, bile is released from the gallbladder into the

small intestine.

The pancreas produces hormones, digestive enzymes, and

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Figure 6.3 Villi and microvilli (illustrated here) act to increase
the surface area of the small intestine, thus increasing the potential
for nutrient absorption. Villi are finger-like projections on the surface
of the intestine, and microvilli are smaller projections stemming
from the villi.

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bicarbonate to deliver to the duodenum to help digestion.

Insulin and glucagon are two pancreatic hormones that work
within the body to control blood glucose levels.

DIGESTION IN THE SMALL INTESTINE
When acidic chyme enters the duodenum, it triggers several

events. The acid, along with short proteins called peptides

and fatty acids in the chyme, causes cells at the beginning of

the duodenum to secrete intestinal fluid. About 1 to 2 quarts

of this digestive fluid is produced each day. The intestinal

juice, which contains some mucus, is alkaline and helps to

neutralize the acidic nature of chyme and protect the duo-

denum from the effects of the acid. The chyme also causes the

release of two hormones from the duodenum. The hormone

cholecystokinin (CCK) causes the gallbladder to constrict
and pump bile into the small intestine. CCK also causes the

pancreas to secrete digestive enzymes into the duodenum.

The hormone secretin causes the pancreas to secrete large
amounts of bicarbonate into the small intestine. The bicar-

bonate neutralizes most of the hydrochloric acid from the

stomach. After the neutralization occurs, the small intestine

contents are alkaline, creating the conditions needed for the

digestive enzymes to work.

Digestive enzymes break starch, proteins, triglycerides,

and nucleic acids into intermediate size pieces. Pancreatic

amylase breaks down starch. The bicarbonate from the

pancreas creates the alkaline conditions needed for amylase

and other enzymes to function. Amylase does not break

starch into glucose monosaccharide units, but into smaller

pieces, including the disaccharide maltose. Thus far in the

digestive process, the disaccharides, such as sucrose (from

table sugar and fruits) and lactose (from milk, such as the

chocolate shake in our example), that are ingested have not

been broken down. Dietary triglycerides are broken apart

by pancreatic lipase.

59The Stomach and Small Intestine

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Picture Credits

13: Lambda Science Artwork
20: © SIU/Visuals Unlimited
25: Lambda Science Artwork
27: Lambda Science Artwork
31: Lambda Science Artwork
34: Lambda Science Artwork
36: Lambda Science Artwork
42: Lambda Science Artwork
43: Lambda Science Artwork
47: Lambda Science Artwork

51: Lambda Science Artwork
53: © G.W. Willis, MD/Visuals Unlimited
58: Lambda Science Artwork
61: Lambda Science Artwork
65: Lambda Science Artwork
72: Lambda Science Artwork
77: AP Graphics
78: Courtesy USDA
79: Lambda Science Artwork

page:

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About the Author

Robert J. Sullivan Ph.D., MT (ASCP), is an Associate Professor of
Medical Laboratory Sciences at Marist College in Poughkeepsie, New
York. Dr. Sullivan teaches in both the medical laboratory science and the
biology curriculums. His research interests include the toxic effects of
heavy metals in alternative medicines, the use of medical laboratory
assays to evaluate the nutritional status of athletes, and international
issues in laboratory medicine.

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