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The Digestive Tube (Alimentary Canal) in Mammals Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

The Wondrous Small Intestine

Within humans, the small intestine is a small-diameter, extensively folded tube that averages about 20 feet (6 meters) in length. We have called this the wondrous small intestine, because of all the wonderful and amazing feats of digestion and absorption that occur within its walls.

The first segment of the small intestine is called the duodenum (dew- AH -den-um), which comes from Medieval Latin for “presence of 12.” Thus, the name, duodenum, reflects the fact that ancient anatomists measured its length as equal to about 12 fingers, placed side-by-side. A close look at Figure 19.4 also shows how critical the duodenum is as a common meeting point for chyme and various digestive secretions.

The duodenum receives chyme from the pylorus of the stomach, and secretions from the liver, gall bladder, and pancreas, as well. The liver is a large, brown, many-lobed organ that produces and secretes bile, as well as many other useful substances. Bile is a brownish-green detergent substance that emulsifies (ih- MUL -sih-feyes) fat within the small intestine. Emulsification (ih- mul -suh-fuh- KAY -shun) is literally the “process of” (-tion) “milking out” (emulsif) one non-mixable fluid substance from another one. Consider, in this case, the large globules of partially digested fat that do not mix very well with the soupy chyme entering the duodenum from the stomach. Bile from the liver acts to emulsify the large fat globules, breaking them apart (or, in a sense, “milking them out” of the rest of the chyme). As a result, a separate foam of tiny fat droplets is created within the small intestine. [ Study suggestion: Pour some liquid detergent onto a bunch of greasy plates, and then observe what happens. In what way does this liquid detergent act somewhat like bile?]

Bile is secreted continuously, day and night, into the right and left hepatic (heh- PAT -ik) or “liver” ducts. These ducts carry the bile into the cystic ( SIS -tic) duct . Cyst means “bladder” or “sac,” while chole ( KOH -lee) is Latin for “bile or gall.” Hence, the compound word, cholecyst ( KOH -luh-sist), translates into English as “gall bladder” or “bile sac.”

The cholecyst (gall bladder) is a muscular-walled sac that receives the bile from the liver and stores it temporarily. When the duodenum becomes swollen with fatty chyme, a hormone is released that stimulates the walls of the gall bladder to contract. A load of bile is squirted out of the cholecyst, much like a slug of brownish-green pea soup or gravy being squeezed out of a rubber balloon.

The bile squirts into the cystic duct, and then into the common bile duct, which carries it the rest of the way down into the duodenum. Here, then, the emulsification of fat takes place.

As shown in Figure 19.4, the pancreatic (pan-kree- AT -ik) duct extends from the pancreas and merges with the base of the common bile duct. The pancreatic duct is the main passageway for the pancreatic juice. Surrounding both of them at their point of union is the hepatopancreatic (heh- PAT -oh-pan-kree- AT -ik) sphincter ( SFINK -ter). The hepatopancreatic sphincter is a ring of smooth muscle that regulates the emptying of both the common bile duct (the hepatic or “liver” portion) and the pancreatic duct into the small intestine. When this sphincter (muscular ring) relaxes, bile and pancreatic juice flow through the duodenal (dew- AH -deh-nal) papilla (pah- PIL -lah). The duodenal papilla is a “little nipple or pimple” (papill) -like projection with a hole in its center. Bile from the liver and pancreatic juice from the exocrine gland portion of the pancreas drip into the duodenum through the hole in the duodenal papilla.

The pancreatic juice contains sodium bicarbonate (buy- CAR -buh- nayt ), symbolized chemically as NaHCO 3 , as well as a variety of digestive enzymes. These enzymes include amylases (starch-splitters), lipases ( LIE -pay-sez) or “fat-splitters,” and proteases ( PROH -tee- ay -sez) or “protein-splitters.” The lipases, for example, complete the chemical digestion of fat or lipids, after they have been emulsified by bile into a fatty foam. The resulting products, such as fatty acids and the substance, glycerol ( GLIH -sir- ahl ), are then absorbed across the walls of the small intestine, and into the bloodstream. Similarly, the proteases continue the chemical breakdown of proteins into amino acids, which are also absorbed into the bloodstream. And the amylases in the small intestine generally finish the chemical breakdown of carbohydrates into simple sugars such as glucose, which are then absorbed.

“Getting the Goodies”: Nutrition and the Digestive System The Digestive Tube (Alimentary Canal) in Mammals The Wondrous Small Intestine

Fig. 19.4 The duodenum and its friendly neighbors.

Reflecting its critical role in the absorption of nutrients, there are several important modifications to the mucosa (mew- KOH -sah), the “mucous” (mucos) membrane lining the small intestine. The mucosa of the duodenum, jejunum, and ileum is thrown into thousands of villi ( VIL -ee). Review of Figure 19.4 shows that each single villus ( VIL -us) is literally named for its resemblance to a little bump or curved “tuft of hair” (vill). The surface of each villus, in turn, is covered with dozens of microvilli ( MY -kroh- vil -ee). The microvilli are little bumps upon each villus, like “tiny tufts of hair.” The numerous villi and microvilli, by throwing the mucosa up into hundreds of tiny bumps, vastly increases the amount of surface area available for absorption. As a result, the absorption of nutrients into the bloodstream from the small intestine is extremely efficient.

Revisiting the liver once more, we can say that it has numerous other critical body functions, in addition to the production of bile. First among these is detoxification (dee- TAHKS -ih-fih- KAY -shun) – “the process of taking poison out of” the bloodstream. The liver cells help detoxify (dee- TAHKS -ih-feye) drinking alcohol, for instance, so that it is broken down into sugar and water without toxifying (“poisoning”) the brain! [ Study suggestion: When we say that a person is drunk, what does that imply, with regards to the associated function of the liver?]

Summarizing all of the above, we can say that the duodenum, as the first segment of the small intestine, receives chyme from the stomach, bile from the liver and cholecyst, and pancreatic juice from the pancreas. As a result, the chemical digestion of all three basic types of foodstuffs – carbohydrates, lipids, and proteins – is essentially completed within the small intestine. Lying downstream from the liver and pancreas entry point – the duodenal papilla – are the jejunum (jeh- JOO -num) and the ileum ( IL -ee-um).

The jejunum and ileum basically complete the processes of chemical digestion and absorption of nutrients that began in the duodenum.

We thus have the following simple summary equation:

SMALL INTESTINE = Duodenum + Jejunum + Ileum

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