Introduction to The Digestive Tube (Alimentary Canal) in Mammals
The digestive tube is alternately called the alimentary ( al -uh- MEN -tur-ee) tract. The reason, of course, is that the digestive tube has many of its major functions “related to” (-ary) “nourishing” (aliment) the rest of the body. Let us now, therefore, examine the parts of the human digestive tube in sequence, along with their related functions. (Start looking at Figure 19.2). And in so doing, we shall also be learning about the digestive tube in other mammals.
Fig. 19.2 Major features of the human digestive tube.
The Oral Cavity, Pharynx, And Esophagus
Food ingested by a human (or other mammal) immediately enters the mouth (oral cavity) where physical digestion of all three major types of foodstuffs (carbohydrates, lipids, and proteins) begins. Physical digestion is just the mechanical breaking apart of ingested food, using the teeth, lips, and gums.
In humans, chemical digestion of carbohydrates also begins within the oral cavity. This is due to the presence of “spit” or saliva (sah- LIE -vah), secreted into the mouth by the salivary glands. The saliva contains various digestive enzymes, such as salivary amylase ( AM -ih- lace ), or “starch” (amyl) “splitter.” Salivary amylase begins the chemical digestion (breakdown) of complex carbohydrates, such as starch, into double-sugars. [ Study suggestion: Eat a plain saltine cracker. At first, it is quite dull, reflecting its complex starch content. But as you chew it and mix it with your saliva, notice that it begins to taste sweet. To what specific chemical should you give credit for this dramatic change?]
By the time a person is done chewing food and mixing it with saliva, the general result is a food bolus ( BOH -lus). A food bolus is a soft “ball” (bol) of partially digested food.
When done chewing, the person uses the tongue and flips the food bolus into the back of the oropharynx (portion of the throat behind the mouth). The bolus pushes the epiglottis shut, then slides down into the esophagus. The upper portion of the esophagus is lined by voluntary striated (cross-striped) muscle. Hence, the first part of swallowing is voluntary. (“So, why did I just gulp down that piece of delicious apple?” you might well ask yourself. “Because I darn well wanted to, that’s why!”)
However, the lower 2/3 of the esophagus is lined by smooth, involuntary muscle. This means that the latter part of swallowing is not under our conscious control. Therefore, once a swallowed food bolus has entered the lower esophagus, you just have to let it go down into your stomach! (“Oh, oh!” you might suddenly question yourself. “Didn’t I just see half a worm in that chunk of apple?” Too late! You’ve already swallowed it! You can’t back out, now!)
The Stomach And Its Adaptations
In humans, the stomach is a capital J-shaped pouch that acts as a temporary storage place for ingested food. The body is the large central area of the stomach, while the pylorus (pie- LOR -us) or “gatekeeper” is a small room at the far end of the stomach.
The epithelial cells lining the “stomach” (gastr) secrete the gastric ( GAS trick) juice. The gastric juice is especially rich in hydrochloric ( HIGH -droh- klor -ik) acid, abbreviated as HCl, and pepsin ( PEP -sin). Being an extremely strong acid, HCl breaks down rapidly and donates many H + ions, making it highly reactive and corrosive. Thus, hydrochloric acid begins the chemical digestion of lipids and proteins, as well as continuing the digestion of carbohydrates. Pepsin is an enzyme that helps break down proteins, as well.
Due to the action of the gastric juice, the food bolus from the esophagus is now changed into chyme ( KIGHM ). The chyme is a thick, soupy mass of partially digested material. Since it is almost a liquid, chyme is like a “juice” (chym) that leaves the stomach through a muscular ring called the pyloric (pie- LOR -ik) sphincter.
“If the stomach is full of so much HCl, then why doesn’t it digest itself?” Part of the answer is that the stomach secretes a highly alkaline (basic) layer of mucus, a protective “slime” (muc) that coats the lining and neutralizes acid that contacts it.
There is one type of foodstuff that HCl and pepsin hardly touch, however. That foodstuff is cellulose ( SELL -yuh- lohs ). Cellulose is a “carbohydrate” (-ose) composed of the walls of many “little cells” (cellul) found in plants. Wood, cotton, and grass, for instance, are largely made up of cellulose. “Why don’t we just send the kids out in the backyard to graze on our grass?” a sarcastic father might jokingly ask. “After all, then we wouldn’t have to mow it, and they’d get big as cows!”
A comparison to the anatomic features of the cow stomach may help explain why this dad is really off the mark! As Figure 19.3 reveals, the cow stomach consists of four different chambers. These chambers are called the rumen ( ROO -men), reticulum, omasum (oh- MAY -sum), and abomasum ( ab -uh- MAY -sum). Because the first chamber, the rumen, is so important, it serves as the foundation for the name of the entire group of herbivores (plant-eating mammals) to which cows, deer, goats, and sheep belong. This group is called the ruminants ( REW -muh-nunts) – animals that are “chewers of cud.”
Cud represents a mouthful of food that has been swallowed and then brought back up into the mouth from the rumen (first stomach chamber) of a ruminant. The cud is then given a slow, thorough, second chewing before it is swallowed, again. But simple re-chewing isn’t good enough, by itself. This is because cows, like humans, don’t produce enzymes capable of chemically digesting plant cellulose (as found in grass). To get around this problem, there are special bacteria and protists (Chapter 7) living in the first two stomach chambers, the rumen and reticulum. Now, the microorganisms within these chambers do have enzymes that digest cellulose in grass and break it down into simple sugars that the cow’s stomach can further handle, then absorb. The repeated chewing of cud containing millions of these micro-organisms simply crunches up the cellulose plant walls, thereby allowing the enzymes to digest them even more efficiently.
Fig. 19.3 The four stomach chambers of the cud-chewing cow.
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.
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
The Colon: Our Large Intestine
The last major section of the digestive tube is the colon ( KOH -lun) or “large intestine.” The colon (large intestine) is a wide-diameter, folded tube, about 6 feet (2 meters) in length in an average-sized human adult (see Figure 19.5). The colon begins with the cecum ( SEE -kum), a “blind” ( cec ) or dead-ended pouch that has the vermiform ( VER -mih-form) appendix or “worm-like attachment” hooked to its base. The vermiform appendix is basically a solid attachment of modified lymphatic tissue that plays a minor role in the body’s immune or self-defense system.
Liquid chyme from the ileum of the small intestine pushes through the ileocecal ( il -ee-oh- SEE -kul) sphincter. Once within the cecum, the chyme begins to undergo an extensive drying out process, wherein large amounts of water and salt (H 2 O) are absorbed. In addition, there are beneficial bacteria in the colon that produce a variety of B-vitamins, as well as sulfur-containing amino acids, which are also absorbed.
Due to this drying out process, chyme is solidified into feces ( FEE -seez) within the colon. Besides H 2 O, feces also contain a significant percentage of fecal ( FEE -kal) bacteria and dietary fiber (actually undigested cellulose material).
Looking at Figure 19.5, you might well ask yourself, “How does chyme/feces in the cecum, which is way down at the bottom of the large intestine, move up to the other parts of the colon?” Good question! The answer is, by mass peristalsis (pear-ih- STAHL -sis). Peristalsis literally means “a constriction” (-stalsis) “around” (peri-). Formally defined, then, peristalsis is a constriction (narrowing) around a particular point of some tube, due to the contraction of a circular ring of smooth muscle around the tube. Mass peristalsis is thus the simultaneous constriction or narrowing of a large number of directly neighboring points along the large intestine wall, such that the heavy mass of the feces is pushed on to the next section of the colon. [ Study suggestion: Either in your imagination, or for real, get a large tube of toothpaste. Turn it upside down, and remove the cap. Now, make a ring around the upper part of the tube using both your hands. Count one . . . two . . . three . . . . Constrict your fingers hard around the tube! What do you observe? This constriction at a single point is a crude model of peristalsis. Think. How could you use the same basic elements to model mass peristalsis in the colon? If you phoned up several of your friends, what could you do?]
Fig. 19.5 The human colon (large intestine).
After the cecum, next in sequence are the ascending colon (which goes upward), transverse colon (which runs sideways), and the descending colon (which goes downward). Sigmoid ( SIG -moyd) means “S-resembling.” Hence, the sigmoid colon is the S-resembling portion of the large intestine coming right after the descending colon.
The sigmoid colon snakes down into the rectum ( WRECK -tum), a muscular-walled, “straight” (rect) tube that empties feces into the anus. There are two sphincters within the rectum. The higher one, called the internal anal sphincter, is not under our conscious control. The internal anal sphincter tends to open whenever mass peristalsis has moved some feces down into the upper portion of the rectum.
Fortunately, for us, there is also a ring of voluntary striated (cross-striped) muscle, positioned in the lower portion of the rectum. This muscular ring is called the external anal sphincter. Its contraction and relaxation is very much under our conscious control (at least, ever since we were first “potty-trained”)! Therefore, we can usually choose the time and place where we will consciously relax this lower sphincter and carry out defecation (egestion).
Let us end by summarizing in words the parts of the large intestine:
COLON = Cecum + Ascending colon + Transverse colon + Descending colon + Sigmoid colon + Rectum
Practice problems for these concepts can be found at: Nutrition And The Digestive System Test
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