The Digestive Tube (Alimentary Canal) in Mammals Help
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.
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.