10.5: The Mouth—The First Chamber of Digestion

The digestive tract—almost 30 feet long in an adult—can be envisioned as a series of chambers (see Fig. 10-1). Each chamber is a processing unit separated from the next by a muscular ring which opens to let the digesting food pass through and then closes to keep it from going back.

Our lips can be seen (quite unromantically) as a muscular ring leading to our mouth. Lips are made of tissue that is highly sensitive, especially to heat and pressure. Lips give us information about prospective food. Along with the help of the eyes and nose—under which approaching food must first pass—we learn enough decide whether or not to open our lips to let the food enter.

To watch the system in action, offer a new food to a small child. The head snaps back to let the eyes focus. Then the lips purse tight (backed up by the tightening of powerful jaw muscles) until the nose can sniff. That sniff carries aroma high into the nose where much of our tasting ability resides. Finally, the tip of the tongue protrudes to sample, and at last the mouth may open. But if the taste doesn’t live up to the advance billing, how quickly the food comes back out.

The mouth holds an intricate system for responding to food. Salivating is an initial response. The aroma, or even the thought of food, triggers the release of saliva from the salivary glands.

Saliva provides the moisture and lubrication needed for food to slip comfortably through to the stomach. Taste and perception of texture may cause the release of still more saliva, as in the case of very dry food, or when a strong acid is perceived and more saliva is needed to dilute it. The next time you taste a vinegar (acid) salad dressing, note how quickly your saliva flows. Or simply think about biting into a lemon. The mere memory of acidity is enough to flood the mouth.

Throughout the digestive tract, there’s a pattern of preparing for what’s coming. Seeing, smelling, even thinking about food brings saliva into the mouth, ready for the anticipated food. Indeed, this same stimulation may even begin the secretion in the stomach of a hormone which starts the stomach’s production of acid.

This triggering of saliva in the mouth and acid in the stomach is but one part of a complex cascade of chemical messages and responses triggered during digestion. Many of the chemical messages are transmitted by hormones; some of them and their actions are listed in Table 10-3.

Saliva contains a number of substances, including a digestive enzyme (salivary amylase) that begins the digestion of starch, and a lubricant (mucin) that makes the food “slippery.” We secrete about a quart-and-a-half of saliva a day.

As the food enters the mouth, the jaw begins to move—forward and back, up and down, side to side—to work the cutting-and-grinding edges of the teeth. Meanwhile, the muscular tongue and cheeks help to churn the food and keep returning it to the teeth for more cutting and grinding, while mixing it with saliva.

Figure 10-1: Mouth to Anus Pathway.

A Matter of Taste

The old idea that we taste only with our tongue was disproven long ago. Taste buds are tiny receptor organs buried in the surface of the tongue and are almost closed over except for individual small open pores. A few taste buds aren’t even on the tongue but in the lining of the mouth and at the back of the throat.

Plentiful as they are, these organs recognize only five basic “tastes”—sweet, salty, sour, bitter, and umami. All other “tastes” are really smelled. We acknowledge this when we bemoan the tastelessness of food when we have a “stuffy nose.”

Aromas from food are perceived by a small colony of smell receptors high up in the nose. Attached to these smell receptors are nerves that connect to the brain and report on the odors of food. Typically, smell receptors are much more numerous in other animals than in man, explaining why a St. Bernard dog can do a much better job of sniffing things out than we can.

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Umami is the Japanese word for savory/delicious, and is the taste of certain amino acids, like the glutamate in MSG (monosodium glutamate). It has a “meaty” flavor and drives a “craving” for protein-rich foods.

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Tastes can be perceived only when “tasty” chemicals are dissolved. When your mouth is dry, it’s hard to taste the saltiness of a potato chip—a crystal of salt is too large to fit into a taste bud. As the salt dissolves in the saliva, it becomes small enough to fit.

Aromas are really combinations of a number of smell sensations detected by the smell receptors in the nose. The process of recognition is subtle and complex. The brain assembles a combination of scent signals, and refers the particular combination to its memory bank for interpretation. Imagine the refinement of learning necessary for a wine taster to take a sip, bubble air inwardly from the lips to lift the aromatics to the smell receptors high in the nose, and say, “Definitely a claret…, French…, Chateau Margaux 1976.”

Many other factors influence taste and aroma. Temperature is one. Note how much sweeter ice cream tastes melted than when frozen. Tastes are also interactive. A dash of salt enhances the sweetness in a food—which explains why some people sprinkle salt on their watermelon. Salt cuts down the sour taste of certain acids, such as vinegar. In turn, certain acids make salty tastes more pronounced. Salad dressings often include both vinegar and salt. A popular fast food (particularly in Canada and England) is fish and chips sprinkled with salt and vinegar.

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Durian, a fruit from Southeast Asia, is known for its horrible smell (“smelly socks,” “sewage”) and its exquisite taste (“rich almond-flavored custard”).

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All the senses enter into the final perception of what we call “taste.” We find food appealing by touch (the crust on the roll feels crisp), temperature (ice cold beer), color (a lovely red tomato), sound (crunchy celery), and even the sensation of “melting” (cookies that melt in the mouth). All these qualities have much to do with which foods we take into our mouths.

Table 10-3: Some Gastrointestinal Hormones.

Production Site	Hormones	Trigger	Target	Action at Target
Stomach	Gastrin Ghrelin Obestatin	Stretching stomach Low calorie intake Unknown	Stomach Brain Brain	Secrete acid, pepsin Increased appetite Decreased appetite
Small Intestine	CCK (cholecystokinin) Secretin GIP (glocuse-dependent insulinotrophic peptide) PYY (Peptide YY)	Protein, fat Stomach acid, protein Glucose Eating	Gallbladder Pancreas Pancreas Stomach Pancreas Brain	Contract to release bile Secrete enzymes Secrete bicarbonate Inhibit acid secretion Secrete insulin Decreased Appetite

Swallowing isn’t as simple as it seems. The process starts with a closing of the lips and a voluntary push of the food towards the throat by the tongue. This act touches off a series of reflexes that close off alternate routes for the lump of food about to be swallowed into the esophagus.

The soft palate (a rearward extension of the roof of the mouth) rises to close off the nasal passages to prevent the food from going up into the nose. Simultaneously, the windpipe is blocked by the epiglottis, located at the top of the windpipe, which moves upward and forward to close the flap over the windpipe. This produces the familiar movement of the “Adam’s apple.” If food should accidentally enter the windpipe, the body uses violent coughs—sharp expulsions of air from the lungs—to try and blow the intruding matter away.

These patterns help explain some familiar experiences—why you may choke if you’re talking or laughing while eating; why it’s hard to swallow with your mouth open—as when you try to swallow the saliva that collects while the dentist works in your mouth; and why you can’t keep drinking without pausing for breath—as a baby can.

Babies can breathe and swallow at the same time, as when they are nursing or sucking a bottle. The baby’s short lower face causes the epiglottis (the flap that sits up over the windpipe) to extend above the level of the soft palate (the rearward extension of the roof of the mouth) and into the back of the nasal cavity. Thus, the windpipe doesn’t have to be shut off when the baby swallows because it’s already protected. The milk can just go around the epiglottis and down the esophagus.

Beyond the Mouth

In a very simplified way, the digestive tract beyond the mouth can be seen as a kind of tube. And this is how early investigators saw it—a long, simple tube of uneven width made up of four major chambers: the esophagus, stomach, small intestine, and colon (large intestine). But this “tube” isn’t so simple (see Fig. 10-2).

The digestive tube is multilayered. It not only has the layer of cells that make up the intricate and absorptive inner lining, but also layers of muscles—circular muscles that provide a squeezing motion and longitudinal muscles that contract lengthwise. The coordinated contraction of these muscles results in “waves” of motion (peristalsis) that propel the contents of the digestive tract steadily downward from esophagus to anus.

The squeezing motion of the circular muscles also performs the important task of mixing the food being digested. This mixing aids the chemical reactions of digestion, and increases the contact of the digestive products with the absorptive surface of the tube. The digestive tract’s movement, blood flow, and secretions are controlled by its own nervous system containing about 100 million nerve cells.¹

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Peristalsis: Wave-like contractions of digestive tract muscles that move the digestive material downward.

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Figure 10-2: The Digestive Tract.