10.7: The Churning, Acid Stomach
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Once food reaches the bottom of the esophagus, it passes through a circular band of muscle called the cardiac sphincter (located near the heart) and into the stomach. The stomach is about 10 to 12 inches long and about 4 inches wide, a kind of muscular elastic bag that can stretch when food enters and then later return to its normal size.
The stomach is the widest part of the digestive tract. It looks quite different than the esophagus, but it’s fairly similar in structure. As we shall see with respect to each digestive chamber, the layers of tissue are modified to enable each chamber to do its special work.
Sphincter: A ring-shaped muscle that can open or close to let substances pass—or not pass.
Two common complaints one hears about the stomach are that it’s “acid,” or that distress makes it “churn.” But the fact is that these characteristics refer to the normal state of the stomach during digestion. Necessarily so.
In addition to its ballooning shape, the stomach differs from the esophagus in two other major ways. One is a dramatic increase in muscularity.
In addition to the circular and longitudinal muscles, the stomach has diagonal muscles which make it possible for the stomach to twist and churn. This kneading effect actually breaks up food. The second key change is in its lining. In the esophagus, this lining did little more than supply mucus, both to ease the food along its way and to protect itself from physical damage. In the stomach, the lining is much thicker, and multitudes of glands appear.
The millions of glands in the folded, velvety lining of the stomach secrete many substances, including digestive enzymes, hydrochloric acid, mucus, and a protein called intrinsic factor. Intrinsic factor is needed for vitamin B12 absorption. It attaches to B12, and the complex is absorbed later in the lower part of the small intestine.
Stomach secretions total about 3 quarts per day, and the hydrochloric acid makes the mixture very acid. Food, in effect, goes through an acid bath. The acid is strong enough to eat into meat—and strong enough to kill large numbers of bacteria.
Very little absorption of nutrients occurs in the stomach. But alcohol can be absorbed here. It only needs to make contact with the stomach lining. Thus, alcohol’s effects are felt sooner when consumed on an empty stomach.
The stomach secretes an enzyme (alcohol dehydrogenase) that breaks down alcohol. Women have less of this enzyme in their stomach than men.2 In addition, a woman’s smaller body size and higher body fat, means higher blood-alcohol levels and more risk of damage to her heart and liver (the only other organ with this enzyme) when she drinks the same amount as a man.
Alcohol is first broken down to acetaldehyde, which is normally broken down quickly by a second enzyme (aldehyde dehdrogenase). Many Asians have a genetic variation of this enzyme that’s less efficient, causing higher levels of acetaldehyde.3,4 This results in flushing (“red face”) and a more severe hangover.
Excess Acidity
Contrary to popular belief, acid foods have little to do with excess acidity in the stomach. By the time acid foods such as citrus fruits have been diluted by saliva and mixed with other foods, they aren’t acid enough to increase the acidity of the stomach. Most ulcers in the stomach (and duodenum) are caused by H. pylori infection (see Small Intestine discussion of duodenal ulcers).
The pain associated with stomach acid isn’t usually experienced in the stomach at all, but rather in the lower end of the esophagus. Here the strong muscular squeezing of a full stomach, or an excess of stomach gas, may splash stomach acid up past the cardiac sphincter (the muscular ring between the stomach and esophagus) onto the lining of the esophagus. This splashing of acid can cause pain because the esophagus isn’t as well protected as the stomach against acid.
Because the lower esophagus is near the heart, the pain is described as “heartburn.” Heartburn is really “esophagus burn.”
When this happens, the sphincter isn’t doing its job of preventing this back-tracking. The stretching of the stomach by a large meal and/or a high-fat meal can cause the sphincter to relax inappropriately. Thus, heartburn often can be avoided by eating smaller, lower-fat meals, and not lying down after eating.
A self-prescribed, over-use of antacids is unwise. The advice is to not use an antacid product for more than two weeks without consulting a physician. A continuing need for an antacid suggests a problem that should be investigated by a physician.
GERD (Gastroesophageal Reflux Disease) is the chronic (more than twice a week) backing up of acid. It can cause abnormal cells in the esophageal lining, which sometimes results in cancer of the esophagus. It’s important to get GERD treated by a physician.
Vomiting
Vomiting brings up the contents of the upper digestive tract (usually from the stomach up, but sometimes from as far down as the duodenum). The vomiting reflex is relayed through a vomiting center in the brain, and involves a squeezing of the stomach by a downward movement of the diaphragm together with a contraction of the abdominal muscles.
Its strongest triggers are irritation and marked stretching of the stomach. But other things like motion sickness, the morning sickness of early pregnancy, or a blow to the head can cause vomiting. Even emotional upsets can cause it.
An empty stomach is more susceptible to vomiting, so eating a little can help ward it off. Prolonged vomiting can cause serious problems, especially for infants and the elderly.
Vomiting can be induced by initiating the gagging reflex, as it often is by those suffering from the eating disorder bulimia (see Chap. 3).
Burps and Belches
Gas from the stomach is a common concern, at least as a social embarrassment. The usual cause has nothing to do with digestion at all, but the harmless result of swallowing air. We swallow air not only with our food, but with two other substances that we swallow almost continually— excess saliva (especially if we feel nervous or hungry) and mucus from the nose. We swallow about 2,400 times a day.
The nose is protected by a thin blanket of mucus. This mucus originates near the start of the nasal passages, descends down the back of the throat, and is swallowed. Among other things, this nasal flow washes away bacteria that, when swallowed, can be killed by stomach acid.
When enough gas has accumulated—from air-swallowing or from chemical reactions of gastric secretions and food—and when the pressure from the filling of the stomach or from muscular churning is great enough, the bubble of gas breaks upwards into the esophagus. And we burp.