The stomach is an expanded J-shaped organ in the upper left region of the abdominal cavity (Fig. 15-7). In addition to the two muscle layers already described, it has a third, inner oblique (angled) layer that aids in grinding food and mixing it with digestive juices. The left-facing arch of the stomach is the greater curvature, whereas the right surface forms the lesser curvature. The superior rounded portion under the left side of the diaphragm is the stomach’s fundus.
Sphincters A sphincter is a muscular ring that regulates the size of an opening. There are two sphincters that separate the stomach from the organs above and below. Between the esophagus and the stomach is the lower esophageal sphincter (LES). This muscle has also been called the cardiac sphincter because it separates the esophagus from the region of the stomach that is close to the heart. We are sometimes aware of the existence of this sphincter when it does not relax as it should, producing a feeling of being unable to swallow past that point. Between the distal, or far, end of the stomach and the small intestine is the pyloric sphincter. The region of the stomach leading into this sphincter, the pylorus, is important in regulating how rapidly food moves into the small intestine.
Figure 15-7 Longitudinal section of the stomach. The stomach’s interior is visible, along with a portion of the esophagus and the duodenum.
Functions of the Stomach The stomach serves as a storage pouch, digestive organ, and churn. When the stomach is empty, the lining forms many folds called rugae. These folds disappear as the stomach expands. (The stomach can stretch to hold one half of a gallon of food and liquid.) Special cells in the lining of the stomach secrete substances that mix together to form gastric juice. Some of the cells secrete a great amount of mucus to protect the stomach lining from digestive secretions. Other cells produce the active components of the gastric juice, which are:
* Hydrochloric acid (HCl), a strong acid that helps break down protein and destroys foreign organisms
* Pepsin, a protein-digesting enzyme produced in an inactive form and activated only when food enters the stomach and HCl is produced. Chyme, from a Greek word meaning “juice,” is the highly acidic, semiliquid mixture of gastric juice and food that leaves the stomach to enter the small intestine.
The Small Intestine
The small intestine is the longest part of the digestive tract (Fig. 15-8). It is known as the small intestine because, although it is longer than the large intestine, it is smaller in diameter, with an average width of approximately 2.5 cm (1 inch).
After death, when relaxed to its full length, the small intestine is approximately 6 m (20 feet) long. In life, the small intestine averages 3 m (10 feet) in length. The first 25 cm (10 inches) or so of the small intestine make up the duodenum (named for the Latin word for “twelve,” based on its length of twelve finger widths). Beyond the duodenum are two more divisions: the jejunum, which forms the next two-fifths of the small intestine, and the ileum, which constitutes the remaining portion.
Figure 15-8 The small and large intestines.
Functions of the Small Intestine The duodenal mucosa and submucosa contain glands that secrete large amounts of mucus to protect the small intestine from the strongly acidic chyme entering from the stomach. Mucosal cells of the small intestine also secrete enzymes that digest proteins and carbohydrates. In addition, digestive juices from the liver and pancreas enter the small intestine through a small opening in the duodenum. Most of the digestive process takes place in the small intestine under the effects of these juices. Most absorption of digested food, water, and minerals also occurs through the walls of the small intestine. To increase the organ’s surface area for this purpose, the mucosa is formed into millions of tiny, fingerlike projections, called villi (Fig. 15-9), which give the inner surface a velvety appearance. The epithelial cells of the villi also have small projecting folds of the plasma membrane known as microvilli. These create a remarkable increase in the total surface area available in the small intestine for absorption. Each villus contains blood vessels through which most digestion products are absorbed into the blood. Each one also contains a specialized lymphatic capillary called a lacteal through which fats are absorbed into the lymph.
Figure 15-9 A villus of the small intestine. Each villus has blood vessels and a lacteal (lymphatic capillary) for absorption of nutrients.
The Large Intestine
The large intestine is approximately 6.5 cm (2.5 inches) in diameter and approximately 1.5 m (5 feet) long (see Fig. 15-8). It is named for its wide diameter, rather than its length. The outer longitudinal muscle fibers in its wall form three separate surface bands (see Fig. 15-8). These bands, known as teniae coli draw up the organ’s wall to give it its distinctive puckered appearance. (Spelling is also taeniae; the singular is tenia or taenia).
Subdivisions of the Large Intestine The large intestine begins in the lower right region of the abdomen. The first part is a small pouch called the cecum. Between the ileum of the small intestine and the cecum is a sphincter, the ileocecal valve, that prevents food from traveling backward into the small intestine. Attached to the cecum is a small, blind tube containing lymphoid tissue; its full name is vermiform appendix (vermiform means “wormlike”), but usually just “appendix” is used.
The second portion, the ascending colon, extends upward along the right side of the abdomen toward the liver. It bends near the liver at the right colic (hepatic) flexure and extends across the abdomen as the transverse colon. It bends again sharply at the left colic (splenic) flexure and extends downward on the left side of the abdomen into the pelvis, forming the descending colon. The distal part of the colon bends backward into an S shape forming the sigmoid colon (named for the Greek letter sigma), which continues downward to empty into the rectum, a temporary storage area for indigestible or nonabsorbable food residue (see Fig. 15-8). The narrow portion of the distal large intestine is the anal canal, which leads to the outside of the body through an opening called the anus.
Functions of the Large Intestine The large intestine secretes a great quantity of mucus, but no enzymes. Food is not digested in this organ, but some water is reabsorbed, and undigested food is stored, formed into solid waste material, called feces or stool, and then eliminated. At intervals, usually after meals, the involuntary muscles within the walls of the large intestine propel solid waste toward the rectum. Stretching of the rectum stimulates contraction of smooth muscle in the rectal wall. Aided by voluntary contractions of the diaphragm and the abdominal muscles, the feces are eliminated from the body in a process called defecation. An anal sphincter provides voluntary control over defecation (see Fig. 15-8). While the food residue is stored in the large intestine, bacteria that normally live in the colon act on it to produce vitamin K and some of the B-complex vitamins. As mentioned, systemic antibiotic therapy may destroy these symbiotic (helpful) bacteria living in the large intestine, causing undesirable side effects.