General Anatomy and Digestive Processes

Objectives

When you have completed this section, you should be able to

• list the functions and major physiological processes of the digestive system;

• distinguish between mechanical and chemical digestion;

• describe the basic chemical process underlying all chemical digestion, and name the major substrates and products of this process;

• list the regions of the digestive tract and the accessory organs of the digestive system;

• identify the layers of the digestive tract and describe its relationship to the peritoneum; and

• describe the general neural and chemical controls over digestive function.

Digestive Function

The digestive system is the organ system that processes food, extracts nutrients from it, and eliminates the residue. It does this in four stages:

1. ingestion, the selective intake of food;

2. digestion, the mechanical and chemical breakdown of food into a form usable by the body;

3. absorption, the uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood or lymph; and finally

4. defecation, the elimination of undigested residue.

The digestion stage itself has two facets, mechanical and chemical. Mechanical digestion is the physical breakdown of food into smaller particles. It is achieved by the cutting and grinding action of the teeth and the churning contractions of the stomach and small intestine. Mechanical digestion exposes more food surface to the action of digestive enzymes. Chemical digestion is a series of hydrolysis reactions that break dietary macromolecules into their monomers (residues): polysaccharides into monosaccharides, proteins into amino acids, fats into glycerol and fatty acids, and nucleic acids into nucleotides. It is carried out by digestive enzymes produced by the salivary glands, stomach, pancreas, and small intestine. Some nutrients are already present in usable form in the ingested food and are absorbed without being digested: vitamins, free amino acids, minerals, cholesterol, and water.

Digestion involves the processes of motility, secretion, and membrane transport. Motility refers to the muscular contractions that break up food, propel it through the canal, mix it with digestive enzymes, and eliminate the waste. Secretion releases enzymes, hormones, and other products that carry out or regulate digestion. Membrane transport includes all the mechanisms such as active transport and facilitated diffusion that absorb nutrients and transfer them to the blood and lymph.

General Anatomy

The digestive system has two anatomical subdivisions, the digestive tract and the accessory organs (fig. 25.1). The digestive tract is a tube extending from mouth to anus, measuring about 9 m (30 ft) long in the cadaver. It is also known as the alimentary2 canal. It includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. Part of this, the stomach and intestines, constitute the gastrointestinal (GI) tract. The accessory organs are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

The digestive tract is open to the environment at both ends. Most of the material in it has not entered any body tissues and is considered to be external to the body until it is absorbed by epithelial cells of the alimentary canal. In the strict sense, defecated food residue was never in the body.

Most of the digestive tract follows the basic structural plan shown in figure 25.2, with a wall composed of the following tissue layers, in order from the inner to the outer surface:

Mucosa Epithelium Lamina propria Muscularis mucosae Submucosa Muscularis externa Inner circular layer Outer longitudinal layer

1gastro = stomach + entero = intestines + logy = study of

Saladin: Anatomy & I 25. The Digestive System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 25 The Digestive System 941

Oral cavity Tongue— Teeth

Sublingual gland

Submandibular gland

Diaphragm

Liver

Gallbladder Duodenum

Bile duct -

Ascending-

colon

Small intestine

Cecum

Appendix

Parotid gland

Oral cavity Tongue— Teeth

Sublingual gland

Submandibular gland

Diaphragm

Gallbladder Duodenum

Bile duct -

Ascending-

colon

Small intestine

Cecum

Appendix

Bile Duct Anatomy

Esophagus

Stomach Pancreas

Transverse colon

Descending colon

Sigmoid colon

Rectum Anal canal Anus

Figure 25.1 The Digestive System.

Parotid gland

Pharynx

Esophagus

Stomach Pancreas

Transverse colon

Descending colon

Sigmoid colon

Rectum Anal canal Anus

Figure 25.1 The Digestive System.

Serosa

Areolar tissue Mesothelium

Slight variations on this theme are found in different regions of the tract.

The mucosa, lining the lumen, consists of an inner epithelium, a loose connective tissue layer called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosae (MUSS-cue-LERR-is mew-CO-see). The epithelium is simple columnar in most of the digestive tract, but of the nonkeratinized stratified squa-mous type from the oral cavity through the esophagus and in the lower anal canal, where the tract is subject to more abrasion.

The submucosa is a thicker layer of loose connective tissue containing blood vessels, lymphatic vessels, a nerve plexus, and in some places, glands that secrete lubricating mucus into the lumen.

The muscularis externa consists of usually two layers of smooth muscle near the outer surface. Cells of the inner layer encircle the tract while those of the outer layer run longitudinally.

The serosa is composed of a thin layer of areolar tissue topped by a simple squamous mesothelium. The serosa begins in the lower 3 to 4 cm of the esophagus and ends with the sigmoid colon. The oral cavity, pharynx, most of the esophagus, and the rectum are surrounded by a fibrous connective tissue layer called the adventitia.

The esophagus, stomach, and intestines have a nervous network called the enteric3 nervous system, which regulates digestive tract motility, secretion, and blood flow. Two nerve networks make up this system: the sub-mucosal (Meissner4) plexus in the submucosa and the myenteric (Auerbach5) plexus between the two layers of the muscularis externa. These plexuses are considered to be part of the parasympathetic nervous system. Their functions will be discussed shortly.

Relationship to the Peritoneum

The stomach and intestines are enfolded and suspended from the body wall by extensions of the peritoneum (figs. A.9 and A.10, pp. 38-39). Most of the digestive tract is within the peritoneal cavity, but some portions of it are retroperitoneal—notably the duodenum (the first part of the small intestine), most of the pancreas, and parts of the large intestine.

Along the dorsal midline of the abdominal cavity, the parietal peritoneum turns inward and forms a sheet of tissue, the dorsal mesentery, extending to the digestive tract. The membrane then folds around the digestive tract to form the serosa. In some places it continues beyond the digestive organs as a sheet of tissue called the ventral mesentery, which may hang freely in the abdominal cavity or attach to the ventral abdominal wall or other organs.

Along the right superior margin (lesser curvature) of the stomach, a ventral mesentery called the lesser omen-tum extends from the stomach to the liver (fig. 25.3). Another membrane, the greater omentum, hangs from the left inferior margin (greater curvature) of the stomach and loosely covers the small intestine like an apron. At its inferior margin, the greater omentum turns back on itself, passes upward, and forms serous membranes around the spleen and transverse colon. Beyond the transverse colon, it continues as a mesentery called the mesocolon, which anchors the colon to the posterior abdominal wall.

3 enter = intestine

4Georg Meissner (1829-1905), German histologist

5Leopold Auerbach (1828-97), German anatomist

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

942 Part Four Regulation and Maintenance

942 Part Four Regulation and Maintenance

Tract Cross Section
Figure 25.2 Tissue Layers of the Digestive Tract. Cross section of the esophagus just below the diaphragm.

The omenta have a loosely organized, lacy appearance due partly to many holes or gaps in the membranes and partly to an irregular distribution of fatty tissue. They also contain many lymph nodes, lymphatic vessels, blood vessels, and nerves. The omenta adhere to perforations or inflamed areas of the stomach or intestines, contribute immune cells to the site, and isolate infections that might otherwise give rise to peritonitis.

Regulation of the Digestive Tract

The motility and secretion of the digestive tract are controlled by neural, hormonal, and paracrine mechanisms. The neural controls include autonomic reflexes called short reflexes and long reflexes. In short (myenteric) reflexes, stretching or chemical stimulation of the digestive tract acts through the myenteric nerve plexus to stimulate contractions in nearby regions of the muscularis externa, such as the peristaltic contractions of swallowing. Long (vagovagal) reflexes act through autonomic nerve fibers that carry sensory signals from the digestive tract to the central nervous system and motor commands back to the digestive tract. Parasympathetic fibers of the vagus nerves are especially important in stimulating digestive motility and secretion by way of these long reflexes.

The digestive tract also produces numerous hormones such as gastrin and secretin, and paracrine secretions such as histamine and prostaglandins, that stimulate digestive function. The hormones are secreted into the blood and stimulate relatively distant parts of the digestive tract. The paracrine secretions diffuse through the tissue fluids and stimulate nearby target cells.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. What is the term for the serous membrane that suspends the intestines from the abdominal wall?

2. Which physiological process of the digestive system truly moves a nutrient from the outside to the inside of the body?

3. What one type of reaction is the basis of all chemical digestion?

4. Name some nutrients that are absorbed without being digested.

Saladin: Anatomy & I 25. The Digestive System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 25 The Digestive System 943

Liver -

Gallbladder-

Transverse -colon

Ascending-colon

Gallbladder-

Greater— omentum

Small-

Greater— omentum

intestine

Greater omentum

(retracted)

Stomach

-Lesser omentum

Jejunum

Descending colon

Sigmoid colon

Jejunum

Descending colon

Mesocolon Adherence

-Mesocolon

- Mesentery

Sigmoid colon

Figure 25.3 Serous Membranes Associated with the Digestive Tract. (a) The greater and lesser omenta. (b) Greater omentum and small intestine retracted to show the mesocolon and mesentery. These membranes contain the mesenteric arteries and veins.

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Responses

  • Catriona
    What hangs from left inferior margin of the stomach and loosely covere the small intestine?
    5 years ago
  • mildred
    What is the physiological processes of the digestive system?
    3 years ago

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