Anatomy and Physiology

The alimentary tract, sometimes referred to as the gastrointestinal or GI tract, is the passageway running from the mouth to the anus. Like the skin, it is one of the body's boundaries with the environment, and it is one of the major routes into the body for invading microbial pathogens. The alimentary tract and its accessory organs, the saliva-producing glands, liver, and pancreas, together compose the alimentary system, the main purpose of which is to provide nourishment for the body. The upper part is composed of the mouth, salivary glands, esophagus, and stomach, and the

598 Chapter 24 Alimentary System Infections lower part includes the intestines, pancreas, and liver. Figure 24.1 illustrates the relationships among the various parts of the alimentary system. The main functions of the system are:

■ To grind food in the mouth into small particles that can readily react with digestive juices that break down large, complex molecules into absorbable components

■ To move the food through the esophagus to the stomach, where it undergoes preliminary treatment with acid and enzymes

■ To discharge stomach contents at a controlled rate into the small intestine, where they undergo digestion under alkaline conditions and absorption of nutrients into the bloodstream

Parotid gland Mumps

(a) Food ground; digestion initiated by saliva

(b) Conveyed to stomach

Upper — alimentary tract

(c) Acid digestion

Pylorus

Gallbladder Cholecystitis

Parotid gland Mumps

(a) Food ground; digestion initiated by saliva

(b) Conveyed to stomach

Upper — alimentary tract

(c) Acid digestion

(d) Alkaline digestion and absorption

(e) Resorption of water and nutrients; elimination of feces

Upper small intestine (duodenum) Duodenal ulcer

Small intestine joins large intestine

Appendix Appendicitis

Lower alimentary tract

Microvilli

Figure 24.1 The Alimentary System To the right of the human figure is a schematic representation of the major components of the system. Terms in color under the body part labels name some of the disease conditions that can affect that part. For example,mumps is a viral disease affecting the parotid gland; cholecystitis is an infection of the gallbladder.

Openings of intestinal glands

Epithelial cell

Intestinal • gland

Upper small intestine (duodenum) Duodenal ulcer

(d) Alkaline digestion and absorption

Lower alimentary tract

(e) Resorption of water and nutrients; elimination of feces

Small intestine joins large intestine

Appendix Appendicitis

Microvilli

Figure 24.1 The Alimentary System To the right of the human figure is a schematic representation of the major components of the system. Terms in color under the body part labels name some of the disease conditions that can affect that part. For example,mumps is a viral disease affecting the parotid gland; cholecystitis is an infection of the gallbladder.

Openings of intestinal glands

Epithelial cell

Intestinal • gland

■ To move undigested material into the large intestine, where water is absorbed from the intestinal contents along with any remaining vitamins, minerals, or nutrients

■ To discharge the waste as feces.

The Mouth

The mouth is mainly a grinding apparatus, the tongue and the cheeks moving bites of food to be ground up by the teeth (figure 24.2). The outer portion of the teeth, the enamel, is a hard, protective layer. Pits and crevices normally present on the enamel surfaces of the teeth collect food particles and offer protected sites for microbial colonization. Beneath the enamel lies the dentin, a softer, more easily penetrated layer. The gingival crevice, the space between the tooth and gum, is important because inflammation at this site can lead to loss of the tooth.

Salivary Glands

Salivary glands are located under the tongue and laterally in the floor of the mouth. There are two additional saliva-producing glands, called the parotids, one on either side of the face below the ears. About 1,500 ml of saliva is secreted from the salivary glands each day. It moistens and lubricates the mouth, contains the enzyme amylase, which begins the breakdown of starches present in food, and is extremely important in protecting the teeth from decay because it is saturated with calcium and contains buffers that help neutralize acids. It also contains antibacterial substances including lysozyme, the enzyme that attacks the pepti-doglycan of bacterial cell walls, lactoferrin, a substance that binds iron ions critically required by bacteria, and specific IgA antibodies that inhibit bacterial attachment. ■ lactoferrin, p. 375 ■ IgA, p. 401

Enamel

Enamel

Dentin

Pulp

Gingival crevice

Gingiva (gum)

Periodontal membrane

Root canal

Nerve

Vein Artery

Bone

Figure 24.2 Structure of a Tooth and Its Surrounding Tissues

Dentin

Pulp

Gingival crevice

Gingiva (gum)

Periodontal membrane

Root canal

Nerve

Vein Artery

Figure 24.2 Structure of a Tooth and Its Surrounding Tissues

24.1 Anatomy and Physiology 599

The Esophagus

The tongue and throat muscles work together in swallowing to propel food from the mouth and throat into the esophagus, a collapsible tube about 10 inches long, located behind the windpipe. The esophagus, like the intestines, has a muscular wall that contracts rhythmically, in a process called peristalsis, to move food and liquid. The lower portion of the esophagus normally closes to prevent regurgitation of stomach contents. The esophagus rarely becomes infected except in individuals with AIDS or other immunodeficiencies.

The Stomach

The lower portion of the esophagus leads into the stomach, an elastic saclike structure with a muscular wall. Some of the cells that line it produce hydrochloric acid, while others produce pepsinogen, which becomes the protein-splitting enzyme pepsin upon contact with acid. The stomach itself is protected from the acid and enzymes by a thick layer of mucus secreted by the stomach lining. Stomach emptying is controlled by a complex system of nerve impulses and hormones acting on a muscular valve, the pylorus, which determines the rate at which the stomach contents enter the intestine.

The Small Intestine

Although it is only 8 or 9 feet long, the small intestine has an enormous surface area, about 30 square feet, which facilitates nutrient absorption. The inside surface of the intestine is covered with many small, fingerlike projections called villi, each 0.5 to 1 mm long. Each of these villi are, in turn, covered with cells that have cytoplasmic projections called microvilli (see figure 24.1). Attached to the microvilli are many intestinal digestive enzymes. This epithelial surface has both secretory and absorptive functions. Tiny intestinal glands continuously secrete large amounts of fluid into the lumen, the space enclosed by the walls of the intestine. This fluid is then reabsorbed along with the products of food digestion.

Small intestine function depends on active transport ofnutri-ents and electrolytes across the plasma membranes of the epithelial cells. For example, sodium ions (Na+) are taken up by the cell when glucose or amino acids are absorbed, while hydrogen ions (H+) and bicarbonate ions (HCO3:) are secreted to adjust the pH ofthe intestinal contents. Movement of these substances is accompanied by water molecules. Digestion takes place as the cells secrete juices rich in enzymes such as the ones that break peptides into amino acids, and disaccharidases that convert complex sugars into simpler ones. The major part of nutrient absorption, including absorption of monosaccharides, amino acids, fatty acids, and vitamins, takes place in the small intestine. Some minerals, such as iron, can only be absorbed there. In addition, the small intestine reabsorbs approximately 9 liters of fluid per day. The lining cells of the small intestine are continuously shed and replaced by new cells, so that cells poisoned by a microbial toxin are soon replaced. Intestinal epithelial cells are completely replaced every 9 days, one of the fastest turnover rates in the body.

The Pancreas

Some cells of the pancreas produce hormones, and others produce digestive enzymes. The hormones are released directly into

Bone

600 Chapter 24 Alimentary System Infections the bloodstream and help regulate various body functions, as for example, insulin helps regulate the amount of glucose in the blood. On the other hand, about 2 liters of pancreatic digestive juices discharge directly into the upper portion of the small intestine each day. These fluids, as well as digestive juices of the small intestine itself, are alkaline and neutralize the stomach acid as it passes into the intestine.

The Liver

The liver aids digestion, neutralizes poisons, degrades medications, and removes a normal breakdown product of hemoglobin from the bloodstream. This yellowish product, called bile pigment, is discharged with the bile, the fluid produced by the liver. Each day, about 500 ml of bile flows through a system of tubes into the upper small intestine. Severe liver disease and obstruction of the bile ducts can produce jaundice, a yellow color of the skin and eyes caused by buildup of bile pigment in the blood. The gallbladder is a saclike structure in which bile is concentrated and stored. Substances in the bile called bile salts help the intestine absorb oils and fats and the fat-soluble vitamins A, D, E, and K. The normal brown color of feces results from the action of intestinal bacteria on bile pigment.

The liver also inactivates poisonous substances that enter the bloodstream. For example, ammonia produced by intestinal bacteria could poison the body if it were not detoxified by the liver. The liver also chemically alters and excretes many medications; if the liver is damaged, as it might be by a viral infection, lower medication doses might be needed to avoid the buildup of toxic levels.

The Large Intestine

The main function of the large intestine is to recycle water and to absorb nutrients. Because the small intestine absorbs so much water, only 300 to 1,000 ml of fluid normally reach the large intestine per day. From this volume of fluid, the large intestine absorbs water, electrolytes, vitamins, and amino acids. The semisolid feces, composed of indigestible material and bacteria, remain. Infection of the large intestine can interfere with absorption, and stimulate the painful peristaltic contractions known as "stomach cramps."

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