Excludes1 newborn esophageal reflux (P78.83)

Functions of the Digestive System

The digestive system (Fig. 5-1) provides the nutrients needed for cells to replicate themselves continually and build new tissue. This is done through several distinct processes: ingestion, the intake of food; digestion, the mechanical and chemical breakdown of food; absorption, the process of extracting nutrients; and elimination, the excretion of any waste products. Other names for this system are the gastrointestinal (GI) tract, which refers to the two main parts of the system (the stomach and intestines) and the alimentary canal, which refers to the function of the tubelike nature of the majority of the digestive system, which starts at the mouth and continues in varying diameters to the anus.

Anatomy and Physiology

Overview

The digestive system begins in the oral cavity, passes through the thoracic cavity in the mediastinum, crosses the diaphragm into the abdominopelvic cavity, and finally exits at the anus. Several glands and organs located in the oral and abdominopelvic cavity are instrumental in carrying out the functions of the digestive system.

Most of the alimentary canal is in four coats, or tunics: the mucosa, the submucosa, the muscularis, and the serosa (Fig. 5-2). The inner tunic is the mucosa, which secretes gastric juices, absorbs nutrients, and protects the tissue through the production of mucus, a thick, slimy emission. This membrane is lined with a single layer of epithelial tissue that is attached to a platelike layer of connective tissue, the lamina propria. You might want to note that the combining form lamin/o, used to mean a “thin plate,” appears throughout many body systems. The term propria is from Latin and means “one’s own, or special” and is most likely used to designate this particular lamina from the many others in the body. The submucosa, the tunic underneath the tunica mucosa, holds blood, lymphatic, and nervous tissues that nourish, protect, and communicate. The next tunic is the muscularis, two layers of circular and longitudinal muscles that contract and relax around the tube in a wavelike movement termed peristalsis. If peristalsis is absent or delayed, the movement of food through the tract is impaired, causing disorders like constipation. The outermost tunic has different names in the digestive system, depending on whether it occurs within or outside of the peritoneal cavity. If outside, an outer tunic covering that binds a structure together is called the adventitia (also tunica externa). The tunic within the peritoneal cavity that emits a slippery fluid to counteract friction is termed the serosa. The serosa and visceral peritoneum are synonymous. All of these four layers are then attached to the body wall in the peritoneum by a rich vascular membrane which is an extension of the visceral peritoneum termed the mesentery.

submucosa

sub- = under

mucos/o = mucus

peristalsis

 peri- = surrounding

 -stalsis = contraction

viscera = viscer/o

 Exercise 1:

Overview of the Digestive System

Match the anatomy to its function.

____ 1. muscularis ____ 2. ingestion ____ 3. mesentery ____ 4. peristalsis ____ 5. serosa ____ 6. mucosa ____ 7. lamina propria ____ 8. absorption ____ 9. alimentary ____ 10. digestion ____ 11. tunic/tunica ____ 12. submucosa ____ 13. elimination

A excretion of waste products

B process of extracting nutrients

C breakdown of food

D vascular extension of visceral peritoneum

E wavelike movement in digestive tract

F tunic under tunica mucosa

G innermost mucus-secreting tunic

H intake of food

I small platelike connective tissue in mucosa

J pertaining to nutrition

K muscular tunic

L tunic secreting watery fluid

M coat, covering

Oral Cavity

Food normally enters the body through the mouth, or oral cavity (Fig. 5-3). The digestive function of this cavity is to break down the food mechanically by chewing (mastication) and lubricate the food to make swallowing (deglutition) easier.

The oral cavity begins at the lips, the two fleshy structures surrounding its opening. The upper lip is termed the labium superioris and the lower lip, the labium inferioris. The vermilion borders of each are the margins between the lip and surrounding skin. The term “vermilion” shares a combining form with the vermiform appendix. Here verm/o refers to the dark red color of a worm, while in the appendix, the term is used to describe its shape. The frenulum of each lip is the small fold of tissue on the inside of each lip that restrains its movement: the superior and inferior labial frenula. (The term frenulum is derived from Latin, meaning a “bridle” as one would use to restrain the movement of a horse.) The small vertical depression above the upper lip (and under the nose) is called the philtrum.

The sides of the face and inside of the mouth are bounded by the cheeks that are covered by skin on the outside, a mucous membrane on the inside, and muscles, fat, nerves, and glands in between. Several glands secrete mucus in the oral cavity: buccal, molar, palatine, and labial. The buccal glands are located throughout the inner cheek wall, while the molar glands are on the cheek near the back teeth. The labial glands are located inside the lips and surrounding the mouth, and the palatine glands surround the soft roof in the back of the mouth.

The tongue, the muscular organ in the oral cavity, is responsible for tasting, chewing, swallowing, and speaking. It is attached in the front to the floor of the mouth by the frenulum lingua, a small fold of tissue under the tongue and in the back to the hyoid bone. The tongue is coated in a mucous membrane studded with thousands of tiny projections called papillae. In between the papillae are nervelike cells called taste buds that have receptors for the five known tastes: sour, sweet, salty, bitter, and savory (umami). The lingual tonsil is lymphatic tissue located at the base of the tongue that serves a protective function against pathogens attempting to enter via the mouth. The anterior hard palate and posterior soft palate form the roof of the mouth. The uvula is a tag of flesh that hangs down from the medial surface of the soft palate. It has a role in the production of speech and the initiation of the gag reflex.

The upper and lower jaws (maxilla and mandible) hold approximately 32 permanent teeth that are set in the fleshy gums (gingivae) of the alveolar ridges of each bone. The thin, hard outer covering of the tooth is the enamel, while the dentin is the calcified second layer of the tooth (Fig. 5-4). The pulp is the center of the tooth with a blood and nerve supply. Cementum is a bonelike substance that covers the part of the tooth that is below the gums. The crown of the tooth is the visible enamel, the root is the area below the gums, and the neck is the area where both of these meet. The teeth are named by their function, location, or appearance. The central and lateral incisors are the front teeth that initially tear food to be chewed on the back teeth, the molars (derived from Latin for grinding). In between the incisors and the molars, the teeth are named for the number of points (cusps), either as cuspids or bicuspids (also called premolars because they are in front of the molars). Another name for the cuspids is the canines, because of the perceived similarity to the dentition of dogs. The upper cuspids were also called eyeteeth, because in the past, it was thought that the eyes and these teeth shared the same nerve supply. Molars have between 3-5 cusps, but their name is taken from their function, not the number of pointed projections. Periodontal ligaments anchor the teeth in their sockets to the alveolar bone of the upper or lower jaw. Note that the reason that periodontal disease causes a loosening of the teeth is that the tooth/bone bond has been compromised or destroyed.

The three pairs of salivary glands provide saliva, a substance that moistens the oral cavity and aids in chewing and swallowing. Saliva begins the chemical digestive process by initiating the digestion of starches. The glands are named for their locations: parotid, near the ear; submandibular, under the lower jaw; and sublingual, under the tongue.

Throat

The throat, or pharynx, is the passageway that connects the oral and nasal cavities with the esophagus (Fig. 5-5). It can be divided into three main parts: the nasopharynx, the oropharynx, and the hypopharynx. The nasopharynx is the most superior part of the pharynx, located behind the nasal cavity. The oropharynx is the part of the throat directly adjacent to the oral cavity, and the hypopharynx (also called the laryngopharynx because of its proximity to the larynx, which is the voice box) is the part of the throat directly below the oropharynx. The piriform recess (sinus or fossa) is the pear-shaped cavity in the hypopharynx near the opening to the voice box. This site is significant because food has a propensity for becoming lodged there.

Esophagus

The esophagus is a muscular, mucus-lined, approximately 12-inch tube that extends from the throat to the stomach. It carries a masticated lump of food, a bolus, from the oral cavity to the stomach by means of peristalsis. The glands in the lining of the esophagus produce mucus, which aids in lubricating and easing the passage of the bolus to the stomach. The muscle that must relax before the food enters the stomach is known by three names: the lower esophageal sphincter (LES), the gastroesophageal sphincter, or the cardiac sphincter (so named because of its proximity to the heart). Sphincters are ringlike muscles that appear throughout the digestive and other body systems. These muscles may be either voluntary or involuntary in their action.

Peritoneum

The peritoneum is a double-sided membrane that holds many of the organs inside the abdominopelvic cavity. The outer side of the membrane, near the body wall, is termed the parietal peritoneum, whereas the inner side, near the organs, is the visceral peritoneum. The visceral peritoneum is the serosal layer that coats the abdominopelvic viscera with its serous fluid, facilitating movement between the organs. Ascites, for example, is an abnormal accumulation of this peritoneal fluid in the abdominopelvic cavity.

Not all of the organs in the abdominopelvic cavity lie within the peritoneum. Some—for example, the aorta, kidneys, ureters, duodenum, and pancreas—are outside and behind the peritoneum in the retroperitoneum.

The organs that are within the peritoneum (intraperitoneal), however, have additional structures that serve to support and supply them: mesenteries, (visceral) ligaments, and folds. The mesenteries are extensions of the visceral peritoneum that stretch out to hold many of the abdominal organs and serve as a channel for blood vessels, nerves, and lymphatic vessels traveling to and from the organs in question. Mesenteries are named for the organs that they hold (e.g., mesocolon—mesentery that surrounds the colon, mesoappendix—mesentery that surrounds the appendix). Ligaments and folds attach one structure to another or provide support for organs in the peritoneal cavity.

The peritoneal cavity is divided into two main regions: the greater sac and the lesser sac. These two regions are connected by an opening termed the epiploic foramen (also called the foramen of Winslow). The greater sac is the main cavity of the peritoneal cavity, while the lesser sac is formed by two separate folds termed omenta. The omenta (sing. omentum) are folds of peritoneum that extend from the stomach, further compartmentalizing the peritoneal cavity and serving as sites of fat deposition, protecting against trauma and infection, and providing an immune support function (Fig. 5-6). The greater omentum extends from the greater curvature of the stomach, covers the intestines, and merges into the parietal peritoneum. The lesser omentum (also termed the omental bursa) extends from the lesser curvature of the stomach and connects to the liver.

Stomach

The stomach, an expandable saclike vessel located between the esophagus and the small intestines, has three main functions (Fig. 5-7). It begins the process of digesting proteins by storing the swallowed food and mixing it with gastric juices and hydrochloric acid to further the digestive process chemically. This mixture is called chyme. The smooth muscles of the stomach contract to aid in the mechanical digestion of the food. A continuous coating of mucus protects the stomach and the rest of the digestive system from the acidic nature of the gastric juices. Finally, the partially digested mixture is moved to the small intestines.

The stomach is divided into three main sections: the fundus, the body, and the pylorus (also called the gastric antrum). The fundus is the area of the stomach that abuts the diaphragm. This section of the stomach has no acid-producing cells, unlike the remainder of the stomach. The body, or corporis, is the central part of the stomach, and the pylorus (pl. pylori) is at the distal end of the stomach, where the small intestine begins. The pylorus is divided into the pyloric antrum, the pyloric canal, and the pyloric sphincter. The pyloric sphincter regulates the gentle release of food from the stomach into the small intestine. The portion of the stomach that surrounds the esophagogastric connection is the cardia (so named because of its proximity to the heart). When the stomach is empty, it has an appearance of being lined with many ridges. These ridges, or wrinkles, are called rugae (sing. ruga).

Small Intestine

Once the chyme has been formed in the stomach, the pyloric sphincter relaxes a bit at a time to release portions of it into the first part of the small intestine, called the duodenum (Fig. 5-8). The small intestine gets its name, not because of its length (it is about 20 feet long), but because of the diameter of its lumen (a tubular cavity within the body). The second part of the small intestine is the jejunum and the distal part is the ileum. The duodenojejunal flexure is the border between the first two sections of the small intestines.

Multiple circular folds in the small intestines, called plicae, contain thousands of tiny projections called villi (sing. villus), which contain blood capillaries that absorb the products of carbohydrate and protein digestion. The villi also contain lymphatic vessels known as lacteals that absorb lipid substances from the chyme.

The suffix -ase is used to form the name of an enzyme. It is added to the name of the substance upon which the enzyme acts: for example, lipase, which acts on lipids, or amylase, which acts on starches. The chemical suffix -ose indicates that a substance is a carbohydrate, such as glucose.

Large Intestine

In contrast to the small intestine, the large intestine (see Fig. 5-8) is only about 5 feet long, but it is much wider in diameter. The primary function of the large intestine is the elimination of waste products from the body. Some synthesis of vitamins occurs in the large intestine, but unlike the small intestine, the large intestine has no villi and is not well suited for absorption of nutrients. The ileocecal valve is the exit from the small intestine and the entrance to the colon. The first part of the large intestine, the cecum, has a wormlike appendage, called the vermiform appendix (pl. appendices), dangling from it. Although this organ does not seem to have any direct function related to the digestive system, it is thought to have a possible immunological defense mechanism.

No longer called chyme, whatever has not been absorbed by the small intestines is now called feces. The feces pass from the cecum to the ascending colon, bending at the hepatic flexure to cross the abdomen at the transverse colon, bending downward at the splenic flexure to become the descending colon, and then on to the S-shaped sigmoid colon. The teniae coli are the muscular bands that contract lengthwise and form the haustra—the bulges in the colon. The rectosigmoid junction marks the beginning of the last straight part of the large intestine, the rectum and its junction with the anus (the anorectal junction), where the feces are held until released from the body completely through the external and internal anal sphincters. The internal sphincter is an involuntary muscle, while the external sphincter is voluntary. The process of releasing feces from the body is called defecation or a bowel movement (BM).

Accessory Organs (Adnexa)

The accessory organs are the gallbladder (GB), liver, and pancreas (Fig. 5-9). These organs secrete fluid into the GI tract but are not a direct part of the tube itself. Sometimes accessory structures are referred to as adnexa.

The four lobes (right, left, quadrate, caudate) that form the liver virtually fill the right upper quadrant of the abdomen and extend partially into the left upper quadrant directly inferior to the diaphragm.

The liver forms a substance called bile, which emulsifies, or mechanically breaks down, fats into smaller particles so that they can be chemically digested. Bile is composed of bilirubin, the waste product formed by the normal breakdown of hemoglobin in red blood cells at the end of their life spans, and cholesterol, a fatty substance found only in animal tissues (Fig. 5-10). Bile is released from the liver through the right and left hepatic ducts, which join to form the hepatic duct. The cystic duct carries bile to and from the gallbladder. When the hepatic and cystic ducts merge, they form the common bile duct, which empties into the duodenum. Collectively, all of these ducts are termed bile vessels. Bile is stored in the gallbladder, a small sac found on the underside of the right lobe of the liver. When fatty food enters the duodenum, a hormone called cholecystokinin is secreted, causing a contraction of the gallbladder to move bile out into the cystic duct, then the common bile duct, and finally into the duodenum.

The pancreas is a gland located in the upper left quadrant. It is involved in the digestion of the three types of food molecules: carbohydrates, proteins, and lipids. The pancreatic enzymes are carried through the pancreatic duct, which empties into the common bile duct. Pancreatic involvement in food digestion is an exocrine function because the secretion is into a duct. Pancreatic endocrine functions (secretion into blood and lymph vessels) are discussed in Chapter 15.