26.1 Introduction to the Digestive System PDF

Page 1027 26.1 Introduction to the Digestive System The digestive system consists of the organs composing the gastrointestinal tract (e.g., stomach, small intestine) and the accessory digestive structures (e.g., liver, pancreas), which function to obtain nutrients from our diet. Here, we examine (a)...

Page 1027 26.1 Introduction to the Digestive System The digestive system consists of the organs composing the gastrointestinal tract (e.g., stomach, small intestine) and the accessory digestive structures (e.g., liver, pancreas), which function to obtain nutrients from our diet. Here, we examine (a) how the digestive system is organized, (b) the general functions that are used to obtain nutrients, (c) the details of the gastrointestinal tract wall, (d) how, in general, the numerous digestive organs are regulated, and (e) the associated serous membranes. 26.1a Organization of the Digestive System LEARNING OBJECTIVES 1. List the organs that make up the gastrointestinal (GI) tract. 2. List the accessory digestive organs and structures involved in the digestive process. The digestive system has two separate categories of organs: those composing the gastrointestinal tract, and the accessory digestive organs ( figure 26.1). The gastrointestinal (gas′trō-in-tes′tin-ăl; gastro = the stomach) (GI) tract is also called either the digestive tract or the alimentary canal. The GI tract organs essentially form a continuous tube that includes the oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, and large intestine. It ends at the anus. It is typically about 30 feet in length in an adult cadaver—although, due to smooth muscle tone (see section 10.10c), it is significantly shorter in a living individual. The entire length of the GI tract is lined internally with a mucous membrane (see section 5.5b). Within the lumen (inner space) of the GI tract, ingested food is broken down into smaller components that can then be absorbed along its length. Thus, the disassembly of molecules for absorption occurs within the lumen of the GI tract, and optimal digestion and absorption are dependent upon regulating and maintaining the environmental conditions within this space. Note that materials within the lumen of the GI tract are not considered part of the body until they are absorbed. Figure 26.1 Digestive System. The digestive system is composed of the gastrointestinal (GI) tract and accessory digestive organs that assist the GI tract in the process of digestion. APR Module 12: Digestive: Animations: Digestive System Overview Accessory digestive organs are connected to the GI tract and typically develop as outgrowths from the tract. These organs assist in the breakdown of food. Accessory digestive glands produce secretions that empty into the lumen of the GI tract and include the salivary glands, liver, and pancreas. Other accessory digestive organs are not glands. They include the teeth and tongue, which participate in the chewing and swallowing of food, and the gallbladder, which concentrates and stores the secretions (i.e., bile) produced by the liver. WHAT DID YOU LEARN? 1 How is the gastrointestinal (GI) tract distinguished from accessory digestive organs? List the structures that compose each category. Page 1028 26.1b General Functions of the Digestive System LEARNING OBJECTIVE 3. List and describe the six general functions of the digestive system. The digestive system performs six main functions: ingestion, motility, secretion, digestion, absorption, and elimination of wastes. Ingestion (in-jes′chŭn; ingero = to carry in) is the introduction of solid and liquid nutrients into the oral cavity. It is the first step in the process of digesting and absorbing nutrients. Motility (mō-til′i-tē) is a general term describing both voluntary muscular contractions (by skeletal muscle) and involuntary muscular contractions (by smooth muscle) for mixing and moving materials through the GI tract. Secretion (se-krē′shŭn) is the process of producing and releasing substances that facilitate both digestion and the movement of contents within the GI tract. Secretions are produced by both the accessory digestive glands (salivary glands, liver, pancreas) and the wall of the GI tract. Digestion is the breakdown of ingested food into smaller components that may be absorbed from the GI tract. Digestion is categorized as either mechanical digestion or chemical digestion. Mechanical digestion is the breaking of ingested material into smaller pieces without changing its chemical structure (i.e., no enzymes are involved). This is similar to an ice cube being crushed into ice chips. Chemical digestion involves the activity of specific enzymes (see section 3.3) to break down complex molecules into smaller molecules so that they can be absorbed. (Note that chemical digestion is also performed by bacteria within the large intestine.) Absorption (ab-sōrp′shŭn) involves membrane transport of digested molecules, electrolytes, vitamins, and water across the epithelial lining of the GI tract into the blood or lymph. Absorption occurs primarily within the small intestine. Elimination is the expulsion of indigestible components through the anal canal. WHAT DID YOU LEARN? 2 What is the primary difference between mechanical digestion and chemical digestion? 26.1c Gastrointestinal Tract Wall LEARNING OBJECTIVES 4. List and describe the four tunics (layers) that make up the gastrointestinal wall. 5. Briefly explain the process of absorption. 6. Distinguish the action of the muscularis mucosae of the mucosal tunic from that of the muscularis tunic. The GI tract from the esophagus through the large intestine is a hollow tube composed of four general layers, called tunics. From innermost (adjacent to the lumen) to outermost, the four general tunics are the mucosa, submucosa, muscularis, and adventitia or serosa ( figure 26.2). Figure 26.2 Tunics of the Abdominal GI Tract. (a) The wall of the abdominal GI tract has four tunics: the mucosa, submucosa, muscularis, and adventitia or serosa. (b) Substances need only to cross the epithelium of the mucosa through membrane transport processes to be absorbed into the blood capillaries or lymphatic capillaries. (c) Motility includes both mixing, which is a type of muscular contraction of the muscularis that facilitates the blending of materials within the GI tract, and propulsion, which moves material through the lumen of the GI tract. APR Module 12: Digestive: Histology: Duondenum: Low Magnification: Mucosa of Duodenum Mucosa The mucosa (mū-kō′sa) is the inner-lining mucous membrane. It typically consists of an epithelium, an underlying layer called the lamina propria, and a thin layer of muscularis mucosae. The epithelium of the mucosa is in contact with the contents within the lumen. It is a simple columnar epithelium for most of the GI tract (stomach, small intestine, large intestine). Recall from section 5.1c that this type of epithelium allows for secretion and absorption. The portions of the GI tract that must withstand abrasion (such as the esophagus) are lined by a nonkeratinized, stratified squamous epithelium. The underlying lamina propria consists of areolar connective tissue that contains small blood and lymph vessels and fine, small branches of nerves. Absorption occurs when substances are moved through the simple columnar epithelial cells that line the GI tract wall and are absorbed into blood or lymphatic capillaries located within the lamina propria ( figure 26.2b). The muscularis (mŭs-kū-lā′ris) mucosae is directly adjacent to the submucosa and is composed of a thin layer of smooth muscle. Contractions of this smooth muscle layer cause slight movements in the mucosa to gently “shake things up,” which (a) facilitates the release of secretions from the mucosa into the lumen and (b) increases contact of materials in the lumen with the epithelial layer of the mucosa for more efficient absorption. Submucosa The submucosa is composed of areolar and dense irregular connective tissue, the relative amounts of each vary depending upon the specific region of the GI tract. Many blood vessels, lymph vessels, nerves, and glands are within the submucosa. Fine branches of the nerves extend into the mucosa and, along with their associated autonomic ganglia, are collectively referred to as the submucosal nerve plexus, or Meissner plexus. The autonomic motor neurons within this plexus innervate both the smooth muscle and glands of the mucosa and submucosa. The areolar connective tissue of both the lamina propria of the mucosa and the submucosa house mucosa-associated lymphoid tissue (MALT). In the small intestine (and especially in its last portion, the ileum), larger aggregates of lymphoid nodules in the submucosa are called Peyer patches. MALT protects us from potentially harmful agents by preventing ingested microbes from crossing the GI tract wall and entering the body (see section 21.4d). Muscularis The muscularis is composed of smooth muscle tissue. The smooth muscle cells are arranged in an inner circular layer, which contains muscle cells oriented circumferentially within the GI tract wall, and an outer longitudinal layer, which is composed of muscle cells oriented lengthwise within the GI tract wall. Fine branches of nerves and their associated autonomic ganglia are located between these two layers of smooth muscle, and are collectively referred to as the myenteric (mī-en-ter′ik; mys = muscle, enteron = intestine) nerve plexus, or Auerbach plexus. The motor neurons within this plexus control contractions of the muscularis. Page 1029 INTEGRATE LEARNING STRATEGY 26.1 The relative orientation of the two layers of smooth muscle within the muscularis tunic can be mimicked by forming an “O” with your left hand (as the inner circular layer) and then placing your right hand with the fingers oriented upward along the outer edge of your left hand (as the outer longitudinal layer). Terri Bidle The function of the muscularis is motility. If you think of the GI tract as a hollow tube, then contractions of the circular layer constrict the lumen of the tube, whereas contractions of the longitudinal layer shorten the tube. The collective contractions of these smooth muscle layers are associated with two primary types of motility: mixing and propulsion ( figure 26.2c): Mixing is a “backward-and-forward” motion that blends secretions with ingested material within the GI tract, but does not result in directional movement of the lumen contents. Mixing includes mixing waves (by the stomach) and segmentation (by the small intestine). Propulsion, in comparison, is the directional movement of materials through the GI tract, and it occurs by the muscularis of the GI tract by peristalsis. Peristalsis (per′i-stal′sis; stalsis = constriction) is the sequential contraction of the muscularis within the GI tract wall that moves like a wave within the different regions of the GI tract (the esophagus, stomach, small intestine, and large intestine). Peristalsis results in one-way movement of the lumen contents from the esophagus to the anus. Additionally, the inner circular muscle layer is greatly thickened at several locations along the GI tract to form a sphincter. A sphincter is typically positioned between regions of the GI tract. These rings of smooth muscle relax (open) and contract (close) to (a) control the movement of materials into the next section of the GI tract and (b) prevent its backflow. The pyloric sphincter, for example, regulates the movement of material from the stomach into the small intestine. Adventitia or Serosa The outermost tunic may be either an adventitia or a serosa. An adventitia (ad-ven-tish′ă) is composed of areolar connective tissue with dispersed collagen and elastic fibers (see section 5.2a). A serosa (se-rō′să) is the adventitia plus an outer covering of a serous membrane called the visceral peritoneum. Only those digestive organs that are intraperitoneal (as described in section 26.1e) have a serosa as their outermost tunic. Page 1030 Some organs of the GI tract deviate from the typical pattern of the tunics described here. For example, the esophagus has a nonkeratinized stratified squamous epithelium in its mucosa to protect its lining, and the stomach has three layers of smooth muscle in the muscularis to help with the mechanical digestion of swallowed food. Being familiar with the basic tunic pattern, and then discovering how an organ may deviate from this pattern, provides clues as to the organ’s function. WHAT DID YOU LEARN? 3 What specific layer(s) must substances cross to enter the blood or lymphatic capillaries during their absorption? 4 How does peristalsis differ from mixing? 5 What purpose is served by muscular sphincters at various locations along the length of the GI tract? 26.1d Overview of the Regulation of the Digestive System LEARNING OBJECTIVES 7. Describe the general function of the enteric nervous system and autonomic nervous system in the regulation of the digestive system. 8. Compare short reflexes and long reflexes that regulate the digestive system. 9. Describe the general function of hormones that regulate the processes of digestion. Digestive processes are regulated by the nervous and endocrine systems. Here we (a) provide a general description of the enteric nervous system and the autonomic nervous system in regulating digestion through nerve reflexes, (b) discuss the general function of hormones that regulate digestion through endocrine reflexes, and (c) describe the receptors that monitor changes for these reflexes. Enteric Nervous System Recall from section 15.5b that the enteric nervous system (ENS) is an array of both sensory neurons and motor neurons, which extends from the esophagus to the anus. This network of neurons is composed of neurons that form both the submucosal nerve plexus and the myenteric nerve plexus within the GI tract wall ( figure 26.2a). It innervates the smooth muscle and glands of the GI tract and mediates the complex coordinated reflexes for the mixing and propulsion of materials through the GI tract. (Some researchers consider the ENS a completely separate nervous system both because of the number of neurons that compose it and because it can function independently of the central nervous system [CNS].) Autonomic Nervous System The GI tract wall is also innervated by both the parasympathetic and sympathetic divisions of the autonomic nervous system (ANS). The parasympathetic and sympathetic axons synapse with smooth muscle and glands of the GI tract wall (to control these structures directly) and with neurons within the ENS (to regulate these structures indirectly). In general, parasympathetic innervation promotes GI tract activity: It stimulates GI motility and release of secretions, and relaxes GI tract sphincters (see section 15.3a). In contrast, sympathetic innervation opposes GI tract activity: It inhibits GI tract motility and release of secretions, contracts GI tract sphincters, and vasoconstricts blood vessels within the GI tract wall. Thus, any conditions that activate the sympathetic division (e.g., exercise, anger, stress) may slow or interfere with digestion (see section 15.4c). INTEGRATE CONCEPT CONNECTION Three of the four cranial nerves (see sections 13.9 and 15.3a) containing parasympathetic axons are involved in regulating digestive activities: The facial nerve (CN VII) extends to the sublingual and submandibular glands to stimulate salivary secretions. The glossopharyngeal nerve (CN IX) extends to the parotid gland to stimulate salivary secretions. The vagus nerve (CN X) innervates most of the organs of the digestive system (e.g., stomach, small intestine, portions of the large intestine, pancreas, liver) and stimulates their digestive activities. Page 1031 Nerve Reflexes Both the ENS and ANS control the GI tract wall through nerve reflexes. In response to stimulation, either a short reflex or a long reflex is initiated. A short reflex is a local reflex that only involves the ENS (and does not involve the central nervous system). Sensory input detected by either baroreceptors or chemoreceptors is relayed to neurons within the ENS to alter smooth muscle contraction and gland secretion of the GI tract wall. These reflexes function in coordinating small segments of the GI tract to changes in stimuli. A long reflex involves sensory input relayed to the central nervous system (CNS), which serves as the integration center. Autonomic motor output is then relayed to alter smooth muscle contraction and gland secretion of the GI tract wall. Note that autonomic motor output is often relayed to other structures, including the accessory digestive organs (e.g., salivary glands, pancreas, liver). The results are coordinated smooth muscle contractions and secretory activity of potentially many different components of the digestive system. Hormonal Control Several primary hormones participate in the regulation of the processes of digestion (see section 17.3). These include circulating hormones, which are released into the blood (e.g., gastrin released from the stomach, which stimulates stomach motility and its release of digestive secretions; and secretin and cholecystokinin released from the small intestine, which inhibit stomach motility and its release of digestive secretions). In addition, local hormones are released and influence adjacent cells. (e.g., histamine released from endocrine cells within the stomach stimulates H+ release from adjacent parietal cells). The specific functions of the primary hormones are described throughout this chapter and summarized in table 26.1. Table 26.1 Primary Hormones That Control Digestion Hormone Stimulus for Release Primary Targets(s) and Effects STOMACH HORMONES Gastrin Somatostatin* Bolus in stomach— Stomach: Stimulates stomach motility and especially if bolus release of gastric secretions from parietal contains protein cells and chief cells Increased stomach Modulates the function of nearby acidity enteroendocrine cells and exocrine cells (e.g., inhibits release of acidic secretions by parietal cells) SMALL INTESTINE HORMONES Cholecystokinin (CCK) Chyme entering Stomach: Inhibits stomach motility and duodenum—especially if gastric secretions chyme contains fatty Gallbladder: Stimulates release of bile acids Pancreas: Stimulates release of enzyme-rich pancreatic juice Hepatopancreatic sphincter: Causes relaxation Ileocecal valve: Causes relaxation Secretin Chyme entering Stomach: Inhibits stomach motility and duodenum—especially if gastric secretions acidic chyme Liver: Stimulates secretion of HCO3− (an alkaline secretion) Pancreas: Stimulates secretion of HCO3− (an alkaline secretion) from pancreatic ducts Glucose-dependent Increase in glucose Pancreas: Primarily regulates release of insulinotropic peptide (GIP); within small intestine insulin from beta cells formerly gastric inhibitory peptide Hormone Stimulus for Release Primary Targets(s) and Effects Motilin Increased amounts Small intestine: Regulates migrating motility released later in the complex to move contents within lumen intestinal phase from the small intestine into the large intestine LIVER HORMONES Hepcidin Increased iron content in chyme Small intestine: Inhibits absorption of iron Receptors Recall from section 14.6a that the general components of reflexes include receptors, which detect stimuli. The receptors that initiate GI reflexes include baroreceptors, which detect stretch of the GI tract wall, and chemoreceptors, which monitor the chemical content of the material within the lumen, including the presence of protein and acid (see section 16.1d). WHAT DID YOU LEARN? 6 How is a short reflex distinguished from a long reflex? 7 Identify an example of both a circulating hormone and a local hormone that regulates digestive processes. 26.1e Serous Membranes of the Abdominal Cavity LEARNING OBJECTIVES 10. Describe the structure of the serous membranes associated with the GI tract. 11. Distinguish between intraperitoneal and retroperitoneal organs. 12. Explain the function of the mesentery, and describe the five individual mesenteries of the abdominopelvic cavity. The serous membrane associated with the abdominopelvic cavity is the peritoneum (per′i-tōnē′ŭm; periteino = to stretch over) and was first introduced in section 1.5e. The peritoneum consists of two serous membrane layers that are continuous with one another along the posterior abdominal wall. The parietal peritoneum is the serous membrane that lines the inner surface of the abdominal wall, whereas the visceral peritoneum is the serous membrane that covers the surface of internal organs within the abdominopelvic cavity. Between the parietal and visceral peritoneum is a potential space called the peritoneal cavity, which contains serous fluid. This fluid, which is produced by both the parietal peritoneum and visceral peritoneum, lubricates both the internal abdominal wall and the external organ surfaces. It allows the abdominal organs to move freely and reduces friction resulting from this movement. Page 1032 INTEGRATE CONCEPT CONNECTION The peritoneum is very similar to the pleura (see section 23.4c) and pericardium (see section 19.2b). These membranes have an outer parietal layer lining the inside of the cavity (parietal pleura) or inside of the pericardial sac (parietal pericardium) and an inner visceral layer that covers the external surface of the organ (visceral pleura and visceral pericardium). The space between the parietal and visceral layers is where serous fluid is secreted, and this fluid acts as a lubricant to prevent friction between the layers as the organs move. WHAT DO YOU THINK? 1 Are there any organs within the peritoneal cavity? Explain your answer. Intraperitoneal and Retroperitoneal Organs Organs within the abdomen that are completely surrounded by visceral peritoneum are called intraperitoneal (in′tră-per′i-tō-nē′ăl) organs. Thus, the outermost layer of each of these organs is a serosa (not an adventitia) (see section 26.1c). They include the stomach, most of the small intestine, parts of the large intestine (cecum, vermiform appendix, transverse and sigmoid colon) and most of the liver. In comparison, retroperitoneal (re-trō-per′i-tō-nē′ăl) organs lie outside the parietal peritoneum directly against the posterior abdominal wall, so only their anterolateral portions are covered with the parietal peritoneum. Note that these organs are not completely enveloped by a visceral peritoneum. Thus, the outermost layer of retroperitoneal organs is an adventitia (not a serosa). Retroperitoneal digestive organs include the pancreas, esophagus (abdominal portion), most of the duodenum (the first part of the small intestine), parts of the large intestine (ascending and descending colon), and the rectum (see figure 1.9). INTEGRATE LEARNING STRATEGY 26.2 Organs that are retroperitoneal are positioned against the posterior wall outside of the parietal peritoneum. These organs are associated with the digestive system, urinary system, and cardiovascular system. A common medical mnemonic used to remember the retroperitoneal organs is SAD PUCKER: S = Suprarenal glands (adrenal glands) A = Aorta and inferior vena cava D = Duodenum (most of) P = Pancreas U = Urinary bladder and ureters C = Colon (ascending and descending) K = Kidneys E = Esophagus (abdominal portion) R = Rectum Mesentery The general term mesentery (mes′en-ter-ē) refers to the double layer of peritoneum that attaches to the posterior abdominal wall and supports, suspends, and stabilizes the intraperitoneal GI tract organs. Blood vessels, lymph vessels, and nerves that supply the GI tract are sandwiched between the two layers of a mesentery. Because a mesentery contains multiple tissues, some anatomists have suggested classifying this structure as an organ (see section 5.5a). There are several types of mesenteries, each associated with specific organs (see figure 1.9 and figure 26.3): The greater omentum (ō-men′tŭm) extends inferiorly like an apron from the inferolateral surface of the stomach (greater curvature) and covers most of the abdominal organs. It often accumulates large amounts of adipose connective tissue; thus, it is referred to as the “fatty apron” and insulates the abdominal organs and stores fat. The lesser omentum connects the superomedial surface of the stomach (lesser curvature) and the proximal end of the duodenum to the liver. The falciform (fal′si-fōrm; falx = sickle) ligament is a flat, thin, crescent-shaped peritoneal fold that attaches the liver to the internal surface of the anterior abdominal wall. The mesentery proper, which is sometimes referred to as the mesentery, is a fan-shaped fold of peritoneum that suspends most of the small intestine (the jejunum and the ileum) from the internal surface of the posterior abdominal wall. The mesocolon (mez′ō-kō′lon) is a fold of the peritoneum that attaches the large intestine to the posterior abdominal wall. The mesocolon has several distinct sections, each named for the portion of the colon it suspends. For example, transverse mesocolon is associated with the transverse colon, whereas sigmoid mesocolon is associated with the sigmoid colon. Figure 26.3 Serous Membranes. Many abdominal organs are held in place or are insulated by double-layered serous membranes called mesenteries, which include (a) the falciform ligament, the greater and lesser omenta, and (b) the mesentery proper and the mesocolon. McGraw-Hill Education/Christine Eckel APR Module 12: Digestive: Dissection: Abdominal cavity: Anterior: Greater omentum Now that an overview of the main functions and features of the GI tract has been presented, we are ready to proceed with a more detailed exploration through the system, beginning with the upper GI tract. INTEGRATE CLINICAL VIEW 26.1 Peritonitis Peritonitis is an inflammation of the peritoneum and is associated with abdominal pain. The most common cause is perforation of the GI tract, which allows the contents of the GI tract to leak out and come in contact with the peritoneum. Perforations may result from within—for example, ulcers of the stomach or duodenum that “eat through” the tunics of the GI tract wall, a sharp object that has been ingested and pierces through the GI tract wall, or a ruptured appendix (which is the most common cause). Perforations may also result from without—for example, from a gunshot wound or during abdominal surgery. If the inflammatory response (see section 22.3e) is unsuccessful in eliminating infectious agents, surgery and extensive antibiotic therapy are required. Without medical intervention, peritonitis that spreads can be fatal. Page 1033 WHAT DID YOU LEARN? 8 Make a table that lists the digestive organs and whether these organs are intraperitoneal or retroperitoneal. 9 Where is the greater omentum located?