Digestion Chapter 15 PDF

Chapter 15: Digestion 513 Lesson 15.1 **Alimentary Canal** Introduction The components of the **alimentary canal**, also known as the **gastrointestinal tract**, include the oral cavity, esophagus, stomach, small intestine, and large intestine (Figure 15.1). Ingested food moves through these co...

Chapter 15: Digestion 513 Lesson 15.1 **Alimentary Canal** Introduction The components of the **alimentary canal**, also known as the **gastrointestinal tract**, include the oral cavity, esophagus, stomach, small intestine, and large intestine (Figure 15.1). Ingested food moves through these components, each of which has a distinct function, to undergo mechanical and chemical digestion. Nutrients are absorbed by epithelial cells lining certain portions of the canal, and indigestible materials and waste are excreted from the body at the distal end of the canal. This lesson details the alimentary canal components and functions. **Figure 15.1** Components of the alimentary canal. 15.1.01 Oral Cavity and Esophagus Initial mechanical digestion of food occurs via mastication (ie, chewing) by the teeth in the **oral cavity** (Figure 15.2). While mastication occurs, saliva and mucus secreted by salivary glands ( [exocrine glands](javascript:void(0)) located in the oral cavity) lubricate ingested food. Saliva, a fluid substance, contains [enzymes](javascript:void(0)) for [initial](javascript:void(0)) [chemical digestion](javascript:void(0)) of some [macromolecules](javascript:void(0)). These enzymes include **lingual lipase**, which hydrolyzes triglycerides into free fatty acids, glycerol, monoglycerides, and diglycerides, and **salivary amylase**, which hydrolyzes the polysaccharide starch into the disaccharide maltose. Chapter 15: Digestion 514 In the oral cavity, the muscles in the tongue and cheeks compress mechanically digested food into a bolus (ball) that can be swallowed. This bolus passes from the oral cavity to the pharynx (throat) and past the upper esophageal sphincter (sphincters are rings of muscle that control transit through tubular structures, see Concept 15.1.06). Next, the bolus enters the **esophagus**, a passageway for food to be carried to the stomach. **Figure 15.2** Anatomy of the oral cavity and esophagus. 15.1.02 Stomach After passing through the esophagus, masticated food exits through the **lower esophageal sphincter** (sometimes called the cardiac sphincter because it is near the heart), a muscular ring located at the junction of the esophagus and the **stomach** that controls the passage of food into the stomach (Figure 15.3). In addition to being the first opportunity for temporary food storage in the alimentary canal, the sac-like stomach functions in chemical and mechanical digestion of the food bolus. In the stomach, the food bolus is converted to **chyme**, a semifluid, partially digested mixture of water, hydrochloric acid (HCl), digestive enzymes, food nutrients (eg, proteins, carbohydrates, fats) and indigestible food components.  Chapter 15: Digestion 515 **Figure 15.3** Anatomy of the stomach. Different cells of the stomach secrete different products (Table 15.1). Some cells secrete products that form a solution known as **gastric juice**, which aids in stomach function. Other cells secrete products that allow the stomach to efficiently form chyme without harming the stomach itself. Cells of the stomach include G cells, parietal cells, chief cells, and mucous cells. **G cells** produce **gastrin**, a hormone that helps regulate secretion of hydrochloric acid by parietal cells. Hydrochloric acid creates an [acidic](javascript:void(0)) environment (pH 1--3) in the stomach, which serves to activate [digestive proteolytic enzymes](javascript:void(0)), [defend](javascript:void(0)) against pathogens, and denature [proteins](javascript:void(0)). **Parietal cells** produce **hydrochloric acid** and **intrinsic factor**, a glycoprotein that aids in the [absorption](javascript:void(0)) of vitamin B12 in the ileum of the small intestine. **Chief cells** produce **pepsinogen**, an inactive form (ie, zymogen) of [pepsin](javascript:void(0)). When activated by the low pH of the stomach, the [proteolytic](javascript:void(0)) enzyme pepsin breaks down polypeptides into smaller peptides. Chief cells also produce **gastric lipase**, which serves to hydrolyze lipids in the stomach. **Mucous cells** produce **mucus**, which forms a [protective physical barrier](javascript:void(0)) for the stomach wall to guard against autodigestion (self-destruction) in the stomach\'s acidic, proteolytic environment. **Bicarbonate ions** released from stomach epithelial cells form a chemical barrier (ie, acid-neutralizing buffer) under the mucus barrier. Chapter 15: Digestion 516 **Table 15.1** Summary of secretions produced by various types of stomach cells. **Stomach cell type** **Secretory product** **Function of secretory product** G cells Gastrin Signals parietal cells to secrete hydrochloric acid Hydrochloric acid Primary component of gastric juice; activates proteolytic enzymes, kills microbes, and denatures (unfolds) proteins Parietal cells Intrinsic factor Aids in the absorption of vitamin B12 in the ileum Pepsinogen Zymogen (inactive form) of pepsin, which cleaves polypeptides into smaller peptides when activated by the low pH of gastric juice Chief cells Gastric lipase Carries out hydrolysis of lipids in the stomach Mucous cells Mucus and bicarbonate Protect the stomach walls from autodigestion by gastric juice, which contains acid and proteases Once the food bolus has been converted to chyme, it passes through the **pyloric sphincter** into the **small intestine**. 15.1.03 Small Intestine The **small intestine**, so named because it is smaller in diameter than the large intestine, is the site in which [macromolecular digestion](javascript:void(0)) is completed, and it is the primary site from which substances useful to the body are [absorbed](javascript:void(0)). As shown in Figure 15.4, there are [three subdivisions](javascript:void(0)) of the small intestine: the **duodenum**, the **jejunum**, and the **ileum**. The **pyloric sphincter** controls the flow of chyme from the stomach into the duodenum, and digestion is largely, if not entirely, completed in the duodenum and jejunum. Nutrient absorption occurs throughout the small intestine, with the specific nutrients and amounts absorbed varying in the three small intestine subdivisions. Chapter 15: Digestion 517 **Figure 15.4** Anatomy of the small intestine. In the duodenum, the presence of meal-derived fats within chyme and the acidity of chyme itself stimulate the release of bile and pancreatic secretions that digest fat and counteract the acid load (ie, raise the pH). **Bile** is a nonenzymatic solution produced by liver cells and stored in the gallbladder. When secreted into the duodenum, bile promotes the [neutralization](javascript:void(0)) of acidic chyme arriving from the stomach and the mechanical digestion of fats (ie, emulsification). Likewise, the pancreas assists both in neutralizing chyme (by secreting bicarbonate) and in digesting fats (by secreting lipases). Bile and pancreatic secretions enter the duodenum of the small intestine through the common bile duct (Figure 15.5).  Chapter 15: Digestion 518 **Figure 15.5** Digestion of lipids in the small intestine. In addition to the bile duct\'s digestive secretions and the mechanical mixing that occurs in the small intestine to make nutrients available, the surface of the small intestine itself strongly influences nutrient absorption. Large **circular folds** in the intestine\'s epithelial lining increase surface area and slow the movement of chyme through the intestinal tract. In addition to these circular folds, **villi** (fingerlike projections extending from the lining) and **microvilli** (smaller fingerlike extensions of individual absorptive cells) maximize the time and surface area available for nutrient absorption (Figure 15.6). The microvilli-covered epithelial surface where digestion and absorption of nutrients occur is called the brush border of the small intestine. A diagram of a human body Description automatically generated Chapter 15: Digestion 519 **Figure 15.6** The small intestine is specialized to maximize surface area for nutrient absorption. Nutrient absorption is also influenced by various intestinal enzymes. Complementing pancreatic lipases are lipases secreted from small intestinal cells. Proteolytic enzymes, which hydrolyze polypeptides, are synthesized and secreted from the microvilli-covered brush border and participate in protein digestion within the small intestine. **Dipeptidase** is a proteolytic enzyme that functions to directly digest proteins, while **enteropeptidase** serves to convert pancreatic trypsinogen in the small intestine into its active proteolytic form, **trypsin**. The small intestine also secretes enzymes that hydrolyze disaccharides (**disaccharidases**). In addition to enzymes, the small intestine releases several hormones important in digestion. **Secretin** is a hormone released by the small intestine that functions to promote pancreatic enzyme and bicarbonate release into the duodenum and inhibit gastric acid secretion by parietal cells of the stomach, thereby  Chapter 15: Digestion 520 regulating digestive tract pH. Secretin also slows digestive tract motility, allowing sufficient time for digestive enzymes to interact with chyme. **Cholecystokinin** (CCK) is another intestinal hormone that functions to promote pancreatic enzyme and bile release. The small intestine absorbs nutrients and most of the water from the chyme it receives from the stomach. Fat digestion products (eg, free fatty acids, monoglycerides) are ultimately absorbed via microvilli of small intestine epithelial cells, as are mono-, di-, and trisaccharides from digested carbohydrates and small peptides and amino acids from digested proteins. Material unable to be digested in the small intestine passes to the large intestine through the **ileocecal sphincter** (also known as the ileocecal valve). 15.1.04 Large Intestine The final portion of the alimentary canal is the **large intestine**, wider yet shorter than the small intestine but similarly consisting of [three subdivisions](javascript:void(0)): the **cecum**, **colon**, and **rectum** (Figure 15.7). The cecum, the first segment of the large intestine, is a small pouch connected to the appendix. The colon is subdivided into four sections: the ascending colon, transverse colon, descending colon, and sigmoid colon, from proximal (near) to distal (far). The colon is responsible for reabsorption of electrolytes (eg, sodium ions, chloride ions) and water from indigestible material. As this reabsorption occurs, undigestible material is compacted into solid waste (feces), which is stored in the rectum prior to elimination from the body. **Figure 15.7** Large intestine anatomy. The large intestine contains a diverse array of bacterial species (ie, **gut flora**) that aid in digestive processes. For example, some of these bacteria can process food items that would otherwise be indigestible, such as certain carbohydrates. Gut bacteria metabolize these undigested carbohydrates into short-chain fatty acids that can be absorbed and used by the body for energy. In addition, some bacterial species in the large intestine synthesize certain vitamins. A diagram of a human colon Description automatically generated Chapter 15: Digestion 521 15.1.05 Peritoneum The **peritoneum** lines the abdominal cavity and is made of two tissue layers. As shown in Figure 15.8, the **peritoneal cavity** is the space enclosed by the peritoneum. It is described as a potential space, or an area between two adjacent structures that may be pressed against one another. **Figure 15.8** Peritoneum and peritoneal cavity.  Chapter 15: Digestion 522 15.1.06 Peristalsis The movement of material through the alimentary canal relies on rhythmic muscular contractions rather than gravity. These wavelike muscular movements are termed **peristalsis** and occur due to the action of [smooth muscle](javascript:void(0)) (Figure 15.9). In addition to peristaltic waves, the movement of food through the digestive system is reliant on the relaxation of **sphincter muscles**. These rings of muscle (eg, cardiac sphincter, pyloric sphincter, ileocecal sphincter) divide the alimentary canal into segments with distinct functions. A sphincter is typically constricted so its center is closed but can relax to allow the natural passage of substances based on physiological requirements. **Figure 15.9** Peristalsis. A diagram of a human stomach Description automatically generated