Digestion Chapter 15: Accesory Organs PDF

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 15: Digestion 524 Lesson 15.2 **Accessory Digestive Organs** Introduction In addition to the organs of the alimentary canal, which are discussed in Lesson 15.1, there are several accessory organs important to the dig...

Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 15: Digestion 524 Lesson 15.2 **Accessory Digestive Organs** Introduction In addition to the organs of the alimentary canal, which are discussed in Lesson 15.1, there are several accessory organs important to the digestion and subsequent absorption of nutrients liberated by digestion. These organs include the **liver**, **gallbladder**, and **pancreas** (Figure 15.10) and are detailed in this lesson. **Figure 15.10** Accessory organs to the digestive system. A diagram of human organs Description automatically generated Chapter 15: Digestion 525 15.2.01 Liver The **liver** is located in the upper right quadrant of the abdominal cavity, just below the diaphragm, and performs various functions (Figure 15.11). **Figure 15.11** The liver is an accessory digestive organ. Prior to food consumption and the initiation of digestion, the liver plays an essential role in whole-body glucose homeostasis, supplying glucose to the blood for uptake by the tissues of the body. Upon [food](javascript:void(0)) [ingestion](javascript:void(0)), concentrations of small molecules derived from carbohydrate, protein, and/or fat digestion increase in the blood. These molecules are delivered to the liver for processing. In addition, insulin is released by the pancreas (discussed in more detail in Concept 15.2.03), which signals the liver to take up glucose and reduce the production of glucose from gluconeogenesis and glycogenolysis. As the site of bile production, the liver is crucial for fat digestion (Figure 15.12). **Bile** (also discussed in Concept 15.2.02) is a solution released into the duodenum of the small intestine to help mechanically (ie, by physical, nonenzymatic means) digest lipids by breaking down large lipid globules into micelles (smaller droplets) during emulsification.  Chapter 15: Digestion 526 **Figure 15.12** The liver is involved in bile production and release. The liver is also an important organ in the breakdown and detoxification of many drugs and waste products. Drugs are exogenous compounds that may have toxic effects when present in the body at elevated concentrations. The body produces endogenous waste products (eg, bilirubin, ammonia) that must be modified in the liver to avoid adverse effects. Macromolecules such as plasma proteins (eg, clotting factors and albumin), fats, ketone bodies, and cholesterol are produced by liver cells. Additionally, the liver stores several molecules (eg, glycogen), minerals (eg, iron), and vitamins. 15.2.02 Gallbladder The gallbladder is an accessory digestive organ. It does not synthesize molecules (eg, digestive compounds, enzymes); rather, the gallbladder is the storage reservoir for bile produced by the liver. A diagram of a human body Description automatically generated Chapter 15: Digestion 527 **Bile** is an alkaline fluid that facilitates fat digestion. Cholesterol, bile acids, and bile pigments (eg, bilirubin) are contained within bile. Before bile acids are released from the liver, they are conjugated with additional compounds to form bile salts, which are amphipathic (ie, containing hydrophobic and hydrophilic regions), a property essential for fat digestion. On a molecular level, hydrophobic regions of bile salts associate with fat globules, whereas hydrophilic regions associate with the aqueous environment. This allows bile salts to act as detergents and break down large lipid globules into smaller spherical structures called **micelles** (Figure 15.13). The formation of small micelles from larger lipid globules serves to increase lipid surface area for hydrolysis by lipases. **Figure 15.13** Emulsification of lipids by bile salts. In the duodenum of the small intestine, the presence of meal-derived fats within chyme and the acidity of chyme itself stimulate bile release from both the liver and the gallbladder. Bile secretion into the duodenum promotes the [neutralization of chyme](javascript:void(0)) and the physical digestion of fats (ie, emulsification). Emulsification is an example of mechanical digestion, a nonenzymatic process that physically breaks down food particles into smaller pieces. 15.2.03 Pancreas The pancreas, composed of various cell types (eg, alpha cells, beta cells, delta cells), has **paracrine**, **exocrine**, and **endocrine** functions. Cells with [paracrine function](javascript:void(0)) secrete substances that exert effects on neighboring cells, and cells with [exocrine function](javascript:void(0)) secrete substances (eg, saliva, sweat, enzymes) through a duct and onto an [epithelial](javascript:void(0)) surface. In comparison, cells with [endocrine function](javascript:void(0)) secrete [hormones](javascript:void(0)) into the bloodstream to cause an effect in a different part of the body.  Chapter 15: Digestion 528 Endocrine cells of the pancreas secrete [insulin](javascript:void(0)) and glucagon, which are hormones involved in the [regulation of blood glucose](javascript:void(0)), into the bloodstream (Figure 15.14). Exocrine cells of the pancreas secrete digestive enzymes and bicarbonate into the small intestine to assist in digestive processes and to neutralize the acidity of chyme. **Figure 15.14** Involvement of the pancreas in the regulation of blood glucose. A diagram of the internal organs Description automatically generated with medium confidence Chapter 15: Digestion 529 The control of blood glucose is important in the maintenance of homeostasis. Food ingestion increases the blood glucose level and stimulates pancreatic beta cells to release **insulin**. Insulin is a [glucose-](javascript:void(0)) [regulating hormone](javascript:void(0)) that decreases the concentration of glucose in the blood. Insulin regulates glucose concentration in the blood by *promoting* glucose uptake by tissues (eg, adipose, muscle) and decreasing liver glucose production (ie, via gluconeogenesis, glycogenolysis), while at the same time promoting glucose storage in glycogen molecules (ie, [glycogenesis](javascript:void(0))). Insulin sensitivity refers to the biological response elicited by a fixed quantity of insulin. An organism is considered insulin-sensitive if only a minimal amount of insulin is needed to induce a reduction in blood glucose levels. In contrast, insulin-resistant organisms need substantially more insulin to stimulate cells to take up the same amount of glucose from the blood (Figure 15.15). **Figure 15.15** Insulin resistance. **Glucagon**, a hormone released by alpha cells of the pancreas in response to low blood glucose levels, [opposes](javascript:void(0)) the effects of insulin. Release of glucagon results in increased [gluconeogenesis](javascript:void(0)) and [glycogenolysis](javascript:void(0)) and decreased glycogenesis. Another hormone important to digestion is **somatostatin**, which is released by pancreatic delta cells. This hormone has a generalized inhibitory effect on digestive function and has been shown to suppress insulin and glucagon release. Figure 15.16 summarizes the cells of the pancreas and their products.  Chapter 15: Digestion 530 **Figure 15.16** Cells of the pancreas and their products. The exocrine pancreas assists in chemical digestion, which is carried out by acids and enzymes and involves the cleavage of chemical bonds within macronutrients to form simpler compounds that can be absorbed. The formation of these compounds occurs through the secretion of enzymes into the pancreatic duct, which empties into the duodenum. These enzymes aid in the digestion of chyme in the lumen of the small intestine. Enzymes secreted by the pancreas include **pancreatic lipase**, which acts to chemically digest lipids, and **pancreatic amylase**, which facilitates polysaccharide hydrolysis to form disaccharides. The pancreas also secretes [proteolytic digestive enzymes](javascript:void(0)) (eg, trypsinogen, chymotrypsinogen). A duodenal enzyme, enteropeptidase, converts the zymogen **trypsinogen** to its active form, **trypsin**. Trypsin activates other pancreatic zymogens (eg, converts **chymotrypsinogen** to **chymotrypsin**) and functions in continued peptide digestion. A diagram of a cell Description automatically generated Enter word / phrase to search UBook text Automatic ZoomActual SizePage Width100%50%75%100%125%150%200%300%400% Chapter 15: Digestion 532 Lesson 15.3 **Control of Digestion** Introduction Both digestion and the subsequent absorption of ingested nutrients are complex processes involving many different organs. Therefore, nutrient digestion and absorption are tightly regulated by both the endocrine system (via hormones) and the nervous system (via innervation). This lesson covers endocrine control of digestion and the enteric nervous system. 15.3.01 Endocrine Control of Digestion The balance between intake and utilization of dietary nutrients is regulated by numerous factors, including hormones released from cells throughout the body (eg, intestine, adipose tissue, pancreas). Some of these hormones promote desire for food intake (ie, appetite), whereas others promote a feeling of satiety (ie, fullness or dietary satisfaction, Figure 15.17). **Leptin** is one hormone that responds to changing energy availability to influence nutrient intake and metabolism. When the body is in an energy-rich state (eg, after a meal), the hormone leptin is released by white adipose tissue. Elevated adipose tissue stores are an indicator of elevated long-term energy stores and, in general, the greater the adipose tissue stores, the higher the leptin levels in the serum. Leptin release triggers feelings of satiety by communicating to the hypothalamus that energy stores are elevated, thereby suppressing appetite. In contrast, a fasting or energy-poor state triggers release of the hormone **ghrelin** by gastric cells in the stomach wall. Ghrelin release triggers feelings of hunger by communicating to the hypothalamus that energy stores are diminishing, thereby increasing appetite and triggering food-seeking behavior. Chapter 15: Digestion 533 **Figure 15.17** Ghrelin and leptin are hormones involved in appetite regulation. Other hormones involved in the digestive system are discussed in other lessons and are summarized in Table 15.2.  Chapter 15: Digestion 534 **Table 15.2** Hormones with functions in digestion. **Hormone** **Site of secretion** **Site of function** **Function** Gastrin G cells of the stomach wall Parietal cells of the stomach wall Signals parietal cells to release HCl; promotes stomach motility Secretin Duodenal epithelial cells Pancreas, stomach Promotes pancreatic enzyme and bicarbonate release into duodenum; inhibits HCl secretion by parietal cells Cholecystokinin (CCK) Duodenal epithelial cells Pancreas, gallbladder, stomach Promotes pancreatic enzyme and bile release; decreases stomach motility; promotes satiety Insulin Pancreatic beta cells Adipose, muscle, liver Promotes glucose uptake by tissues; decreases liver glucose production; promotes liver glycogen formation Glucagon Pancreatic alpha cells Adipose, liver Promotes increased gluconeogenesis and glycogenolysis; decreases glycogenesis; promotes fat release into bloodstream Somatostatin\* Pancreatic delta cells Pancreas Generalized inhibitory effect on digestive function; suppresses insulin and glucagon release Leptin White adipose tissue Hypothalamus Triggers feelings of satiety; supresses appetite Ghrelin Endocrine cells of the stomach wall, pancreas Hypothalamus Triggers feelings of hunger; increases appetite \* Somatostatin is also produced by the hypothalamus, where its release inhibits growth hormone secretion by the anterior pituitary 15.3.02 Enteric Nervous System The **enteric nervous system** (ENS) is a specialized division of the nervous system found within the digestive system. As shown in Figure 15.18, the ENS consists of the **submucosal nerve plexus** and the **myenteric nerve plexus**. These plexuses innervate the walls of the gastrointestinal tract and regulate digestive function by controlling digestive secretions and triggering [peristalsis](javascript:void(0)). The ENS does not require input from the [central nervous system](javascript:void(0)); however, the [autonomic nervous system can modulate ENS function. Specifically, the parasympathetic division of the autonomic nervous system stimulates digestive activity by modulating ENS function. In contrast, the sympathetic division of the autonomic nervous system inhibits digestive activity by decreasing both gut motility and blood flow to the gastrointestinal tract in the face of an immediate stressor](javascript:void(0))