Chapter 9: The Digestive System Flashcards
Intracellular digestion involves the
oxidation of glucose and fatty acids to make energy.
Extracellular digestion occurs in the
lumen of the alimentary canal.
Mechanical digestion is the
physical breakdown of large food particles into smaller food particles.
Chemical digestion is the
enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches.
The pathway of the digestive tract is:
oral cavity → pharynx → esophagus → stomach → small intestine → large intestine → rectum
The accessory organs of digestion are the
salivary glands, pancreas, liver, and gallbladder.
The enteric nervous system is in the
wall of the alimentary canal and controls peristalsis.
Its activity is upregulated by the parasympathetic nervous system and downregulated by the s_ympathetic nervous system._
Ingestion and Digestion
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To supply the body with nutrients, we must ingest (eat) food. Several hormones are involved with feeding behavior, including antidiuretic hormone (ADH or vasopres-sin), aldosterone, glucagon, ghrelin, leptin, and cholecystokinin. ADH and aldoste-rone trigger the sensation of thirst, encouraging the behavior of fluid consumption. Glucagon, secreted by the pancreas, and ghrelin, secreted by the stomach and pancreas, stimulate feelings of hunger. Leptin and cholecystokinin do the opposite, stimulating feelings of satiety. Digestion begins in the oral cavity and continues in the stomach and the first part of the small intestine, known as the duodenum.
Oral Cavity
The oral cavity plays a role in both mechanical and chemical digestion of food. Mechanical digestion in the mouth involves the breaking up of large food particles into smaller particles using the teeth, tongue, and lips. This process is called masti-cation (chewing). Chewing helps to increase the surface area-to-volume ratio of the food, creating more surface area for enzymatic digestion as it passes through the gut tube. It also moderates the size of food particles entering the lumen of the alimentary canal; food particles that are too large create an obstruction risk in the tract.
Chemical digestion is the breakdown of chemical bonds in the macromolecules that make up food. This relies on enzymes from saliva produced by the three pairs of salivary glands. Saliva also aids mechanical digestion by moistening and lubricating food. The salivary glands, like all glands of the digestive tract, are innervated by the parasympathetic nervous system. The presence of food in the oral cavity triggers a neural circuit that ultimately leads to increased parasympathetic stimulation of these glands. Salivation can also be triggered by signals that food is near, such as smell or sight. Saliva contains salivary amylase, also known as ptyalin, and lipase. Salivary amylase is capable of hydrolyzing starch into smaller sugars (maltose and dextrins), while lipase catalyzes the hydrolysis of lipids. The amount of chemical digestion that occurs in the mouth is minimal, though, because the food does not stay in the mouth for long. Our muscular tongue forms the food into a bolus, which is forced back to the pharynx and swallowed.
Pharynx
The pharynx is the cavity that leads from the mouth and posterior nasal cavity to the esophagus. The pharynx connects not only to the esophagus, but also to the larynx, which is a part of the respiratory tract. The pharynx can be divided into three parts: the nasopharynx (behind the nasal cavity), the oropharynx (at the back of the mouth), and the laryngopharynx (above the vocal cords). Food is prevented from entering the larynx during swallowing by the epiglottis, a cartilaginous structure that folds down to cover the laryngeal inlet. Failure of this mechanism can lead to aspira-tion of food and choking.
Esophagus
The esophagus is a muscular tube that connects the pharynx to the stomach. The top third of the esophagus is composed of skeletal muscle, the bottom third is composed of smooth muscle, and the middle third is a mix of both. What does this mean in terms of nervous control? While the top of the esophagus is under somatic (voluntary) motor control, the bottom—and most of the rest of the gastrointestinal tract, for that matter—is under autonomic (involuntary) nervous control. The rhythmic contraction of smooth muscle that propels food toward the stomach is called peristalsis. Under normal circumstances, peristalsis proceeds down the digestive tract. However, certain factors such as exposure to chemicals, infectious agents, physical stimulation in the posterior pharynx, and even cognitive stimulation, can lead to reversal of peristalsis in the process of emesis (vomiting). Swallowing is initiated in the muscles of the oropharynx, which constitute the upper esophageal sphincter. Peristalsis squeezes, pushes, and propels the bolus toward the stomach. As the bolus approaches the stomach, a muscular ring known as the lower esophageal sphincter (cardiac sphincter) relaxes and opens to allow the passage of food.
Stomach
There are three main energy sources: carbohydrates, fats, and proteins. As mentioned earlier, the chemical digestion of carbohydrates and fats is initiated in the mouth. No mechanical or chemical digestion takes place in the esophagus, except for the con-tinued enzymatic activity initiated in the mouth by salivary enzymes. Thus, digestion that occurs prior to the entrance of the bolus into the stomach is minimal compared to the digestion that occurs in the stomach and small intestine.
The stomach is a highly muscular organ with a capacity of approximately two liters. In humans, the stomach is located in the upper left quadrant of the abdominal cavity, underneath the diaphragm. This organ uses hydrochloric acid and enzymes to digest food, creating a fairly harsh environment. Therefore, its mucosa is quite thick to pre-vent autodigestion. The stomach can be divided into four main anatomical divisions, as shown in Figure 9.2: the fundus and body, which contain mostly gastric glands, and the antrum and pylorus, which contain mostly pyloric glands. The internal cur-vature of the stomach is called the lesser curvature; the external curvature is called the greater curvature. The lining of the stomach is thrown into folds called rugae. The mucosa of the stomach contains gastric glands and pyloric glands. The gastric glands respond to signals from the vagus nerve of the parasympathetic nervous sys-tem, which is activated by the brain in response to the sight, taste, and smell of food. Gastric glands have three different cell types: mucous cells, chief cells, and parietal cells. Mucous cells produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic (pH = 2) and proteolytic environment of the stomach.
Gastric juice is a combination of secretions from the other two cell types in the gastric glands: chief cells and parietal cells. The chief cells secrete pepsinogen. This is the inactive, zymogen form of pepsin, a proteolytic enzyme. Hydrogen ions in the stomach, secreted by parietal cells as hydrochloric acid, cleave pepsinogen to pepsin. Pepsin digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments. Because pepsin is activated by the acidic environment, it follows that pepsin is most active at a low pH. This is a unique characteristic among human enzymes, as most human enzymes are most active at physiological pH. Stomach acid also kills most harmful bacteria (with the exception of Helicobacter pylori, infection with which is usually asymptomatic but can cause inflammation, ulcers, and even certain gastric cancers). The acidic environment also helps to denature proteins and can break down some intramolecular bonds that hold food together. In addition to HCl, parietal cells secrete intrinsic factor, a glycoprotein involved in the proper absorption of vitamin B12. The pyloric glands contain G-cells that secrete gastrin, a peptide hormone. Gastrin induces the parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents. The digestion of solid food in the stomach results in an acidic, semifluid mixture known as chyme. The combined mechanical and chem-ical digestive activities of the stomach result in a significant increase in the surface area of the now unrecognizable food particles, so when the chyme reaches the small intestine, the absorption of nutrients from it can be maximized. There are a few sub-stances that are absorbed directly from the stomach (such as alcohol and aspirin), but the stomach is mainly an organ of digestion.
Duodenum
The small intestine consists of three segments: the duodenum, the jejunum, and the ileum. The small intestine is quite long, up to seven meters. The duodenum is responsible for the majority of chemical digestion and has some minor involvement in absorption. However, most of the absorption in the small intestine takes place in the jejunum and ileum.
Food leaves the stomach through the pyloric sphincter and enters the duodenum. The presence of chyme in the duodenum causes the release of brush-border enzymes like disaccharidases (maltase, isomaltase, lactase, and sucrase) and peptidases (including dipeptidase). Brush-border enzymes are present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers. The duodenum also secretes enteropeptidase, which is involved in the activation of other digestive enzymes from the accessory organs of digestion. Finally, it secretes hormones like secretin and cholecystokinin (CCK) into the bloodstream.
The disaccharidases digest disaccharides. Maltase digests maltose, isomaltase digests isomaltose, lactase digests lactose, and sucrase digests sucrose. Lack of a particular disaccharidase causes an inability to break down the corresponding disaccharide. Then bacteria in the intestines are able to hydrolyze that disaccharide, producing methane gas as a byproduct. In addition, undigested disaccharides can have an osmotic effect, pulling water into the stool and causing diarrhea. This is why people who are lactose intolerant have symptoms of bloating, flatulence, and possibly diarrhea after ingesting dairy products. Peptidases break down proteins (or peptides, as the name implies). Aminopeptidase is a peptidase secreted by glands in the duodenum that removes the N-terminal amino acid from a peptide. Dipeptidases cleave the peptide bonds of dipeptides to release free amino acids. Unlike carbohydrates, which must be broken down into monosaccharides for absorption, proteins can be broken down into di-and even tripeptides and can be absorbed across the small intestine wall. Enteropeptidase (formerly called enterokinase) is an enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin. Trypsin then initiates an activation cascade, as described later in this chapter. Enteropeptidase can also activate procarboxypeptidases A and B to their active forms. Secretin is a peptide hormone that causes pancreatic enzymes to be released into the duodenum. It also regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas. Secretin is also an enterogastrone, a hormone that slows motility through the digestive tract. Slowing of motility allows increased time for digestive enzymes to act on chyme—especially fats. Finally, cholecystokinin (CCK) is secreted in response to the entry of chyme (specif-ically, amino acids and fat in the chyme) into the duodenum. This peptide hormone stimulates the release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety. Bile is a complex fluid composed of bile salts, pigments, and cholesterol. Bile salts are derived from cholesterol. They are not enzymes and therefore do not directly perform chemical digestion (the enzymatic cleavage of chemical bonds). However, bile salts serve an important role in the mechanical diges-tion of fats and ultimately facilitate the chemical digestion of lipids. Bile salts have hydrophobic and hydrophilic regions, allowing them to serve as a bridge between aqueous and lipid environments. In fact, bile salts are much like the common soaps and detergents we use to wash our hands, clothes, and dishes. In the small intes-tine, bile salts emulsify fats and cholesterol into micelles. Without bile, fats would spontaneously separate out of the aqueous mixture in the duodenum and would not be accessible to pancreatic lipase, which is water-soluble. In addition, these micelles increase the surface area of the fats, increasing the rate at which lipase can act. Ultimately, proper fat digestion depends on both bile and lipase. Bile gets the fats into the solution and increases their surface area by placing them in micelles (mechanical digestion). Then, lipase can come in to hydrolyze the ester bonds holding the lipids together (chemical digestion). CCK also promotes the secretion of pancreatic juices into the duodenum, as shown in Figure 9.3. Pancreatic juices are a complex mixture of several enzymes in a bicarbonate-rich alkaline solution. This bicarbonate helps to neutralize acidic chyme, as well as provide an ideal working environment for the digestive enzymes, which are most active around pH 8.5. Pancreatic juices contain enzymes that can digest all three types of nutrients: carbohydrates, fats, and proteins. The identities and func-tions of these enzymes will be discussed in the next section of this chapter.
Multiple hormones regulate
feeding behavior, including antidiuretic hormone (ADH or vasopressin) and aldosterone, which promote thirst; glucagon and ghrelin, which promote hunger; and leptin and cholecystokinin, which promote satiety.
Feeding behavior: Promotes thirst
- ADH antidiuretic hormone or vasopressin
- Aldosterone
Feeding behavior: Promotes hunger
- Glucagon
- Ghrelin
Feeding behavior: promote satiety
- Leptin
- Cholecystokinin
In the ___, ___ starts the mechanical digestion of food, while ___ and ___ start the chemical digestion of food. Food is formed into a ___ and swallowed.
**oral cavity
mastication
salivary amylase
lipase
bolus**
The pharynx connects the
the mouth and posterior nasal cavity to the esophagus.
The esophagus does what
propels food to the stomach using peristalsis. Food enters the stomach through the lower esophageal (cardiac) sphincter.
Mucous cells produce
bicarbonate-rich mucus to protect the stomach.
Chief cells secrete
pepsinogen, a protease activated by the acidic environment of the stomach.
Parietal cells secrete
hydrochloric acid and intrinsic factor, which is needed for vitamin B12 absorption.
G-cells secrete
gastrin, a peptide hormone that increases HCl secretion and gastric motility.
The stomach has four parts
fundus, body, antrum, and pylorus. The stomach has a lesser and greater curvature and is thrown into folds called rugae. Numerous secretory cells line the stomach.
- Mucous cells produce bicarbonate-rich mucus to protect the stomach.
- Chief cells secrete pepsinogen, a protease activated by the acidic environ-ment of the stomach.
- Parietal cells secrete hydrochloric acid and intrinsic factor, which is needed for vitamin B12 absorption.
- G-cells secrete gastrin, a peptide hormone that increases HCl secretion and gastric motility.
After mechanical and chemical digestion in the stomach, the food
particles are now called chyme.
Food passes into the duodenum through the pyloric sphincter.
The duodenum is the
first part of the small intestine and is primarily involved in chemical digestion.
Disaccharidases are
brush-border enzymes that break down maltose, isomaltose, lactose, and sucrose into monosaccharides.
Disacchridases –> are nezymes that break down and turn into–> monosaccharides
Brush-border peptidases include
aminopeptidase and dipeptidases.
Enteropeptidase activates
trypsinogen and procarboxypeptidases, initiating an activation cascade.
The liver synthesizes
The liver makes bile but this bile is stored in the gallbladder or secreted into the duodenum directly.
bile, which can be stored in the gallbladder or secreted into the duodenum directly.
- Bile emulsifies fats, making them soluble and increasing their surface area.
- The main components of bile are bile salts, pigments (especially bilirubin from the breakdown of hemoglobin), and cholesterol.
2� For each of the cell types below, list the major secretions of the cell and the functions of these secretions.
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3� For each of the following substances, determine whether it is a digestive enzyme or a hormone and briefly summarize its functions.
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