Chapter 26 Digestive System Objectives

Identify the type and location of the two main receptors of the digestive system.

1. Baroreceptors are located within the wall and detect either stretch or pressure in a particular region of the GI tract. 2. Chemoreceptors are embedded within wall and detect presence of specific substances (e.g H, fatty acids) of passing contents within lumen

List the glands found in the small intestine and their secretions.

1. Intestinal glands are invaginations of mucosa that secrete intestinal juice. 2. Goblet cells produce mucus which lubes and protect the intestinal lining. 3. Unicellular glands create enteropeptidase. 4. Submucosal glands are found only in the duodenum and produce thick alkaline mucus to protect against acidic chyme.

List and describe the four tunics (layers) that make up the gastrointestinal wall.

1. Mucosa- Inner layer. The epithelium is in contact with the contents of the lumen and is a simple columnar which helps with reabsorption. The underneath lamina propria is made of areolar CT and contains smaller blood vessels and nerves. 2. Submucosa- Large blood vessels, nerves, and glands are located here. Fine branches of nerves called submucosal nerve plexus innervate smooth muscle and glands of the submucosa. In the small intestine, large groups in the submucosa layer of lymphatic nodules are called Peyer patches. 3. Muscularis- Contain two-layer of smooth muscle called circular layer and longitudinal layer. Muscularis mucosae is a thin layer of smooth muscle that facilitates the release of secretions and increase contact with materials. "shakes things up" Nerves between these two layers control muscle contractions. 4. Serosa- Is made of areolar connective tissue and collagen and elastic fibers located within the peritoneal cavity.

Describe the structure of the serous membranes associated with the GI tract. cavity

1. Parietal peritoneum is the membrane that lines the inner surface of the abdominal wall. 2. Visceral peritoneum is the membrane that reflects over and covers the surface of internal organs. Between these two layers is the peritoneal cavity, which has a lubricating fluid that lubes the abdominal wall to the external organ surface; this reduces friction.

Describe the three components of the lower gastrointestinal tract.

1. Small Intestine is three sections (duodenum, jejunum, and ileum) it receives chyme from the stomach and then mixed with accessory organ secretions. Most chemical digestions and absorption happen here. 2. Accessory organs- secretions from these include bile and pancreatic juice. Bile is produced by the liver and stored then released by the gallbladder. Pancreatic juices contain digestive enzymes released by the pancreas. 3. Large Intestine- This continues reabsorption of water, electrolytes, and vitamins such as B and K. Here is where the digestive process is complete and feces forms.

List the accessory organs and structures involved in the digestive process. 5

Accessory organs help with food breakdown and include 1. teeth and tongue 2. gallbladder, others produce secretions that empty into GI tracts like 3. salivary glands, 4. liver, and 5. pancreas.

Explain why the proteolytic enzymes of the stomach and pancreas are synthesized in inactive forms.

All enzymes that digest proteins from both the stomach and pancreas are released as inactive. Proteolytic enzymes would destroy the cells lining the main and accessory pancreatic duct as they pass through those ducts.

Discuss the structure and development of the teeth.

Also known as dentition. Teeth have an exposed crown and constricted neck and its roots anchor it to the jaw. These roots fit tightly into dental alveoli which are sockets in the mandible. Dentin is the innermost hard layer of the tooth then followed by the upper layer of enamel which forms the crown, and the pulp cavity contains the connective tissue. The root canal opens into the connective tissue and contains the apical foramen which holds nerves and vessels. A baby has 20 deciduous teeth and erupts between 6 months and 30 months after birth, and then are replaces by 32 permanent teeth. We begin with the 2 incisors (designed for slicing food) next to those are 1 canine and have a pointed tip for tearing food and then 2 premolars which are used to crush and grind and finally 3 molars that grind and crush ingested materials.

Describe the chemical digestion of proteins that occurs in the small intestine.

Because of high pH three new inactive enzymes trypsinogen, chymotrypsinogen, and procarboxypeptidase form. Once these three reach the small intestine, they are activated by enteropeptidase (which is made in the small intestine) Trypsin and chymotrypsin break peptide bonds to produce smaller strands of amino acids. Carboxypeptidase removes amino acids from the carboxyl end of a protein. Dipeptidase breaks the final bond between two amino acids so they can be absorbed. Aminopeptidase makes free amino acids from the end of peptides.

Explain how blood and bile flow through the liver.

Blood from portal vessel capillaries mixes with arterial capillary blood in the sinusoids (open channels). Sinusoids are lined with Kupffer cells that engulf potentially harmful substances. Blood is exposed to the hepatocytes, which process the materials in the blood. The blood then collects into a central vein for the return to the heart by hepatic veins. Bile gets made by the hepatocytes as they recycle RBCs. The bile moves in the opposite direction, through a series of small vessels (canaliculi) to bile ducts

Describe and compare long reflexes and short reflexes.

CNS stimulates autonomic motor output is relayed through (facial, glossopharyngeal, and vagus nerve) to different digestive effectors. 1. The result is coordinated secretory and smooth muscle contractions involved in digestive responses, these are described as long reflexes. 2. Some digestive reflexes are not controlled by CNS and happen within neurons housed in the GI tract wall, these are called short reflexes.

Describe the phases that regulate motility and secretion in the stomach.

Cephalic phase involves the cephalic reflex, in which the thought, smell, sight, or taste of food begins an increase of contractile force and secretory activity in the stomach, as well as the contraction of the pyloric sphincter. The gastric phase involves a bolus reaching the stomach. Begins when food enters and baroreceptors detect a stretch in the stomach wall and chemoreceptors detect an increase in pH of gastric contents. Sensory neurons in the medulla oblongata will increase stomach motility and secretory activity. The intestinal phase involves intestinal reflex in which the entry of chyme in the duodenum initiates a decrease in motility and secretory activity in the stomach. CCK decreases motility and secretin decreases secretory activity.

Explain the process of mastication.

Chewing or mastication is the process by which food is crushed and ground by teeth. Mastication is controlled by the medulla oblongata and pons collectively called the mastication center. The primary function for breaking bulk into smaller pieces. It is the first step of digestion, Increasing the surface area so that more chemicals can attach to the pieces and enzymes can help with breakdown and absorption. Mastication also promotes saliva production to soften and moisten food to form a bolus.

Discuss the process by which lipids are absorbed.

Digested monoglycerides, cholesterol, other lipids, and fat-soluble vitamins are contained in micelles. Micelles transport these to columnar cells lining the small intestine. Lipids enter while bile salts are passed on to be reused. Once in fatty acids attach to mono to form a triglyceride again. These lipid molecules are wrapped with a protein called a chylomicron. Chylomicrons pass through lymph capillaries of the small intestine and enter the blood and deliver lipids to the liver and other tissues.

Briefly describe the general process of absorption.

Digested substances are transported from the lumen of the GI tract through the epithelium. These substances only need to cross the epithelium of the mucosa through the membrane transport process to be absorbed into blood capillaries or lymphatic capillaries

Explain the processing in the oral cavity that initiates carbohydrate digestion.

Digestion of starch begins in the oral cavity. Salivary amylase is made and released from the salivary glands. This amylase breaks chemical bonds between glucose and starch to break down specifically the starch molecule. The larger the meal the longer salivary amylase stays active

Explain the role of bile salts in lipid digestion.

Emulsification (breaking ice cube into ice chips) occurs from bile salts. These are produced by the liver and stored in the gallbladder. Bile salts have a polar head and nonpolar tail. They position themselves like an inverted spike ball with the fat in the center. This is what's called a micelle. So bile salts emulsify fats so that pancreatic lipases have better access to the triglyceride and can more efficiently digest it. Cholesterol is the same but it is not digested.

Identify the anatomic structures of the oral cavity.

Food is ingested into the oral cavity where mechanical and chemical digestion occurs. 1. The vestibule of the oral cavity is the space between gums, lips, and cheeks. 2. The buccinator muscles compress the cheeks to hold materials in place during chewing. 3. Cheeks end at the lips which are formed by orbicularis oris muscles. 4. The labial frenulum attaches lips to the gums. 5. The palate forms the top of the oral cavity and palatine folds embedded within assist with swallowing. 6. The uvula is a projection of the soft palate. 7. The fauces represent the opening between the oral cavity and the oropharynx. 8. The tongue is primarily made of skeletal muscle and is covered with numerous papillae that are involved with a sense of taste. 9. The lingual frenulum attaches the bottom of the tongue to the floor of the oral cavity.

Explain another general functional activities of the stomach.

Gastric mixing is a form of mechanical digestions that changes bolus into chyme, which is soup-like. Contractions of the stomach's thick muscular layer churn and mixes the bolus with gastric secretions. Gastric emptying is the movement of acidic chyme from the stomach through the pyloric sphincter into the duodenum. As a wave of peristaltic contractions moves through the pylorus a pressure gradient is made which pushes contents into the small intestine. Retropulsion is the reverse flow of chyme back into the stomach body as the pyloric sphincter closes.

List the major hormones that regulate the processes of digestion.

Gastrin is released in the stomach secretin and cholecystokinin are released from the small intestine.

Describe the digestion of nucleic acids.

Happens in the small intestine. A nucleotide is composed of sugar, a phosphate group, and a nitrogenous base. Deoxyribonuclease and ribonuclease, produced by the pancreas, break the phosphodiester bond between individual nucleotides of DNA and RNA. Phosphatase breaks bonds holding phosphate to the rest of it. And nucleosidase breaks the bond between sugar and a nitrogenous base. All nucleic acid components are absorbed into the blood.

List and describe the six general functions of the digestive system

Ingestion- intro if solids and liquids nutrients into the mouth. 1st step in the process of digesting and reabsorbing nutrients. Motility- Voluntary and involuntary muscle contractions for mixing and moving stuff in the GI tract Secretion- producing and releasing substances that facilitate digestion Digestion- the breakdown of ingested food into smaller pieces that can be absorbed. 1. Mechanical digestion is materials being physically broken down. 2.Chemical digestion involves enzymes that break chemical bonds. Absorption- membrane transport of digested molecules, vitamins, waster from GI to blood. Elimination- the expulsion of indigestible components that are not absorbed.

Describe the structure and function of salivary glands and how the release of saliva is regulated.

Intrinsic salivary glands continuously release small amounts of secretions due to food present. This also contains lingual lipase, which begins the digestion of triglycerides. Multicellular exocrine glands where most saliva comes from are located outside the oral cavity are called extrinsic salivary glands. Parotid salivary glands produced the saliva and are transported by the parotid duct. Submandibular salivary glands are near the floor of the oral cavity and next to the mandible, and produce the most saliva that comes out of the submandibular duct. Sublingual salivary glands extend into tiny little sublingual ducts that open into the oral cavity. Mucous cells and serous cells are housed within the glands and produce saliva. The salivary nuclei within the brainstem regulate salivation. Chemoreceptors and mechanoreceptors in the upper GI tract input the salivary nuclei. These glands respond to spiciness, acidity, the food inside the stomach, and psychological stimulation. Parasympathetic stimulation produces more saliva while sympathetic stimulation stimulates more saliva.

Describe the accessory digestive organs associated with the small intestine and the contributions of each to digestive processes.

Liver is the largest internal organ and its primary digestive function os to produce and release bile into the duodenum. The hepatic artery carries oxygenated blood to the liver; the hepatic portal vein carries nutrient-rich but oxygen-poor blood from capillary beds of the GI tract, spleen, and pancreas to the liver. Hepatocytes are liver cells within the hepatic lobules. The gallbladder is a small muscular sac on the front side of the liver and it stores, concentrates, and releases bile into the duodenum The pancreas is a mixed gland with endocrine and exocrine functions. Exocrine cells function to produce and release numerous digestive enzymes, and HCO3- is released from pancreatic ducts to form a mixture called "pancreatic juice" into the duodenum.

Explain the action of the muscularis tunic.

Muscularis- Contain two-layer of smooth muscle called circular layer and longitudinal layer. Nerves between these two layers control muscle contractions. Muscle sphincters close off lumen at some point to move contents down. The function of the muscularis is to mix and propel contents in the GI tract. The circular layer constricts the tube, while the longitudinal layer shortens the tube. Two types of motility are peristalsis which is the alternating contractions of these movements for purpose of propelling contents and Mixing "back and forward" with no directional movement and is for blending contents together.

Describe the bacterial action that takes place in the large intestine

Normal bacteria flora inhabit the large intestine. These guys are responsible for the chemical breakdown of carbs, proteins, and lipids that remain in the chyme after digestion. These actions produce H+, CO2, hydrogen sulfide, methane, indoles. Some of these substances are responsible for the odor of poop. B and K vitamins are produced by the flora. Feces is the final product and contains water, salts, bacteria, and undigested material, and epithelial cells

Distinguish between 3 intraperitoneal and 4 retroperitoneal organs.

Organs that are in the abdomen and are completely surrounded by the visceral peritoneum are called intraperitoneal organs, such as the 1. stomach, 2. small intestine, and most of the 3. large intestine. Retroperitoneal organs are outside of the parietal peritoneum and directly against the back abdominal wall and only their front side is covered by the parietal peritoneum and includes the 1. pancreas, 2. rectum, and 3. duodenum (first part of the small intestine), and 4. descending, and ascending colons.

Describe the chemical digestion of carbohydrates that occurs in the small intestine.

Pancreatic amylase is made and released by the pancreas as a component of pancreatic juice into the small intestine. This enzyme continues the digestion of starch into shorter strands of glucose, maltose. To complete the whole break down the brush border enzymes dextrinase and glucoamylase which break bonds between oligosaccharides and maltase break bonds between two glucose molecules that make up maltose. Lactase digests lactose to glucose and galactose. And sucrase digests sucrose to glucose and fructose. Glucose, galactose, and fructose monomer are absorbed into the blood and are transported through the hepatic portal vein to the liver. The liver is where fructose and galactose convert into glucose. Glucose circulating the blood will be used for energy or stored as glycogen. Cellulose is a carb used as fiber

Identify the enzyme that initiates protein digestion in the stomach, and explain its activation and action.

Pepsin is from the inactive precursor pepsinogen released by chief cells. HCl that is released from parietal cells causes the low pH in the stomach that actives pepsin and denatures proteins to help their chemical breakdown.

Explain motility within the small intestine.

Segmentation mixes chyme with accessory gland secretions through a "back-and-forward" motion. Peristalsis then propels material within the GI lumen by altering the contraction of circular and longitudinal muscle layers. The gastroileal reflex is started by food entering the stomach by relaxing the ileocecal sphincter. It moves contents from the ileum into the cecum.

Name the three classes of carbohydrates

Simple sugars. 1. Monosaccharides (glucose, fructose, galactose) 2. Disaccharides (sucrose, maltose, lactose) 3. Polysaccharides (starch and cellulose)

Explain one general functional activities of the stomach.

Surface mucous neck cells line and lumen and produce an alkaline mucus to prevent protection against exposure to acidic stomach contents. Mucous neck cells are located deep within gastric pits and create acidic mucus which helps maintain acidic conditions. Parietal cells are responsible for the adding of intrinsic factors and hydrochloric acid into the stomach. Intrinsic factor is needed for Vitamin B12 absorption. And HCl helps with chemical digestion and kills any bad bacteria that enters the stomach. Gastric chief cells secrete packets of zymogen granules, mainly containing pepsinogen and gastric lipase. Pepsinogen activates pepsin where it chemically digests denatured proteins. and gastric lipase helps digest fats. G-cells are cells that secrete gastrin into the blood. This is a hormone that stimulates stomach secretions and motility.

Identify the six organs that make up the gastrointestinal (GI) tract

The GI tract organs form a continuous tube and are 1. Oral cavity and Pharynx, 2. esophagus, and 3. stomach, 4. small intestine, and 5. large intestine and end at the 6. anus. In here food is broken down into smaller pieces and absorbed.

Name the three major regions of the large intestine and four segments of the colon of the large intestine.

The cecum is the first portion of the large intestine and is sac-like. Chyme enters the cecum from the ileum. Projecting off of the cecum is the appendix. The colon beings at the ileocecal valve and ends at the anus. there is the ascending colon, right flexure, transverse colon, left flexure, descending colon, sigmoid flexure, and sigmoid colon. The rectum is the third major region of the large intestine. It is a muscular tube that expands once it accumulates feces, and rectal valves ensure that the poop is retained. The anal canal is lined with stratified squamous epithelium. Extra lube lubricates the anal canal during defecation. Internal anal sphincter and voluntary skeletal muscle external anal sphincter close off the opening. The taeniae coli are the long thin smooth muscle running in the middle and the haustra are the bubble-looking parts of the large intestine.

Explain the function of the mesentery, and describe the five individual mesenteries of the abdominopelvic cavity.

The mesentery is a double layer of the peritoneum that supports, stabilizes, and suspends GI organs. Blood vessels, lymph vessels, and nerves that supply the GI tract are located between these two layers. 1. The greater omentum extends interiorly like an apron and covers most of the abdominal organs. Referred to as "fatty apron" because of all adipose tissue on it and stores extra fat. 2. Lesser omentum connects the stomachs with lesser curvature and duodenum to the liver. 3. Falciform ligament is the crescent fold that attaches the liver to the internal surface of the front abdominal wall. 4. Mesentery proper is fan-shaped and suspends most of the small intestine to the internal surface of the back abdominal wall. 5. Mesocolon is a fold that attaches parts of the large intestine to the back abdominal wall and has different names for the different regions of the large intestine it suspends.

Explain the distinguishing histologic features of the large intestine.

The mucosa is lined with simple columnar epithelium and a lot of goblet cells and lacks villi. But looks like a small intestine due to the many epithelial cells and intestinal glands that extend inward towards the mucosae. These glands secrete mucin to lube the materials and help with the passage through the large intestine.

Discuss the anatomy of the pharynx and esophagus and their complementary activities in the process of swallowing.

The pharynx is a passageway for food and air. The esophagus projects inferiorly through the thorax and connects to the stomach. The esophageal sphincter is a contracted ring where the esophagus and pharynx meet. The esophagus is composed of both skeletal muscle and smooth muscle. Swallowing is the process of moving materials from the oral cavity to the stomach. The voluntary phase of swallowing is controlled by the cerebral cortex and is the initial phase of swallowing. The pharyngeal phase consists of the swallowing reflex that is started by the arrival of bolus at the oropharynx and is involuntary. Swallowing centers in the medulla oblongata receive input from tactile receptors in oro which cause involuntary transport of bolus from the pharynx to the esophagus. The presence of bolus in the lumen stimulates peristaltic waves of muscle contractions to propel the bolus towards the stomach

Describe the anatomy of the small intestine.

The small intestine is a long tube that extends from the pylorus of the stomach to the cecum of the large intestine. The duodenum forms the first part, and the duodenojejunal flexure joins duodenum to jejunum. The jejunum is the middle region of the small intestine. The ileum is the last part and the ileocecal valve controls what enters the large intestine. The submucosal and mucosal tunics of the small intestine create circular folds which increase surface area and slow the movement of the chyme. Villi are finger-like projections of mucosa in the small intestine. Microvilli are extensions of the plasma membrane of cells lining the small intestine, which form a brush border. Brush border enzymes are enzymes embedded in the border to complete chemical digestions.

Describe the gross anatomy and histology of the stomach.

The stomach is a holding sac where mechanical and chemical digestion continues on the bolus. It starts the chemical digestion of protein and fat. The greater curvature is the bottom curve of the stomach and the lesser curvature is the top curve of the stomach. The cardia is a narrow entryway into the stomach from the esophagus. The fundus is the dome-shaped area above/next to the cardia. The body is the largest region of the stomach. The pylorus is a narrow pouch that forms the terminal region of the stomach. The pyloric sphincter surrounds the pyloric orifice and controls what comes into the duodenum. The inside of the stomach is lined with gastric folds which allow for expansion of the stomach and there is simple columnar epithelium with depressions called gastric pits. Gastric glands go deep into mucosa from each gastric pit. Muscularis of the stomach has oblique, middle circular, and longitudinal layers.

Describe the components of the upper gastrointestinal tract.

The upper gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, and duodenum. The exact demarcation between the upper and lower tracts is the suspensory muscle of the duodenum.

Discuss the regulation of the accessory glands associated with the small intestine.

Vagal stimulations during the cephalic and gastric phases activate the pancreas to release pancreatic juice. Cholecystokinin is a hormone released by the small intestine in response to fatty chyme. CCK stimulates smooth muscle contraction of the gallbladder which releases bile into the duodenum, and also stimulates the pancreas to release pancreatic juice and relaxes the hepatopancreatic ampulla. Secretin is released from the small intestine in response to an increase in chyme acidity. Secretion causes the release of an alkaline solution from the liver and pancreas that helps neutralizes the acidic chyme.