Chapter 23

Describe the composition and function of saliva, and explain how salivation is regulated.

Saliva cleanses the mouth, dissolves food chemicals so they can be tasted, moistens food and helps compact it into a bolus, and contains the enzyme amylase that begins the digestion of starchy foods. Most saliva is produced by the major or extrinsic salivary glands that lie outside the oral cavity and empty their secretions into it. It is largely composed of 97 to 99.5% of water and is hypoosmotic. It is slightly acidic (pH 675 to 7.00). Its solutes include electrolytes (Na, K, Cl, PO, and HCO), digestive enzymes salivary amylase and lingual lipase, proteins (mucin, lysozyme, and IgA), and metabolic wastes (urea and uric acid). The average output of saliva is about 1500 ml per day, but can be much higher when salivary glands are appropriately stimulated. Salivation is controlled by the parasympathetic division of the automatic nervous system. When food, chemoreceptors (activated by strongly acidic substances), and mechanoreceptors (activated by any mechanical stimulus in the mouth) in the mouth sends signals to the salivatory nuclei in the brain stem. Thus, causing parasympathetic nervous system activity to increase. Impulses sent via motor fibers in the facial and glossopharyngeal nerves dramatically increase the output of watery, enzyme-rich saliva. Strong activation of the sympathetic division can cause the constriction of blood vessels serving the salivary glands and almost completely inhibits saliva release, causing a dry mouth.

Digestion

Series of catabolic steps in which enzymes secreted into the lumen (cavity) of the alimentary canal break down complex food molecules to their chemical building blocks.

Roots of teeth

Some teeth have 1 root (incisors, canines, and premolars), while others have 2 to 3 roots (molars). Roots stick down into the jaw and as a new tooth grows and pushes the other tooth out, the roots will die.

Define splanchnic circulation

Splanchnic circulation includes those arteries that branch off the abdominal aorta to serves the digestive organs and the hepatic portal circulation.

Vasoactive Intestinal Peptide (VIP)

Stimulates buffer secretion; dilates intestinal capillaries; relaxes intestinal smooth muscle; increases secretion; inhibits acid secretion

Intestinal Gastrin

Stimulates gastric glands and motility

Motilin

Stimulates migrating motor complex

Describe the mechanism of swallowing.

Swallowing (deglutition) involves the coordinated activity of over 22 separate muscle groups. The two main phases involved in deglutition is the buccal phase and pharyngeal-esophageal phase. The buccal phase occurs in the mouth and is voluntary, ending when a food bolus (a bit of saliva) leaves the mouth and stimulates tactile receptors in the posterior pharynx, leading to the pharyngeal-esophageal phase. It is involuntary and is controlled by the swallowing center in the brain stem. Various cranial nerves, most importantly the vagus nerves, transmit motor impulses from the swallowing center to the muscles of the pharynx and esophagus. Once food enters the pharynx, respiration is momentarily inhibited and all routes except the desired one into the digestive tract are blocked off. Solid foods pass from the oropharynx to the stomach in about 8 seconds, and fluids pass in 1 to 2 seconds.

Ingestion

Taking food into the digestive system (eating)

The mucosa

The innermost layer which is a moist epithelial membrane that lines the alimentary canal lumen from mouth to anus. Its major functions include secreting mucus, digestive enzymes, and hormones, absorbing the end products of digestion into the blood, and protecting against infectious disease. It also consists of 3 sublayers, a lining epithelium, a lamina propria, and a muscularis mucosae. The epithelium of the mucosa is a simple columnar epithelium, except for in the mouth, esophagus, and anus where it is stratified squamous, and is rich in mucus-secreting cells. The mucus it produces protects certain digestive organs from being digested by enzymes and eases food passage along the tract. In the stomach and small intestine, the mucosa also contains both enzyme-synthesizing and hormone-secreting cells. The lamina propria underlies the epithelium and is loose areolar connective tissue. Its capillaries nourish the epithelium and absorb digested nutrients. Its isolated lymphoid follicles, part of MALT (mucosa-associated lymphoid tissue) which helps defend us against bacteria and other pathogens. It also doesn't aid in movement of food. The muscularis mucosae is external to the lamina propria and is a scant layer of smooth muscle cells that produces local movements of the mucosa that can enhance absorption and secretion.

Describe the histologic anatomy of the liver and pancreas.

The liver is composed of liver lobules (sesame seed sized). Each lobule is a hexagonal structure consisting of plates of liver cells (hepatocytes). The hepatocyte plates radiate outward from a central vein. The pancreas is a soft, tadpole-shaped gland that extends across the abdomen from its tail to its head. Most of the pancreas is retroperitoneal and lies deep to the greater curvature of the stomach. It also consists of exocrine and endocrine parts. (More info about liver on page 884). The liver as four primary lobes. The right lobe is the largest. There is also the small lobe. The posterior lobes are the caudate lobe and the quadrate lobe. Ona microscopic level the liver is made of sesame seed sized functional unit's caller liver lobules. Each one is hexagonal shaped that consist of liver cells (hepatocytes).

List the major functions of the large intestine.

The major digestive functions of the large intestine are to absorb most of the remaining water from indigestible food residues, store the residues temporarily, and then eliminate them from the body as a semisolid feces (stool). It also absorbs metabolites produced by resident bacteria as they ferment carbohydrates not absorbed in the small intestine.

Describe stomach structure and indicate changes in the basic alimentary canal structure that aid its digestive function.

The adult stomach can vary from 15 to 25 cm long but the diameter and volume can change depending upon the amount of food in the stomach. When empty, the stomach collapses inward, throwing its mucosa and submucosa into large longitudinal folds called rugae. The major regions of the stomach are the small cardial part (cardia) which surrounds the cardinal orifice through which food enters the stomach from the esophagus. The fundus is the stomach's dome-shaped part, tucked beneath the diaphragm, that bulges superolaterally to the cardia. The body (midportion of the stomach) is continuous inferiorly with the funnel-shaped pyloric part. The pyloric antrum (wider and more superior part of the pyloric part) narrows from the pyloric canal (terminates at the pylorus). The pylorus is continuous with the duodenum through the pyloric sphincter (valve), which controls stomach emptying. The convex lateral surface of the stomach is its greater curvature, and its concave medical surface is the lesser curvature. Extending from these curvatures are two mesenteries called omenta, that helps tether the stomach to other digestive organs and the body wall. The lesser omentum runs from the liver to the lesser curvature of the stomach, where is becomes continuous with the visceral peritoneum covering the stomach. The greater omentum drapes inferiorly from the greater curvature of the stomach to cover the coils of the small intestine.

The alimentary canal (gastrointestinal tract/gut)

The continuous muscular tube that winds through the body from the mouth to the anus. It digests food, breaking it down into smaller fragments and absorbs the digested fragments through its lining into the blood. The organs within the alimentary canal consist of the mouth, pharynx, esophagus, stomach, small intestine, and large intestine (leads to the terminal opening/anus). In a cadaver the alimentary canal is approximately 9 m long, but in a living person it is considerably shorter because of its muscle tone.

Describe the functions of the digestive system

The digestive system takes in food, breaks it down into nutrient molecules, absorbs these molecules into the bloodstream, and then rids the body of the indigestible remains. It also converts food into the raw materials that build and fuel our body's cells.

Describe the tissue composition and general function of each of the four layers of the alimentary canal.

The four different layers of the walls of the alimentary canal are mucosa, submucosa, muscularis externa, and serosa. Each layer contains a predominant tissue type that plays a specific role in food breakdown.

Indicate the importance of the hepatic portal system.

The hepatic portal system collects nutrient rich venous blood (may contain toxins and microorganisms) draining from the digestive viscera and delivers it to the liver, where it can be treated before it reaches the rest of the body. The arteries supply the spleen liver stomach large and small intestines.

Identify and describe structural modifications of the wall of the small intestine that enhance the digestive process.

The modifications of the small intestine include circular folds, villi, and microvilli. The circular folds are deep, permanent folds of the mucosa and submucosa. Nearly 1 cm tall, these folds force chime to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption. Villi are fingerlike projections of the mucosa that give it a velvety texture. They are large, over 1 mm tall, and leaf-like in the duodenum and gradually narrow and shorten along the length of the small intestine. Villus contain a dense capillary bed and a wide lymphatic capillary within its core called the lacteal. Digested foodstuffs are absorbed through the epithelial cells into the capillary blood and the lacteal. Microvilli are long, densely packed cytoplasmic extensions of the absorptive cells of the mucosa that give the mucosal surface and brush border. The plasma membranes of the microvilli bear enzymes referred to as brush border enzymes, which complete the digestion of carbohydrates and proteins in the small intestine. Circular folds, which are deep permanent folds of the mucosa and submucosa allow for chyme to slowly move along its path allowing time for full nutrient absorption. Villi which are fingerlike projections of the mucosa contain a dense capillary bed in their cores known as lacteal. The lacteal is where digested food is absorbed into. Microvilli are long densely packer extensions. They bear enzymes known as brush border enzymes which complete digestion of carbs and proteins in the small intestine.

Describe the gross and microscopic anatomy and the basic functions of the mouth and its associated organs.

The mouth (oral cavity/buccal cavity) has boundaries known as the lips anteriorly, cheeks laterally, palate superiorly, and tongue inferiorly. Its anterior opening is the oral orifice. Posteriorly, the oral cavity is continuous with the oropharynx. The walls of the mouth are lined with a thick stratified squamous epithelium. The epithelium on the gums, hard palate, and dorsum of the tongue is slightly keratinized for extra protection against abrasion during eating.

The Pulp Cavity of the tooth

The outer most region of root canal. The canal brings nerve endings, blood vessels, and veins up into the pulp cavity.

The Serosa

The outermost layer of the intraperitoneal organs, is the visceral peritoneum. It is formed of areolar connective tissue covered with mesothelium (single layer of squamous epithelium cells). It is replaced by an adventitia (in the esophagus), which is ordinary dense connective tissue that binds the esophagus to surrounding structures.

Absorption

The passage of digested end products (plus vitamins, minerals, and water) from the lumen of the GI tract through the mucosal cells by active or passive transport into the blood or lymph.

Describe the location and function of the peritoneum.

The peritoneum is the most extensive of the slippery serous membranes that are located within the abdominopelvic cavity. It holds things in place and allows the alimentary canal to expand or contract; protects organs such as the uterus from bacteria and infection. There are two types of peritoneum, visceral and parietal. The visceral peritoneum covers (adjacent to organs) the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall (associated with the outer wall and dorsal part of the peritoneum; wraps around the small intestine). The peritoneal cavity is a slit-like potential space containing a slippery fluid secreted by the serous membranes. The serous fluid allows the mobile digestive organs to glide easily across one another and along the body wall as they carry out their activities. The peritoneum can cave in on itself, the two layers of the peritoneum fuse and form the mesentery. A mesentery is a double layer of peritoneum that provide routes for blood vessels, holds organs in place, and store fat. It is mostly dorsal to the posterior abdominal walls but there are also ventral ones.

Describe the anatomy and basic functions of the pharynx and esophagus.

The pharynx has two parts that are used in the digestive process, the oropharynx and laryngopharynx, which are common passageways for food, fluids, and air. Its histology resembles that of the oral cavity. The mucosa contains a friction-resistant stratified squamous epithelium. The external muscle layer consists of two skeletal muscle layers. The muscles of the inner layer and outer layer (pharyngeal constrictor muscles) can cause contractions that propel food into the esophagus. The esophagus is a muscular tube about 25cm (10 inches) long and is collapsed when not involved in food propulsion. The esophagus takes a fairly straight course through the mediastinum of the thorax, piercing the diaphragm at the esophageal hiatus to enter the abdomen, joining the stomach at the cardial orifice within the abdominal cavity. The esophagus has all four of the basic alimentary canal layers, the esophageal mucosa, submucosa, muscularis externa, and instead of the serosa, it has a fibrous adventitia composed entirely of connective tissue.

Describe the regulation of defecation.

The stretching of the rectal wall initiates the defecation reflex. The parasympathetic spinal reflex causes the sigmoid colon and the rectum to contract and the internal anal sphincter to relax. As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should open or remain constricted to stop passage of feces temporarily. During defecation, the muscles of the rectum contract to expel the feces. It is regulated voluntarily by closing the glottis and contracting our diaphragm and abdominal walls to increase the intra-abdominal pressure (a process called Valsalva's Maneuver).

What is immediately posterior to the Vallate papillae?

The terminal sulcus, a groove that distinguishes the portion of the tongue that lies in the oral cavity (its body) from its posterior portion in the oropharynx (its root).

3 Parts of the Tooth

The tooth consists of 3 parts, the crown, neck, and root.

The hard palate

Underlain by the palatine bones and the palatine processes of the maxillae and It forms a rigid surface against which the tongue forces food during chewing

The Lips and Cheeks

o Keep food between the teeth when chewing o Composed of a core skeletal muscle covered externally by skin o Orbicularis oris muscle forms the fleshy lips o Cheeks are formed by buccinators

The soft palate

A mobile fold formed mostly of skeletal muscle that rises reflexively to close off the nasopharynx when we swallow. It is anchored to the tongue by the palatoglossal arches and to the wall of the oropharynx by the more posterior palatopharyngeal arches (dorsal; directs food downward into pharynx), which together form the boundaries of the fauces (arched area of the oropharynx that contains the palatine tonsils). Projecting downward from the free edge of the soft palate is the fingerlike uvula.

Dentin

A protein that is calcified, not as hard as enamel, it is produced by odontoblast cells and can repair dentin throughout your life, it is found in the crown, neck, and root.

Histamine

Activates parietal cells to release HCl

Oral cavity proper

Area that lies within the teeth and gums

State the roles of bile and pancreatic juice in digestion

Bile is a fat emulsifier that breaks fat into tiny particles to make them more readily digestible. Pancreatic juice includes enzymes that digest chyme as well as bicarbonate that neutralizes stomach acid. These enzymes breakdown all categories of foodstuffs.

Intestinal hormones and paracrines

CCK, GIP, Gastrin, Histamine, Intestinal Gastrin, Motilin, Secretin, Serotonin, Somatostatin, VIP

Serotonin

Causes contraction of stomach muscle

Intrinsic muscles

Confined in the tongue and are not attached to bone. Their muscle fibers, which run in several different planes, allowing the tongue to change its shape (but not position), becoming thicker, thinner, longer, or shorter as needed for speech and swallowing.

Accessory Digestive Organs

Consist of the teeth, tongue, gallbladder, and some large digestive glands (the salivary glands, liver, and pancreas). The teeth and tongue are located in the oral cavity, while the digestive glands and gallbladder lie outside of the GI tract and connect to it by ducts. The accessory digestive glands produce a variety of secretions that help break down foodstuffs.

Describe stimuli and controls of digestive activity.

Digestive activity is provoked by a range of mechanical and chemical stimuli. Receptors involved in controlling GI tract activity are located in the walls of the tract's organs. These receptors respond to several stimuli, most importantly stretching of the organ by food in the lumen, changes in osmolality (solute concentration) and pH of the contents, and the presence of substrates and end products of digestions. The GI tract has a Enteric Nervous system that contains over 100 million neurons. These neurons communicate to regulate digestive system activity. The semiautonomous enteric neurons make up the bulk of the two major nerve plexuses (submucosal and the myenteric). Short reflexes control the patters of segmentation and peristalsis. These happen when the enteric nervous system responds to stimuli from the GI tract. Long reflexes happen when the enteric nervous system sends information to the central nervous system via visceral nerve fibers. They can be initiated by stimuli arising from outside of the GI tract.

Epiglottis

Elastic cartilage; protecting covering of the airway, prevents you from inhaling your food and choking.

Defecation

Eliminates indigestible substances from the body via the anus in the form of feces.

Extrinsic muscles

Extend to the tongue from their points of origin on bones of the skull or the soft palate. These muscles alter the tongue's position, protruding, retracting, and moving it from side to side.

The Submucosa

External to the mucosa, is areolar connective tissue with a lot of elastic fibers containing a rich supply of blood and lymphatic vessels, lymphoid follicles, and nerve fibers which supply the surrounding tissues of the GI tract wall.

Lingual Frenulum

Fold of mucosa that secures the tongue to the floor of the mouth and limits its posterior movements.

The Palate

Forms the roof of the mouth and has two distinct parts: the hard palate anteriorly and the soft palate posteriorly

Vallate papillae

From 8-12 large; located in a V-shaped row at the back of the tongue; resembles the fungiform papillae but has an additional surrounding furrow

Describe how bile and pancreatic juice secretion into the small intestine are regulated.

Hormones (cholecystokinin and secretin) and neural stimuli regulate both the secretion of bile and pancreatic juice and their release into the small intestine. Bile salts are the major stimulus for enhanced bile secretion. After a fatty meal, when the enterohepatic circulation is returning large amounts of bile salts to the liver, its output of bile rises dramatically. Secretin, released by intestinal cells exposed to fatty chime, also stimulates liver cells to secrete bile. When no digestion is occurring, the hepatopancreatic sphincter is closed and the released bile backs up the cystic duct into the gallbladder, where it is stored until needed. Although the liver makes bile continuously, bile does not usually enter the small intestine until the gallbladder contracts.

Name the 4 kinds of teeth

Incisors (cut/nip off food), canines (fang-like, tear/pierce food, more traction), premolars (Biospids, grinding/crushes) and molars (grinding/crushing; large surface area).

Gastrin

Increases HCl secretion; stimulates gastric emptying; stimulates contraction of intestinal muscle; relaxes ileocecal valve; stimulates mass movements.

Mechanical Breakdown

Increases the surface area of ingested food, preparing it for digestion by enzymes. Mechanical processes include chewing, mixing food with saliva by the tongue, churning food in the stomach and segmentation. o Segmentation - rhythmic local constrictions of the small intestine; mixes food with digestive juices and makes absorption more efficient by repeatedly moving different parts of the food mass over the intestinal wall.

List the major processes occurring during digestive system activity

Ingestion, Propulsion, Peristalsis, Mechanical Breakdown, Segmentation, Digestion, Absorption, Defecation

Glucose-dependent insulinotropic peptide (GIP)

Inhibits HCl production; and stimulates insulin release

Secretin

Inhibits gastric gland secretion and gastric motility; increases output of pancreatic juice rich in bicarbonate ions; potentiates CCK's action; increases bile output

Somatostatin

Inhibits gastric secretion of all products; inhibits secretion; inhibits GI blood flow, thus inhibits intestinal absorption; inhibits contraction and bile release

Cholecystokinin (CCK)

Inhibits stomachs secretory activity; potentiates secretin's actions on these organs; increases output of enzymes-rich pancreatic juice; stimulates organ to contract and expel stored bile; relaxes sphincter to allow entry of bile and pancreatic juice into the duodenum

The two muscles that make up the tongue

Intrinsic muscles and Extrinsic muscles.

Crown of tooth

Is covered in white hard enamel (same thing found in bone, hardest thing in the body, produced in complete form before tooth erupts).

The Neck of the tooth

Is the Segway, the tooth is not exposed to the outside and it contains no enamel. Cement is attached to the tooth and periodontal ligament is connected to the bone.

Foliate papillae

Located on the lateral aspects of the posterior tongue

Teeth

Masticates (chewing) -tears/grinds food.

Labial frenulum

Median fold that joins the internal aspect of each lip to the gum

Propulsion

Moves food through the alimentary canal (includes swallowing), which is initiated voluntarily. o Peristalsis - involves alternating waves of contraction and relaxation of muscles in the organ walls; an involuntary process. Its main effect is to squeeze food along the tract, with some mixing occurring.

Fungiform papillae

Mushroom shaped; scattered widely over the tongue surface and has a vascular core that gives it a reddish hue

The Tongue

Occupies the floor of the mouth and is composed of interlacing bundles of skeletal muscle fibers, and during chewing, it grips the food and constantly repositions it between the teeth. It also mixes food with saliva, forming it into a compact mass called a bolus, and then initiates swallowing by pushing the bolus posteriorly into the pharynx.

What does the superior tongue surface bear?

Papillae, peglike projections of the underlying mucosa

Oral vestibule

Recess bounded externally by the lips and cheeks and internally by the gums and teeth

The Muscularis Externa

Responsible for segmentation and peristalsis. It has an inner circular layer and an outer longitudinal layer of smooth muscle cells. In several places along the tract, the circular layer thickens, forming sphincters that act as valves to control food passage from one organ to the next and prevent back flow. It is found between muscle and is typically 2-3 layers. The stomach has 3 layers, and the 3rd layer of muscle runs diagonally.

Define retroperitoneal and name the retroperitoneal organs of the digestive system.

Retro = behind. The retroperitoneal organs are the first part of the small intestine and parts of the large intestine. Organs are that are posterior to the peritoneum and lose their mesenteries are known as retroperitoneal organs. These organs include the pancreas and duodenum (the first part of the small intestine) and parts of the large intestine.

Filiform papillae

Roughens the tongue surface, helping us lick semisolid foods and provides friction for manipulating foods; the smallest and most numerous type; align in parallel rows on the tongue dorsum; contains keratin (stiffens them and gives the tongue its whitish appearance)