Chapter 22 The Digestive System Mrs. Brown's Class Wallace Community College

Describe the peritoneal membranes and the peritoneal cavity.

The abdominopelvic cavity houses the largest serous membrane in the body, the PERITONEAL MEMBRANE or peritoneum. Outer layer is PARIETAL PERITONEUM which lines the inner surface of the body wall. Where the PP folds in to meets the ab. organs, it folds in on itself to become the inner VISCERAL PERITONEUM or SEROSA, which forms outer tissue layer of such organs. Between 2 peritoneal layers is peritoneal cavity, which contains serous fluid. Fluid lubricates organs as they slide past one another.

Describe the absorption of water, electrolytes, and both types of vitamins.

AC absorbs large quantities of water, elect., and vitamins. Much of this absorption occurs in small intestine, but a sig. quanitity occurs in LI too. 8 liters of water absorbed into enterocytes of SI. Most of remaining waterabsorbed into enterocytes of LI, leaving only about 0.1 liter of water to be excreted in feces. Vitamins are chemicals generally provided by diet that are involved in a host of metabolic reactions. Two types: Water-soluble: polar molecules, can urinate out. Most water-soluble vitamins are absorbed in small intestine by diffusing through enterocytes' plasma membranes. Fat-soluble vitamins: which are lipid-based and mostly nonpolar. Fat-soluble vitamins are packaged into micelles with fats and other lipids and are absorbed with them. Can become toxic (Vitamins D and A) and can be stored in tissues.

Describe the digestion and absorption of carbohydrates, proteins, lipids, and nucleic acids. Include the names of all enzymes involved. Define Micelle.

Carbs: Salivary amylase starts in mouth, continues in stomach - mech. Chem digestion resumes in SI, where polysaccharides and oligosaccarides encounter pancreatic amylase. Brush border enzymes break dis. Into mono. --SEE NOTES--- MICELLE: digested lipids coated in bile salts

List the 2 types of cells in a salivary gland and explain what product each produces and when. List the 3 pairs of salivary glands.

The basic secretory cell of the salivary glands are ACINAR CELLS. Two main types of acinar cells in salivary glands: SEROUS CELLS which secrete a water-based fluid with enzymes and other solutes; and MUCOUS CELLS which secrete mucus. The secretions from SEROUS cells are involved in digestive processes and are generally released just before or during eating. Secretions from MUCOUS cells are primarily involved in keeping the oral mucosa moist, and so are released continually. Saliva is secreted through the parotid duct in the oral cavity. Submandibular, sublingual and parotid glands.

Describe the role of the pharynx in regards to digestion.

Only the Oropharynx and Laryngopharynx are part of AC. Oropharynx houses palatine tonsils and lingual tonsils. Tonsils perform defensive functions; protect remainder of AC from any pathogens that enter body via oral and nasal cavities. Primary function of pharynx is PROPULSION in form of swallowing, during which bolus passes through pharynx and into esophagus.

Describe the composition of saliva.

Saliva consists of water, electrolytes (sodium, chloride, potassium ions), and variable amounts of mucus. Also contains: --SALIVARY AMYLASE, first digestive enzyme food encounters; catalyze beginning of carb. digestion, breaks down lg. polysaccharides into smaller ones. --ENZYME LYSOZYME catalyzes perforation of bacterial plasma membranes; allows bacteria-killing substances in saliva to enter and kill bacteria --SECRETORY IgA - antibody immunoglobulin A, or IgA; binds specific antigens on pathogens and mediates their destruction --BICARBONATE IONS - function is to neutralize any acid from stomach that regurgitates into esophagus.

Describe the pancreas and its digestive functions.

Secretes enzymes that are released into ducts of pancreas that empty into AC, makes pancreatic juice, collective secretions of pancreatic acinar and duct cells. Consists of water and multiple digestive enzymes and other proteins. Duct cells also secrete bicarbonate ions, a base, which make pancreatic juice alkaline. Alkaline juice helps neutralize acidic chyme that enters duodenum from stomach; protects deudenum from damage by acid. Its digestive enzymes, secreted by acinar cells, are crucial in chemical digestion. Catalyze reactions that digest carbs, proteins, lipids, and nucleic acids. Pancreatic secretion occurs at a basal rate b/t meals. During eating, Pan. Secretion rises due to Parasympathetic and hormonal stim.

Retroperitoneal

Some abdominal organs are partly or completely outside the peritoneal cavity (behind). (Pancreas can be both)

Describe the function of the gallbladder and trace the pathway of bile to the small intestine.

Stores bile, concentrates it (removing water), and releases it when stimulated. Pathway of bile: 1) Gallbladder to release bile into cystic duct 2) Cystic duct joins common hepatic duct to from common bile duct. 3) Common bile duct joins main pancreatic duct near duodenum to form hepatopancreatic ampulla 4) Ampulla is surrounded by a ring of smooth muscle called hepatopancreatic sphincter which controls emptying of bile and pancreatic fluids into duodenum. 5) Contents of hepatopancreatic ampulla empty into duodenum at major duodenal papilla

Mesenteries

Visceral peritoneum folds over on itself around certain organs (particularly the small intestine) to form structures called mesenteries. The mesenteries support and bind these organs together and keep the small intestine in a particular shape that fits within the abdominopelvic cavity. Mesenteries also house blood vessels, nerves, and lymphatic vessels, anchoring them in place. The mesenteries attached to much of the large intestine is often called the mesocolon.

Describe the other functions of the liver.

1) Liver secretes bilirubin, a waste product that results in breakdown of hemoglobin by spleen; some remains in blood and is secreted by kidneys. 2) Nutrient metabolism - processes nutrients obtained from diet 3) Detoxification - detoxifies substances produced by body, and substances that we eat or drink, some of which are harmful to body 4) Excretion - directly secretes bilirubin in bile along with other substances liver processes, particularly certain antibiotics; also modifies substances so that they can be excreted by kidneys.

Describe the 6 basic processes of the digestive system.

1. INGESTION - Food and water are brought into digestive system by ingestion, which occurs via mouth. 2. SECRETION - Digestive organs contain both endocrine and exocrine glands that secrete mucus, enzymes, acid, hormones to aid in other digestive processes. 3. PROPULSION - How ingested food passes from one digested organ to the next. Rhythmic contractions of smooth muscle called PERISTALSIS and aided by mucus. 4. DIGESTION - food breakdown. Mechanical and chemical 5. ABSORPTION - Once food particles are digested, nutrients move through the wall of the AC into blood or lymphatic vessels by this. Water, electrolytes, vitamins are absorbed into blood in same manner. 6. DEFECATION - Certain materials not digestible or absorbed continue transit through AC until exiting the body as feces. Provides body with a way to eliminate metabolic wastes. (Specialized form of propulsion)

Define alimentary canal and accessory organs and list organs that belong to each.

Alimentary Canal: A continuous tube through which food passes directly. Organs include: oral cavity (mouth), pharynx, esophagus, stomach small intestine, large intestine. Accessory organs are not part of the alimentary canal but assist in digestion in some way. They are located around the AC and include: teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Describe the liver functions that pertain to digestion. Explain the role of bile. Define emulsification.

Bile is a liquid that contains multiple components, including water, elect., and organic compounds. 2 functions: --Required for digestion and absorption of liquids --mechanism by which liver excretes wastes and other substances that kidneys cannot excrete. Liver produces bile continually but generally does not secrete it at a basal rate. One of bile's main organic compounds is bile salts that are amphiphilic, meaning have both polar and non-polar parts. Allows bile salts to interact with both lipids and watery environment of small intestine. Coats lipids in duodenum and physically break them apart into smaller pieces - emulsification. Mech. Digestion nec. For chem. Digestion and absorption of lipids. Bile contains materials that liver excretes, incl. cholesterol, waste products, and toxins such as heavy metals.

Describe motility in the small intestine during fasting and eating.

During fasting, small intestine exhibits slow, rhythmic contractions along its length in a pattern called MIGRATING MOTOR COMPEX. Contractions clear any remaining material, including leftover food and secretions. Requires about 2 hours to push digesting food from duodenum to ileocecal valve. Controlled by both ENS and a hormone called motilin; produced by cells in duodenal mucosa. During eating, peristalsis and segmentation occurs. Peristalsis is accomplished by alternating contractions of longitudinal and circular layers of smooth muscle in muscularis externa. Primary function is to propel chyme toward ileum and ultimately through ileocecal valve to cecum. Segmentation, also known as intestinal churning, involves contractions of only circular layer of smooth muscle. Produces a squeezing motion. Primary functions are mechanical digestion and mixing chyme with intestinal and pancreatic enzymes as well as bile.

Describe the functions of the bacteria within the large intestines.

Houses numerous bacteria that perform important functions such as synthesizing vitamins. Ten times more bacteria in LI than cells in human body; bacteria make up 60% of dry mass of feces. 500 dif. Bacterial species in symbiotic rltp. Functions: --produce vitamins (such a vitamin K for blood clotting) --Metabolize undigested materials (cellulose) --Deter growth of harmful bacteria --Stimulate immune system (MALT)

Intraperitoneal

Organs located entirely within the peritoneal cavity

Describe the major functions of the digestive system.

--To take food into the body and break it down into its component nutrients so that they can be used by body cells. --Also plays a critical role in fluid, electrolyte, and acid-base homeostasis by taking in water and electrolytes via the diet and delivering them to the blood. --Other functions include ingesting vitamins and minerals, producing hormones, and excreting metabolic wastes.

Describe the functions of saliva.

-Moistening, lubricating, cleansing oral mucosa -Lysozyme and IgA deter growth of pathogenic bacteria in oral cavity -Mechanical digestion, achieved by moistening and helping to mix ingested food into a bolus so it can be swallowed -Chemical digestion achieved by actions of salivary amylase -Many food molecules dissolve in water of saliva, and these molecules then stimulate taste receptors on tongue

Explain the process of deglutition, including the 3 phases.

A specialized type of propulsion that pushes a bolus of food from oral cavity through the pharynx and esophagus to the stomach. Swallowing relies on coordinated action of upper AC (soft palate, pharynx, esophagus) Tongue plays a role and is only accessory organ to directly participate in motility. Swallowing almost entirely neural. ANS does not directly control any phase of swallowing. 1. VOLUNTARY PHASE The tongue pushes the bolus posteriorly toward the Oropharynx. (Superiorly a/g hard palate, posteriorly toward Oropharynx.) ONLY stage of swallowing under conscious control. 2. PHARYNGEAL PHASE The bolus enters the Oropharynx; soft palate and epiglottis seal off the nasopharynx and larynx, respectively. Bolus goes from pharynx to esophagus during this phase. Involves involuntary contraction of skeletal muscles such as pharyngeal constrictor muscles and is controlled by swallowing reflex, initiated by medulla oblongata. Bolus pushes epiglottis down, preventing aspiration (entry of food into larynx). 3. ESOPHAGEAL PHASE Peristaltic waves move the bolus down the esophagus to the stomach. This phase begins as the upper esophageal sphincter relaxes. Neurons of the enteric nervous system then stimulate the inner circular and outer longitudinal layers of the muscularis externa to undergo peristalsis and massage the bolus inferiorly towards stomach.

Describe the structure and functions of the esophagus including the roles of the upper esophageal sphincter and the gastroesophageal sphincter.

Esophagus is 10" long, found posterior to trachea. Transports a bolus from pharynx to stomach. Esophageal mucosa contains esophageal glands which secrete mucus to lubricate bolus as it passes through esophagus. At junction of pharynx and esophagus, muscularis externa is modified into a sphincter called UPPER ESOPHAGEAL SPHINCTOR which controls passage of bolus into esophagus. At esophagus' inferior end is another sphincter, GASTROESOPHAGEAL SPHINCTER, also known as lower esophageal sphincter (supposed to close, causes reflux) which regulates passage of bolus into stomach and also prevents contents of stomach from re-entering esophagus. Primary functions of esophagus are propulsion and a small amount of secretion of mostly mucus. During swallowing, skeletal muscle and smooth muscle of muscularis undergo peristalsis; pushes bolus inferiorly.

Discuss the basic functions of the oral cavity, the teeth, and the tongue. Include bolus, deglutition and mastication.

Four digestive processes take place in the oral cavity: ingestion, secretion, chemical and mechanical digestion, propulsion. It houses 2 accessory organs: teeth and tongue, as well as 3 pairs of salivary glands. Together these organs turn ingested food into a moist, chewed mass called a BOLUS. The posterior oral cavity, oro-pharynx, and esophagus then deliver the bolus to the stomach through swallowing or deglutition. MASTICATION grinds it into smaller pieces. Mastication aids digestion by increasing overall surface area of food, giving digestive enzymes more places to catalyze reactions of chemical digestion. Tongue involved during ingestion. Movement of tongue helps turn food to bolus. Tongue pushes food a/g hard palate, assisting in mechanical digestion, and it pushes food posteriorly during swallowing.

Describe roles of the proximal and distal portions of the large intestines and explain the motility of the large intestine.

LI has 2 functional segments: Proximal large intestine consists of ascending and transverse colon. Distal large intestine consists of descending and sigmoid colon, rectum, and anal canal. Proximal LI is primary site of water and electrolyte absorption and bacterial activity. Distal LI performs a small amount of absorption, primarily of water, but its main role is to store fecal material until it is ready to be expelled during defecation. 2 main types of motility of LI: Segmentation (Churning) - circular muscle of hamstrung contracts repeatedly; swirls material around in haustrum. Aids in water and elec. absorption primary controlled by local neurons of ENS; triggered by STRETCH. Propulsive motion known as a MASS MOVEMENT (mass peristalsis) - during a mass movement, multiple HAUSTRA undergo peristalsis; propels their contents toward distal large intestine. Mass movements occur 3-4x day, triggered by food consumption; initiates reflexes controlled by ENS. Distal LI is much less motile. When mass movements occur, initiates ParaSymp.-mediated DEFACATION REFLEX.

Describe the functions of the large intestine and discuss the histology of the large intestines.

LI receives material from SI that was not digested or absorbed, and is a passageway for feces or fecal matter to exit body. Active in absorbing water and electrolytes; critical for maintaining fluid, electrolyte, and acid-base homeostasis. Secretion (primarily mucus), propulsion, and defecation. Mucosa of LI lacks villi and microvilli. Absorption not primary function. Mucosa rich with goblet cells secreting protective and lubricating mucus.

Explain the 2 types of digestion. Explain the process of enzymatic hydrolysis reactions of nutrient molecules.

Mechanical Digestion: food is physically broken into smaller parts. All nutrients go through same mechanisms of mechanical digestion - mastication in mouth, churning in stomach, segmentation in small intestine. Chemical Digestion: chemical bonds between food molecules are broken. CD varies for each nutrient bc digestive enzymes are specific for a single type of bond in a single type of nutrient. Most digestive enzymes catalyze hydrolysis reactions that use a water molecule to break a bond b/t 2 molecules: 1. Explains why so much water is secreted w fluids 2. Water molecules are key components of reactions that chemically break down food molecules 3. Digestive enzymes speed up these reactions by a process known as enzymatic hydrolysis Once nutrients are digested into component molecules, they must enter body.

Explain the motility of the stomach in terms of its receptive function, churning function and emptying function.

Motility enables the stomach to receive food from esophagus, churn incoming bolus into chyme, and control rate at which chyme empties into small intestine. RECEPTIVE FUNCTION - when food or liquid is swallowed, gastroesophageal sphincter and smooth muscle of fundus and body of stomach relax to allow stomach to fill, a process known as RECEPTIVE RELAXATION. CHURNING FUNCTION - after a meal, smooth muscle layers of stomach begin to produce waves of peristalsis; initiated and controlled by a group of specialized cells called gastric pacemaker. (Constant at 3 waves per minute). Peristalsis propels bolus to pylorus, where small amounts of chyme are propelled through pyloric sphincter into duodenum. Remainder of chyme is pushed backward into stomach where peristaltic waves churn and mix chyme, and process repeats. EMPTYING FUNCTION - control movement of chyme into duodenum; different materials pass through pyloric sphincter at different rates. Liquids move rapidly from stomach to duodenum with essentially no delay. Solids must be converted to a nearly liquid state before they are able to enter small intestine. Control of gastric emptying is critical because duodenum must mix incoming chyme thoroughly before it moves to rest of small intestine. Two reasons: --Chyme is acidic, and duodenum must in it with bicarbonate ions to avoid damaging intestinal mucosa. ---Chyme is generally very concentrated and must be diluted with after from pancreatic juice to prevent chyme from drawing water into intestinal lumen by osmosis.

Briefly explain regulation of motility by the endocrine and nervous systems.

Motility, or movement, of the AC is a key process in every region of the canal. In oral cavity, pharynx, superior portion of the esophagus, and the last portion of the large intestine, motility is due to skeletal muscle. In rest of AC, it is smooth muscle. Motility takes several forms: swallowing, churning, peristalsis, and defecation. These move food through the canal or mix it as part of mechanical digestion. Each type of motility is regulated by nervous system and/or endocrine system. ANS: Sympathetic NS inhibits processes; Parasympathetic NS stimulates them. (Long reflexes) ENS: Enteric Nervous System is self-contained branch of ANS (esophagus to anus) works via short reflexes. Endocrine system regulates digestive processes by secreting hormones. Many are paracrine hormones (denoting a hormone that has effect only in the vicinity of the gland secreting it). Enzymes are produced by stomach and small intestine. Hormones bind to target cells to regulate function. Hormones either stimulate or inhibit motility of AC, often with 1 or more neural mechanisms.

Describe the functions of the small intestine and discuss the histology and functions of the small intestine's circular folds, villi, and microvilli.

Secretion, digestion, absorption, and propulsion. Enterocytes produce multiple digestive enzymes, hormones, and mucus. Enzymes are responsible for bulk of chemical digestion. After nutrients are digested chemically, they are absorbed across enterocytes into either blood or lymph, along with water, vitamins, and other substances. Also mixes and propels its contents along its length and into large intestine. Three progressively smaller types of folds; most absorption that occurs takes place in this region. CIRCULAR FOLDS - largest, naked eye. Folds also slow down transit of chyme through small intestine' gives nutrients more time to be digested, and small intestine cells (enterocytes) more time to absorb nutrients. VILLI - Mucosa folds into projections called villi. Each villus consists of a layer of enterocytes surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal. Between villi, mucosa indents to form intestinal crypts. House glands with enteroendocrine cells and mucus-secreting goblet cells. MICROVILLI - smallest folds are found in plasma membrane of enterocytes. Each enterocyte has as many as 3,000 MV; gives appearance of a brush border. Digestive enzymes produced and secreted by enterocytes catalyze reactions that break down disaccharides and peptides.

Describe the histology of the stomach and the cells of the gastric glands. Include the functions of the types of cells the gastric glands contain. Define Chyme.

Stomach has same 4 tissue layers as rest of alimentary canal, exceptions: additional oblique layer of smooth muscle allows stomach to perform churning, pummels food into a liquid called chyme. Mucosa of stomach is heavily indented to form deep structures called gastric pits. Contains columnar cells and numerous goblet cells, both of which secrete a thick mucus that lines and protects cells of stomach from its own secretions. Gastric glands contain both endocrine and exocrine cells, secrete hormones and an acidic gastric juice. The interior of the stomach contains folds called RUGAE which allow the stomach to expand considerably. 4 types of cells in gastric glands: 1. ENTEROENDOCRINE CELLS secrete hormones that influence digestion; gastric stimulates acid secretion 2. CHIEF CELLS secrete Pepsinogen. When pepsinogen encounters an acidic pH, it becomes pepsin which begins protein digestion in the stomach. 3. PARIETAL CELLS secrete HCI (pH of 2) which is responsible for acidic pH of gastric juice. Acid activates pepsinogen, destroys many disease-causing organisms, and stimulates parietal cell production of chemical intrinsic factor; required for intestinal absorption of vitamin B12. 4. MUCOUS NECK CELLS secrete acidic mucus, prevents neutralization of acid produced by parietal cells.

Describe the acid secretion function of the stomach including the 3 regulatory phases. Define enterogastric reflex.

Stomach performs 3 primary functions: secretion, propulsion, digestion. Acid secretion from stomach occurs continuously throughout day between meals, at what is known as basal rate. During eating, secretion can be divided into 3 phases: 1) CEPHALIC PHASE - mediated by sight, smell, taste, or even thought of food under direction of CNS. Prepares stomach to receive food by increasing release of hydrogen ions into it. 2) GASTRIC PHASE - begins when food enters stomach. Presence of food or partially digested protein stimulates acid secretion due to gastrin. 3) INTESTINAL PHASE - responsible for only about 10% of remaining acid secretion, after which it inhibits further acid secretion. A) Triggered by presence of partially digested proteins in fluid entering duodenum; releases more gastrin. B) Stimulatory effect of intestinal phase is brief; as chyme enters duodenum, declining pH and presence of lipids trigger ENTEROGASTRIC REFLEX which decreases vagal activity and acid secretion. The low pH in the duodenum also triggers the production of hormones by the cells of the duodenal mucosa including: C) SECRETIN and GASTRIC INHIBITORY PEPTIDE (GIP) from duodenum - both reduce acid secretion.