AP2 test 1 notes 2017 PDF

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DIGESTION Chapter 23

2017

The digestive system involves structures that ingest food, mix and propel this ingested material to other organs. During this time, the food will be mixed with secretions that will break it into smaller components – some of which will pass into blood vessels and into the body cell. Waste from the ingested food will eventually be eliminated from the body. The digestive system operates on six main functions: 1. Ingestion: Taking into the body food and fluids – normally through the mouth 2. Motility: There is voluntary and involuntary contractions (peristalsis) that mix and move the food downwards through the digestive system 3. Secretions: The production and release of fluids (enzymes, hormones, etc) that facilitate digestion 4. Digestion: The breakdown of ingested food into smaller and smaller components that may be absorbed into the blood. There are 2 types of digestion possible in each digestive structure – MECHANICAL DIGESTION – the physical breakdown of food and CHEMICAL DIGESTION – the breakdown of food through chemical reaction as seen in enzymes, acids, and hormones. (catabolism). 5. Absorption: The movement of digested molecules, and water from the GI tract into the blood. 6. Elimination: The expulsion of indigestible components not used by the body. All of these activities involved nervous system control and integration. The parasympathetic and sympathetic systems are important. Some digestive reflexes do not involve the CNS but are LOCAL and occur only within the GI tract wall. These are referred to as part of the ENTERIC NERVOUS SYSTEM (ENS), which consists of over 100 million neurons. There are 30 neurotransmitters associated with the ENS, including acetylcholine and norepinephrine. Serotonin is also a neurotransmitter that is released by the ENS and has an effect on the digestive system. Over 95% of the serotonin in the body is found within the GI tract, so that medication that increases the serotonin levels, like antidepressants and chemotherapy, may also affect digestion. The organs of the digestive system can be divided into 2 main groups: GI tract +Accessory Organs 1. The GASTROINTESTINAL TRACT (alimentary canal): these are the organs that the food actually passes through mouth pharynx esophagus stomach small intestine and large intestine. Length of G.I. tract in adults = 26 - 28 feet. The UPPER GI TRACT is the mouth through the stomach The LOWER GI TRACT are the intestines 2. ACCESSORY STRUCTURES: These aid in digestion, but the food does not go through them. Examples: teeth, tongue, salivary glands, liver, gallbladder and pancreas. COMPONENTS OF THE DIGESTIVE SYSTEM Upper GI tract = mouth, pharynx, esophagus, stomach 1|Page

MOUTH a. Opening to the digestive tract. b. Vestibule = area of mouth between the cheeks and the gums (gingiva) c. Tongue - structure composed mostly of skeletal muscle, used in speech, but also in digestion to push the food around the mouth. It is attached to the bottom of the mouth by the LINGUAL FRENULUM. Due to cancer of the tongue, a GLOSSECTOMY, tongue removal, may be necessary. d. Teeth: Humans have 2 sets of teeth, deciduous (20 teeth), followed by secondary dentition (32 teeth) and are important in mechanical digestion. e. Salivary glands: There are a number of salivary glands scattered through the oral cavity, but there are 3 major pairs: 1. Parotid Glands: Largest of salivary glands, anterior to the ears with the duct going across the external surface of the masseter muscle. There duct open on either side of the lingual frenulum. 2. Submandibular glands: found under lower jaw (mandible) with opening of ducts on either side of lingual frenulum. 3. Sublingual glands: found under tongue. These 3 sets of exocrine glands will secrete SALIVA into the mouth. Saliva is 99.5% water and under the control of the nervous system. 1000 - 1500 mls. is released each day. The functions of saliva include dissolving the food and therefore helping in mechanical digestion, allowing the TASTEBUDS in the mouth, (a type of CHEMORECEPTOR) are stimulated for taste, soften the food for swallowing and containing SALIVARY AMYLASE, an enzyme that is important to the digestion of carbohydrates. Physiology of digestion in the mouth. 1.Mechanical Digestion: The TONGUE will move the food around the mouth, allowing it to mix with the saliva. The TEETH will break up the food, increasing the surface area and therefore allowing for more digestive enzymes to come into contact with more of the food and also decreasing the size of the food so that it can be swallowed safely. Chewing is also termed MASTICATION. The result of the actions of the teeth and tongue is formation of a soft rounded ball of food called a BOLUS. The bolus, with the help of the tongue, will be passed from the mouth into the pharynx.

2. Chemical Digestion: The mouth is the site for the beginning of the digestion of CARBOHYDRATES by the enzyme, SALIVARY AMYLASE. This enzyme is specialized to digest polysaccharides into disaccharides. Only the monosaccharide form of the carbohydrate can actually enter the cell, so these dissacharides will need to be further digested in another part of the digestive system. 2|Page

Because SALIVARY AMYLASE is an enzyme, it will have an optional T and pH. Optimal T = 98.6F (normal body temp of a human) Optional pH =approximately 6- 7.

(Reminder – Enzyme = A protein that increase the rate of chemical reactions without changing themselves. Each enzyme will have its own SUBSTRATE that it can work on, and will work best at OPTIMAL TEMP. and OPTIMAL pH.) When the bolus is formed, DEGLUTITION (swallowing) occurs pushing it into the OROPHARYNX, (the throat) and with the help of PERISTALSIS, the bolus continues into the esophagus. PERISTALSIS: Involuntary wave like contractions of smooth muscle. In the G.I. tract, these contractions push digesting food downwards. Peristalsis is mostly under the control of the brain, specifically the medulla oblongata. As the bolus moves from the pharynx into the esophagus, it will pass through a SPHINCTER MUSCLE called the UPPER ESOPHAGEAL SPHINCTER (superior esophageal sphincter) sphincter muscle: circular muscle that can constrict an opening. Many sphincters are seen throughout the G.I. tract. ESOPHAGUS a. A collapsible, muscular tube that connects the pharynx to the stomach. b. Peristalsis occurs within this structure, but there are no digestive enzymes that originate from the esophagus. c. The esophagus will go through the DIAPHRAGM, the large muscle that separates the thoracic cavity from the abdominal cavity, at an opening called the ESOPHAGEAL HIATUS. Sometimes this opening causes a weakness in the diaphragm and the portions of abdominal organs (like the lower esophagus and upper stomach) can protrude into the thoracic cavity, causing pain and discomfort. This pathology is called a HIATAL HERNIA. d. The esophagus eventually narrows and the bolus will go through the LOWER ESOPHAGEAL SPHINCTER (also known as inferior esophageal sphincter or CARDIAC SPHINCTER) to the stomach. The food goes from the mouth to the stomach as a BOLUS. Note that although there are no digestive enzymes in the esophagus, chemical digestion is still occurring in the bolus by the salivary amylase from the mouth

SWALLOWING = DEGLUTITION Deglutition has 3 phases, starting in the mouth. It is the process that moves ingested food from oral cavity through esophagus into the stomach. Buccal Phase: occurs after ingestion. Under control of brain, a bolus is formed pushed to the pharynx. This can be controlled voluntarily. Pharyngeal- Phase: This phase is under voluntary control at the beginning but then is under involuntary control. The bolus passes into the pharynx and peristalsis begins to 3|Page

push it downwards. The bolus passes involuntarily through the upper esophageal sphincter into the esophagus. Esophageal Phase: This is also an involuntary phase. The bolus, with the help of peristalsis passes through the esophagus. Through the lower esophageal sphincter into the stomach. STOMACH: a. a J shaped organ located under the diaphragm b. 4 areas to the stomach 1. cardia -surrounds lower esophageal sphincter 2. fundus-rounded portion 3. body- large central portion 4. pylorus- most inferior portion, right around the PYLORIC SPHINCTER which leads to the small intestine

c. Inside of the stomach, the walls are not smooth but are arranged in large folds when the stomach is empty= RUGAE Physiology of the stomach: Note that although the stomach has digestive properties, a person can live without a stomach. It is essentially a holding and mixing center for partially digesting food before it enters the small intestine. Mechanical digestion: When the food enters the stomach, MIXING WAVES will push the bolus around, causing it to be mixed with the digestive enzymes of this organ. The bolus becomes more liquefied and is called CHYME. Chemical digestion: Unlike the mouth and other parts of the digestive system in which the pH is maintained around 6-8, the stomach pH is around 2. Salivary amylase is inactivated in the stomach due to the decrease in pH. A. This acidic pH is due to the secretion of HCL (hydrochloric acid) by the PARIETAL CELLS found in the stomach wall. This HCL will maintain an optimal pH for the enzymes of the stomach to work. It also offers some protection to the person by killing microbes that enter the body through the food that they ingest. Therefore, HCL is important in chemical digestion in stomach. b. PEPSIN: an enzyme that is secreted (in the inactive form of pepsinogen) into the stomach LUMEN by CHIEF CELLS. The optimal pH for this enzyme to function is around 2 It is 4|Page

responsible for beginning the digestion of proteins by decreasing the length and changing the shape of the amino acid chains. Pepsin is secreted in inactive form = PEPSINOGEN, and will be converted to pepsin in the presence of HCL. Features that protect stomach wall from acidic conditions. 1. digesting enzyme include the release of pepsin in an inactive form 2. thick mucus layer over gastric epithelium (mucous cells) 3. high mitotic replacement of damaged stomach epithelium c

GASTRIC LIPASE: enzyme of stomach that digests lipids. Optimal pH is 5-6, and therefore action is limited in stomach by the acidic pH. d. MUCUS: substance that is produced by mucous cells in stomach lining is also important in digestion. It moistens bolus and offers protection to lining of stomach. e. GASTRIN: Hormone of stomach (not an enzyme). Involved with many digestive activates of stomach including relaxation of pyloric sphincter after a meal. f. INTRINSIC FACTOR: THE PRODUCTION OF IF IS THE ONLY ESSENTIAL FUNCTION PERFORMED BY THE STOMACH. This glycoprotein is secreted by parietal cells that allows for the intestinal absorption of vit. B12, needed for the production of RBCs. The biggest problem for people who have their stomach removed (gastrectomy) is the lack of this intrinsic factor, which may lead to a type of ANEMIA (pernicious anemia), a decrease of red blood cells in the blood. This problem is solved by taking vit B12 exogenously. THE LEVEL OF INTRINSIC FACTOR DECREASES WITH AGE, RESULTING IN ANEMIA IN OLDER PEOPLE WHO NEVER HAD THIS PROBLEM WHEN YOUNGER B12 injections may be needed. Within 2-6 hours after eating, the stomach has emptied its contents into the duodenum, the next part of the GI tract. The food enters the stomach as a BOLUS but leave as a thin liquid call CHYME.

The secretory activity of the stomach can be summarized into 3 phases: cephalic, gastric and intestinal phases. cephalic phase: This begins before the food gets to the stomach this reflex is initiated by thought, smell, sight, or taste of food. Eventually, parasympathetic signals from the medulla oblongata travel along the vagus nerve (CN X) to the stomach resulting in increase motility and secretion as the stomach gets ready for food. gastric phase: This phase begins when the bolus reaches the stomach lumen. The activity is stimulated by stretch receptors in the stomach wall as well as chemoreceptors that respond to the presence of protein. Increase in motility and secretions occur, mostly due to parasympathetic

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activity from the medulla oblongata (CN X). The chyme goes through the pyloric sphincter and into the duodenum (the first portion of the small intestine) intestinal phase: The chyme now begins to leave the stomach and fill the duodenum. As the acidic chyme begins to fill the duodenum, inhibitory nerve signals go to the medulla oblongata. This decreases the vagal stimulation to the stomach and therefore decreases the motility and secretory activity of the stomach. Secretory and digestive summary of stomach: The bolus of food enters the stomach from the esophagus, through the cardiac sphincter. There will be an increase in motility and secretions, resulting in the bolus becoming more liquefied- now referred to as chyme. The most vital jobs of the stomach are one of storage as well as production of Intrinsic Factor, important in absorption of Vit B12 which is central to RBC function. Under normal conditions, the beginning of protein and lipid digestions occurs in the stomach – however, both of these activities can occur in the lower GI tract if a gastrectomy has been performed. ABSORPTION IN STOMACH: It’s important to note that before the chyme leaves the stomach, some absorption will occur. The stomach wall is impermeable to most material (most absorption occurs in the small intestine). Some things can go from the lumen into blood vessels in the stomach wall = water, electrolytes, some drugs (ASA), alcohol. Finally, after 2-6 hours (depending on type of food – lipids take longer) chyme leaves the stomach, goes through the pyloric sphincter and into the duodenum, the first part of the small intestine. More pathologies and procedures of the UPPER GI tract: BARRETT ESOPHAGUS= erosion of esophageal mucosa due to chronic GERD. This is a precancerous condition GERD= gastroesophageal reflux disease (heartburn). Lower esophageal sphincter opens and acid from stomach splashes upwards into esophagus. This can cause “heartburn” due to lack of specialized epithelium found in stomach. PYLORIC STENOSIS= a narrowing of the pyloric sphincter, found in some newborns: causing PROJECTILE VOMITING. PEPTIC ULCERS= these are erosions that occur in the wall of the stomach, upper small intestine (duodenum) or the esophagus. They are associated with the presence of Helicobacter pylori bacteria. Antibiotics are usually a successful treatment. GASTRITIS= Inflammation of the stomach. May be results of stress, medications, alcohol BARIATRIC SURGERY= Surgery done on the morbidly obese. Decreases gastric lumen size, leading to less amt of food intake, leading to wt. loss. ENDOSCOPY: Procedure of passing a flexible tube into throat through the esophagus and stomach. A camera can then be passed into these areas, visualizing the tissue and lumen. 6|Page

LOWER GI TRACT: The lower GI tract continues the process of digestion and is very important in the absorption of nutrients. Material that cannot be digested and absorbed will be eliminated. The lower digestive system includes the small intestine (duodenum, jejunum and ileum), the accessory organs associated with the duodenum (liver, gallbladder and pancreas) and the large intestine. SMALL INTESTINE The small intestine is the area of the digestive system where most of the digestion of the food will occur. This is because the small intestine itself contains many digestive secretions and the important accessory organs produce digestive materials and place them within the small intestine. These accessory organs secrete their digestive juices into the duodenum, the first section of the small intestine, following the stomach. So, as the chyme leaves the stomach, it will pass into the duodenum and come in contact with material from the pancreas, gall bladder and liver, the accessory organs. It will also be digested by material from the small intestine itself.

Accessory organs of the small intestine = pancreas, gallbladder, liver PANCREAS: (both exocrine and endocrine organ) 1. Located posterior to the greater curvature of the stomach and connected by 2 ducts to the duodenum, the pancreatic duct (largest) and the accessory duct. 2. About 5 inches long, 1 inch thick. 3. Acini cells make up small exocrine glands within the organ. These structures will secrete pancreatic juice that contains a number of digestive enzymes. This pancreatic juice will be secreted into one of the 2 major ducts that will lead into the duodenum, the pancreatic duct and the smaller accessory duct. 7|Page

Exocrine functions of pancreas: Pancreatic Juice: (from acini cells) a. clear, colorless, composed of water, salts and enzymes b. pH = 7.2 - 8.2 Therefore will stop the action of pepsin. c. Pancreatic juice contains enzymes that are very important in digestion: pancreatic amylase: continues digestion of carbohydrates that was started in the mouth. Trypsin, cymotrypsin and carboxypepsidase: all of these enzymes will be secreted in inactive forms and will continue Pr- digestion started in stomach. Pancreatic lipase: continues the digestion of lipids. Ribonucleases and deoxyribonucleases: digestion of nucleic acids. (part of genetic structures) Endocrine functions of the pancreas: secretes the hormones glucagon and insulin Both of these hormones are important to maintaining the level of glucose in the blood. They are antagonistic hormones. Both insulin and glucagon is produced in small structures of the pancreas called ISLETS OF LANGERHANS, which contained alpha and beta cells. The aim of these ANTAGONISTIC HORMONES is to maintain the blood glucose within homeostatic levels. NORMAL BLOOD GLUCOSE LEVEL:

Background: relationship between glucose, glycogen and homeostasis of blood glucose

GLYCOGEN (stored in liver)

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Glycogenesis (Insulin from beta cells) cells)

Glycogenolysis ( Glucagon from alpha

GLUCOSE (level of blood glucose needs to be maintained at a homeostatic level)

Alpha Cells: produce glucagon when blood glucose level in blood decreases. This hormone will raise the blood glucose level by traveling to the liver and causing the breakdown of glycogen into glucose (glycogenolysis). This glucose is then released into the blood and the blood glucose level rises, maintaining homeostasis. Decrease blood glucose level  Alpha cells in pancreas release GLUCAGON into the blood Blood goes to the liver and stimulates GLYCOGENOLYSIS to occur. Increase in glucose level in the blood (example of homeostasis)

Beta Cells: produce insulin when blood glucose levels increase. This hormone will decrease the blood glucose level by traveling to the liver and causing the bonding of many glucose molecules into glycogen (glycogenesis). Also, insulin will increase the permeability of the cell membrane to glucose, allowing for passage of glucose from the blood into the cell, and therefore decreasing the blood glucose level. Increase in blood glucose level 

Beta cells in pancreas release INSULIN into blood

Blood goes to liver and stimulates liver to go through GLYCOGENESIS 9|Page

Decrease glucose level in the blood (example of homeostasis)

Insulin also has an effect on many cells in the body, increasing permeability of cell membranes to the glucose Decrease of blood glucose level (example of homeostasis)

Pancreatic Pathology: Diabetes Mellitus: The inability of the body to produce insulin or the insulin receptors of cells become hyposensitive to insulin. The result is that the blood glucose levels rise and none or lower amount reach the cells. There are 3 major typesa. TYPE I (insulin dependent) Symptoms appear suddenly, usually in childhood. Person is dependent upon exogenous INSULIN INJECTIONS (needs to be given as injection, because insulin is a protein which would be digested and inactivated if taken as a pill) Thought to be an AUTOIMMUNE DISEASE in which the beta cells are destroyed by the body. b. TYPE II (non-insulin dependent) occurs u...