22 Digestive System. Taft College Human Physiology

Transcription

1 22 Digestive System Taft College Human Physiology

2 Functions of the Digestive System (4 Basic Functions of Digestion) 1. Ingestion - food intake. 2. Digestion - process of reducing foods to their constituent building blocks from larger molecules to smaller ones that may be absorbed and used by the body. a. mechanical or physical digestion - ex. - chewing b. chemical digestion ex. Digestive enzymes (catalysts) 3. Absorption - taking in the nutrients that have been broken down. 4. Defecation or egestion - elimination of unabsorbed residues of the digestive process.

3 Components of the Digestive System 1. Alimentary Tract - tube with various modifications from mouth to anus. All digestion occurs in the tube. All absorption occurs from this tube. Within wall of tract is smooth muscle that serves to propel foods = peristalsis 2. Accessory organs - structures that lie outside of the alimentary tract. These organs empty secretions into the digestive tract by way of ducts. The secretions are necessary for digestion.

4 Control of Feeding (Appetite, Hunger) Feeding is controlled by the hypothalamus. (remember homeostasis = hypothalamus). How the hypothalamus determines we are hungry is not sure, but here are some hypotheses: 1. Gastric sensation hypothesis - filling of stomach or stretching of abdominal wall. Some diet aids attempt to work on this mannerlike a sponge that swells. 2. Glucostat hypothesis - body monitors glucose levels. Glucose uptake by liver cells influence vagus nerve and information is transmitted to lateral hypothalamus (Glucose Receptors). 3. Thermostat hypothesis - intake of food associated with rise in metabolic activity. Therefore, increase in heat. The increase in temperature reduces appetite. Closely tied to caloric need. In cooler temperatures- the more food we eat. 4. General nutritional state hypothesis - body can monitor usage of a particular nutrient (substance, vitamin, minerals).

5 Control of Feeding (Appetite, Hunger) 5. Intake of specific foods hypothesis - body may be able to monitor presence of certain foods or food groups. It is unlikely that a single hypothesis will explain appetite for food by itself. More than likely your interest in food is due to a number of factors. Interesting- tend to have appetite (craving) for different things a different times. Apparently to satisfy nutritional need at that time. 6. Psychological status hypothesis - sometimes we eat out of habit or boredom. Pleasure: food preferences, eat more in cafeteria, appetizers, dessert, Pleasure Centers in Hypothalamus. Social factors: cultural demands to be thin...anxiety. 7. Neurotransmitter and hormones Research has shown that neurotransmitters (serotonin) and hormones (leptin from fat cells and PYY from intestine) are involved in a feeling of fulfillment following eating. Carbohydrates increase serotonin levels in the brain, so one may eat carbohydrates when depressed. Use of drugs such as fenfluramine (FenPhen is a combo drug.) Biological Basis: fenfluramine increases serotoin levels, while phentermine is an amphetamine mimic that promotes a sympathetic response. Both curb appetite.) Taken off the market as heart failure occurred. Research in these areas may be very important in in reducing the alarming increase in childhood obesity in the U.S.

6 Function and Control of Specific Digestive Organs (in order passed by bolus of food) I. Mouth 1. Ingestion begins 2. Digestion a. Mechanical digestion - chewing = mastication. b. Chemical digestion Saliva- produced by the salivary glands, is important as it: 1. Begins the digestion of carbohydrates. Starches are broken down to disaccharides by amylase in the saliva. Only 3-5%, since food spends such short time in mouth. 2. Dissolves soluble foods. 3. Moistens food or lubricates food for swallowing. 4. Cleanses and moistens mouth- helps to prevent tooth decay. Saliva washes away acidic waste of bacteria that promotes tooth decay. 3. Swallowing Movement of bolus of food. A voluntary act using skeletal muscles. Tongue forces food against hard palate (roof of mouth).

7 Function and Control of Specific Digestive Organs (in order passed by bolus of food) II. Pharynx The food/drink moves into the oropharynx and stimulates receptors that go to the medulla oblongata. Involuntary impulses cause the soft palate and uvula to be forced up and close off opening from nose to pharynx. The larynx moves up under the tongue and the vocal cords come together which seals off the respiratory passage. The bolus passes through the laryngopharynx and enters the esophagus in 1-2 seconds.

8 Function and Control of Specific Digestive Organs (in order passed by bolus of food) III. Esophagus 1. Transportation of foods to stomach by peristalsis - wave like contraction of smooth muscle. Peristalsis is controlled by the medulla oblongata so is involuntary response. Takes about 4-8 seconds for bolus, 1 second for liquid to reach stomach. 2. Secretion of mucus by glandular cells acts as a lubricant of the bolus. No digestion or absorption occurs in the esophagus. 3. Vomiting - reverse peristalsis. Usually stimulated by irritation and distention of the stomach. Medulla oblongata vomit center causes abdominal wall muscles and diaphragm to squeeze the stomach and expel its contents through the open esophageal sphincters. Heart burn is due to HCl from the stomach contents entering the inferior part of the esophagus causing burning and irritation. Don t lie down after a meal. Prolonged exposure can lead to esophageal cancer.

9 Function and Control of Specific Digestive Organs (in order passed by bolus of food) IV. Stomach Major functions: 1. Storage (reservoir) 2. Chyme formation = combination of saliva, gastric juice, food, and drink. 3. Protein digestion by HCl and pepsin. (some lipase activity) 4. Little absorption of food. Water, ions, some drugs (alcohol, aspirin).

10 Function and Control of Specific Digestive Organs (in order passed by bolus of food) Digestion in the Stomach The stomach contains many gastric glands that produce gastric juice. Gastric juice = watery solution of HCl and pepsin, lipase, gastrin, and intrinsic factor for B12 absorption. 3 important types of cells in the stomach, each with different functions: 1. Parietal (oxyntic) cells- secrete HCl, hydrochloric acid which: a. Kills microbes in food. b. Denatures proteins. c. Converts pepsinogen to pepsin. 2. Chief (zymogenic) cells- secrete pepsinogen (inactive form) which is converted to pepsin (active form) in the presence of HCl. Breaks certain peptide bonds in proteins. Secrete gastric lipase (breaks down triglycerides). 3. (Mucous) Neck cells- secrete mucus that forms a protective layer barrier that prevents digestion of stomach wall (and absorption of materials).

11 Function and Control of Specific Digestive Organs (in order passed by bolus of food) The Small Intestine Structures 1. 3 sections of small intestine, in order: Duodenum, Jejunum & Ileum 2. Folds in small Intestine - All these folds dramatically increase surface area for absorption. a. Plicae of small intestine b. Villi- finger like projections Villi contain blood and lymph capillaries that will carry the nutrients away. c. Microvilli - absorptive simple columnar epithelium cell surface (w/ 1700 microvilli per cell, which makes for a massive area for absorption. 3. Structures (digestive glands) associated with the small intestine. Pancreas, liver, and gallbladder All produce digestive secretions

12 Function and Control of Specific Digestive Organs (in order passed by bolus of food) The Small Intestine-Function 1. Absorption of nutrients (microvilli is absorption unit). 90% of nutrients absorbed here 2. Digestion - Many digestive enzymes are present. 3. Secretion - of intestinal juice about 1-2 L/day. Slightly alkaline (7.6) mix of water and mucus. Provides a vehicle for chyme to move through the intestine to be absorbed. 4. Protection - lymphoid tissue (Peyer s patches) especially towards the end (ileum) of the small intestine. The colon contains huge numbers of bacteria that must be kept in check.

13 Function and Control of Specific Digestive Organs (in order passed by bolus of food) The Small Intestine Function Digestion in the Small Intestine We shall take a look at digestion of the basic food groups: Carbohydrates, Proteins, & Fats We will see that the pancreas is: 1. The primary source of digestive enzymes that act in the intestine. Pancreatic juice is ducted to duodenum also exocrine gland. 2. Provides the alkalinity (bicarbonate) necessary for creating optimum ph for enzyme activity in the small intestine. Pancreas

14 Digestion of Basic Food Groups End products Note: -ase on end indicates an enzyme that breaks down chemical. Note: significance of pancreas in all food groups.

15 Bile Bile (1 L/d) is produced in liver and stored in gallbladder. Greenish to brownish consisting of water, bile acids, phospholipids, bile pigments. Gives stool & urine its color. Bile serves to emulsify fats. It breaks down lipids so they can be absorbed. Emulsifies large lipid globules into 1mm droplets. If bile backs up due to liver injury or obstruction, bilirubin (from breakdown of heme group in rbc) will back up and deposit in body producing a yellow color = jaundice.

16 Function and Control of Specific Digestive Organs (in order passed by bolus of food) The Small Intestine Function Absorption in Small Intestine Absorption of the end products of digestion takes place through epithelial lining of small intestine. Microvilli offers increased surface area. Most end products of digestion, monosaccharides, amino acids, plus water, vitamins, and ions are absorbed into blood capillaries of the villi. Most dietary fatty acids (long chain) differ in that they are absorbed by the much leakier lacteals of the lymphatic system. Note - Any disruption of this lining may lead to discomfort, diarrhea, and malabsorption. The lining of the small intestine may be affected by chemotherapy, radiation therapy, toxins, or infection. Long chain fatty acid

17 Function and Control of Specific Digestive Organs (in order passed by bolus of food) Function of the Colon (large intestine) No more digestion by you, but some by bacteria of indigestible foods. Can be beneficial (vitamins) or embarrassing (gas). Important for: 1. Water absorption. 2. Electrolyte absorption. 3. Vitamin absorption- bacterial products Vit. K, B complex (B1, B2). Vitamin K is necessary for formation of clotting proteins in the liver. If given wide spectrum antibiotics that kills off bacteria, give Vit. K injections. 4. Formation of feces (stool, solid waste (hopefully)). As feces enter and fill the rectum, stretch receptors initiate a defecation reflex that empties the rectum to the anal canal. Once the reflex is triggered, only the voluntary external sphincter is holding the feces back so voiding can be postponed until an appropriate time.

18 Functions of the Liver 1. Detoxification: detoxifies alcohol, drugs, and other substances or modifies them so they are less toxic. 2. Bile formation 3. Metabolism and storage of many nutrients, vitamins, and minerals 4. Storage reservoir of blood in veins. 5. Cleanses blood from gut of bacteria. 6. Produces blood in fetus. 7. Destroys old RBC. The heme portion is absorbed as bilirubin that is secreted in the bile.

19 Hours a b c d e c a d b e

20 Intermediary Metabolism Intermediary metabolism is all the thousands of different chemical reactions in your cells. Intermediary metabolism mainly manages carbohydrates, proteins, and fats. To burn glucose, lipids, proteins, all use same reactions: 1. Glycolysis 2. Krebs cycle- CO 2 and H 2 0 are wastes 3. Electron transport system (ETS)- needs O 2, = aerobic respiration makes most ATP If no O 2 = anaerobic respiration occurs = fermentation = lactic acid is produced

21 Intermediary Metabolism Carbohydrate metabolism Catabolism = oxidation = cellular respiration = chief source of ATP. CO 2, H 2 0 are wastes. Makes most of ATP, needs O 2 (aerobic respiration). If no O 2 = anaerobic reactions = fermentation = lactic acid. Anabolism Some glucose is converted to glycogen for storage. Can be reconverted to glucose for ATP. A.a., glycerol, lactic acid can be converted to glucose too. Triglycerides Catabolism Broken down into glycerol and fatty acids. Burned for energy in Krebs-ETS. Ketone bodies appear in large amounts during a diabetic crisis. Anabolism Synthesis of triglycerides from glucose and fatty acids = lipogenesis. Stored in adipose tissue.

22 Intermediary Metabolism Protein Catabolism Ammonia converted to urea in liver, excreted in urine. Can be converted to glucose, fatty acids, of ketone bodies. Anabolism Protein synthesis is directed by DNA and carried out by RNA and ribosomes.

23 Disorders of Digestive Tract Diarrhea - defecation of liquid stool. Caused by increased motility and decreased absorption of water by the intestines. Diarrhea can be caused by microbes (toxins or intestinal wall invasion), certain foods (some fruits increase motility) (lactose intolerance, enzyme deficiency with increased osmolarity in lumen), stress (anxiety increases parasympathetic activity to lower bowel with increased motility). Dehydration and electrolyte imbalance may occur if prolonged. Constipation - infrequent of difficult defecation due to lack of motility. Feces remain in colon too long with excessive water absorption so feces become dry and hard. Causes include lack of fiber in diet, improper bowel habits (failure to heed the call), lack of exercise, emotional upset, lack of adequate fluid intake.

24 Disorders of Digestive Tract Ulcers PUD = peptic ulcer disease = a crater like lesion in the gastro/intestinal membrane exposed to acidic gastric juice. Usually occurs in pyloric region (10%) or first part of duodenum (90%). Serious bleeding can occur (GI bleed) with anemia. 3 causes: a. Bacteria = Helicobacter pylori. b. nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin. c. hypersecretion of HCl, e.g. a gastrin producing tumor. Food Poisoning Microbes can cause irritation of the intestines by direct invasion of the intestinal wall or by secreting toxins. Microbes find their way into foods where they multiply. Common causes are Salmonella, Shigella, Campylobacter, Staphylococcus toxin.

25 Disorders of Digestive Tract Botulism Ingested toxin poisoning that is produced by Clostridium botulinum. Found in improperly cooked non acidic foods. The toxin inhibits ACh release so skeletal muscle paralysis results: blurred vision, difficult breathing, swallowing, speech, general weakness.

26 Assignment 8 1. What part of the brain controls appetite? 2. Summarize each of the 6 hypotheses covered in lecture that serve to attempt to explain how appetite may be controlled. 3. Based on your own eating behavior, state which 2 hypotheses you can specifically relate to and why.