chapter 14. organs of the alimentary canal alimentary canal a.k.a. gastrointestinal tract mouth...

Chapter 14

Organs of the Alimentary canal Alimentary canal a.k.a. Gastrointestinal tract Mouth Pharynx Esophagus Stomach Small intestine Large intestine Rectum All food passing through the AI canal is technically outside the body

Tunics of the AI Canal (Esophagus to Lg. Intestines) 1. Mucosa innermost layer that lines the lumen. Mostly surface epithelium and small amounts of CT (lamina propria) and a scanty smooth muscle layer The esophagus epithelium is stratified squamous. The rest of the AI canal is mostly simple columnar 2. Submucosa just below the mucosa. Soft CT layer with blood vessels, nerve endings, GALT (gut-associated lymphoid tissue), and lymphatic vessels 3. Muscularis externa dual smooth muscle layers. The inner circular layer and outer longitudinal layer 4. Serosa outermost layer of the wall. Single layer of serous fluid-producing visceral peritoneum which is continuous with the parietal peritoneum and mesentery

Innervation of the AI Canal The canal wall has two intrinsic nerve plexuses Submucosal nerve plexus Myenteric (intestinal muscle) nerve plexus These are part of the autonomic nervous system Help regulate mobility and secretory activity of the AI canal organs

Sphincters in the AI Canal Sphincters are circular arrangements of muscle fascicles and are found in four places in the GI tract 1. Cardioesophageal sphincter (aka: Cardiac sphincter or esophageal sphincter) found at the entrance to the stomach 2. Pyloric sphincter (aka: pyloric valve) found at the entrance to the small intestines 3. Ileocecal valve found at the entrance to the large intestines 4. Internal and External anal sphincters found in the anus These are controlled by a variety of neural, chemical and mechanical mechanisms to ensure that food is allowed proper time to be digested and/or absorbed in different locations of the GI tract

Sphincters of the AI Canal

Functions of the Digestive System 1. Ingestion active and voluntary process of putting food into the mouth 2. Propulsion- moving food from one digestive organ to the next Ex: Peristalsis in the esophagus and segmentation in the small intestines (which is used more for mixing food with digestive enzymes) 3. Food breakdown: Mechanical digestion 4. Food breakdown: Chemical digestion 5. Absorption 6. Defecation

Food breakdown: Chemical digestion The process of breaking organic food molecules down into building blocks Carbs monosaccharides Proteins amino acids Lipids glycerols and fatty acids All hydrolysis reactions because water is added to each bond to be broken Water is also necessary as a solvent and softening agent for food Each food molecule is broken down by a different enzyme There are many molecules in food that we do not have enzymes for, thus these food pass out of the body either undigested or partially digested by gut bacteria

The Mouth Mechanical digestion begins: mastication of food with teeth. Chemical digestion follows: saliva produced by the salivary glands secretes enzymes in an alkaline solution along with antibodies (IgA) and lysozymes Main enzyme: amylase breaks starch into disaccharides and oligosaccharides Saliva is produced in copious amounts due to presence of food and stimulation of mechanoreceptors Thoughts, smells and sight of food can also stimulate production via cranial nerves VII and IX Sensory papillae on taste buds send taste info to the brain

Food Propulsion Mouth to Stomach Deglutition (a.k.a. swallowing) is a two phase event 1. Buccal phase (voluntary) food is chewed, mixed with saliva, and forms a bolus (food mass). The tongue forces the bolus into the pharynx 2. Pharyngeal-esophageal phase (reflex) food moves down the esophagus. Mostly controlled by vagus nerve activity. All exits are blocked off and food is propelled down the esophagus via peristaltic contractions food pressing on the cardioesophageal sphincter causes it to open and allows food into the stomach The sphincter closes behind to food

The Stomach Three parts: The fundus, body, and pyloric antrum Has large folds called rugae that allow the stomach to stretch and hold up to ~4L of food The muscularis externa has a third oblique (inner) layer of muscle which helps mechanical digestion and mixing Once food is thoroughly mixed with gastric juice, it is known as chyme

The Stomach The lining has gastric glands that produce various secretions (collectively: gastric juice) by different cells: Mucous neck cells (top) make acidic mucous Parietal cells (middle) secrete HCl Chief cells (base) produce pepsinogen (activated by HCl) Enteroendocrine cells (scattered) produce local hormones, ex: Gastrin Intrinsic factor also produced by gastric glands (needed for B 12 absorption) Rennin, also a protease, works mostly on milk proteins in infants, may not be produced in adults

More on the Stomach Before swallowing, cephalic input causes the stomach to produce copious gastric juice Food in the stomach, and a rising pH stimulates release of more gastrin Gastrin in turn causes an increase in pepsinogen, mucus, and HCl production Different nutrients in food can change the rate of gastric emptying. A balanced meal takes 2-4 hrs to pass through, high fat meal may take up to 6: Water, acidic and salty foods increase gastric emptying Simple and some complex carbs increase the rate of gastric emptying Proteins empty less quickly than carbs Fats slow gastric emptying and take the longest to leave the stomach No food is absorbed in the stomach Aspirin, caffeine and alcohol are*

Homeostatic Imbalance Heartburn typically occurs when the cardioesophageal sphincter fails to close completely and inflammation of the esophagus results Sometimes due to Hiatal hernia. This occurs when the fundus protrudes above the diaphragm The sphincter is typically weak and this condition prevents it from working properly Ulcers often result from infection with H. pylori bacteria Emesis vomiting. Occurs when the emetic center of the brain in the medulla is stimulated. Retropulsion of food coupled with a strong abdominal muscle and diaphragm contraction force food back through the sphincter Triggered by many factors

From the stomach to the intestines Once chyme is mixed thoroughly, the stomach begins peristalsis to push food towards the pyloric antrum (pylorus) which holds ~30 mL chyme Each wave of contraction pushes a small squirt of chyme into the intestines, ~3 mL The rest is pushed back into the body for more mixing (retropulsion) As the duodenum of the small intestines is stretched and the acidic food enters, the enterogastric reflex occurs, putting the breaks on gastric emptying Inhibits vagus nerve

The Small Intestines 2-4 m longs, hangs in sausage-like coils, suspended from the posterior abdominal wall by the fan-shaped mesentary Three subdivision: Duodenum (literally twelve fingers widths long) 5% of length Jejunum (empty) 40% of length Ileum (twisted intestine) 55% of length Main site of chemical digestion and absorption Can only handle a small amount of chyme at a time Flow into the intestine is controlled by the pyloric sphincter Up to this point, only carbs and some proteins have been partially digestion. No fats have been digested yet.

Chemical Digestion in the Duodenum Intestine fluid is relatively enzyme poor, though brush border enzymes in the microvilli have important roles ie: break down disaccharides into monosaccharides Intestinal juice itself is enzyme-poor but rich with protective mucus Most digestive enzymes come from the pancreas in enzyme-rich pancreatic juice Some enzymes are made by the intestinal cells, others by the pancreas and liver The acidity of chyme, molecules in the chyme (ie: fat), and stretch receptors trigger the release of hormones from intestinal cells: Secretin causes the pancreas to release pancreatic juice in conjunction with the vagus nerve Cholecyctokinin (CCK) triggers release of bile from gall bladder Gastric inhibitory peptide (GIP) inhibits gastric juice secretion and stimulates insulin secretion

Duodenum The pancreas releases a mixture of enzymes in alkaline solution (pancreatic juice) straight into the duodenum via the pancreatic ducts Bile, formed by the liver and stored in the gall bladder is released into the intestine in the same location. Both accessory organs share the common bile duct, a.k.a.: hepatopancreatic ampulla From there, both bile and pancreatic juice enter the duodenum through the duodenal papilla

Pancreatic Juice Carb digestion enzymes: Pancreatic amylase: Along with the brush border enzymes, this completes digestion of starch Protein digestion enzymes: Trypsin, chymotrypsin, carboxypeptidase and others Lipid digestion enzymes: Lipases (different types) Nucleic acid digestion enzymes: Nucleases Lots and lots of bicarbonate (alkaline buffer) which makes the juice have a pH ~8 Used to neutralize stomach acid and prevent damage to intestinal walls

Small Intestine propulsion and digestion continued Water and nutrients are absorbed along the length of the intestine Most nutrients are absorbed via active transport From the blood, nutrients are shunted to the liver via hepatic portal vein Fats and lipids diffuse into mucosa cells, then form micelles which are taken up by lymph vessels that subsequently dump the lipids into the blood in the jugular vein Peristalsis propels chyme forward, segmentation mixes the chyme further with digestive secretions

Small Intestine Microstructure Three features increase absorptive surface area: 1.Microvilli tiny projections on the plasma membrane of mucosa cells. Also collectively called the brush border 2.Villi finger-like projections of the mucosa that give a velvety appearance and feel Inside each villi is a capillary bed and a lacteal (modified lymph capillary) 3.Plicae circularis deep circular folds of the mucosa and submucosa. These do not disappear when the intestines are stretched with chyme (unlike rugae in the stomach)

Small Intestines Continued Peyers patches lymphatic tissue found in the submucosa toward the end of the small intestine. Since undigested food reaching the large intestine is riddled with bacteria, these lymph patches aide in prevention of bacteria reaching the blood stream

Large Intestine About 1.5 m long, extends from ileocecal valve to anus Food spends 12-24 hrs in here Major function: Remove excess water from indigestible food No digestion enzymes or villi Parts: cecum, appendix, colon, rectum, anal canal Colon: Ascending up on right Transverse Descending down on left Sigmoid s-shaped curve Anus: guarded by purse strings voluntary external sphincter Involuntary internal sphincter

Large Intestine Fauna The large intestine is a complex ecosystem Many bacteria here aide in digestion of some remaining nutrients The gases released by bacterial metabolism (methane and hydrogen sulfide) contribute to feces odor and flatus (500 mL flatus produced/daymore when certain carb-rich foods are consumed) Bacteria also produce some vitamins. Ex: Vit. K, and B vitamins Absorption in the large intestine is limited to most of the remaining water, these vitamins, and some ions Feces = the undigested food residues, mucus, bacteria and some water

Large Intestine Continued The longitudinal layer of the muscularis externa is a three-band cord of muscle called the teniae coli Usually have tone and cause intestine to pucker into saclike pockets called haustra The contractions are called haustral contractions Slow segmenting movements that last ~1 min, every 30 min Mass movements are long, slow, and powerful These flatten large sections of colon and force contents toward the rectum 3 4x daily, usually after a meal Fiber increases the strength of the cotnractions and softens stool

Defecation The rectum is usually empty until a mass movement forces feces into it and the walls are stretched Mechanoreceptors in the rectum walls initial the defecation reflex which is controlled by the sacral region of the spine The walls of the sigmoid colon and the rectum contract and the anal sphincters relax When the feces is forced into the rectum, the brain has enough time to process the input and a conscious decision is made to whether the external voluntary anal sphincter should remain open or be constricted Defecation can be temporarily delayed to a more convenient time If delayed, the reflex contractions end within seconds and the rectal walls relax until the next mass movement

Homeostatic Imbalances Gallstones crystallization of cholesterol in gall baldder Jaundice failure of bile to enter the small intestines, too much bile in the blood Hepatitis and cirrhosis Pancreatitis Vitamin K deficiency from lack of bile or pancreatic juice secretion Diverticulitis Diarrhea and constipation

Nutrition and Digestion Major nutrients: carbs, lipids, proteins, water Minor nutrients: vitamins, minerals All used for growth, maintenance, and repair Some foods are nutrient rich, or dense. Others are nutrient poor Typical Absorption per day: 100 g Carbs, 60+ g fat, 50-100 g a.a.s, 3-5 g mins/vits, 7-8 L water Capability for absorption per day: 1000s g Carbs, 500 g fat, 500-700 g a.a., 20+ L water Mypyramid.gov 2005 US Govt came up with the new food pyramid to display the proper amount of nutrients one should consume based on physical activity, age, and sex. The website also gives customizable tools for determining your personal nutrient needs

Carbohydrates Sugars, fiber and starches All derived from plants except milk sugars Simple sugars (glucose, fructose, galactose) give a sweet taste Starches (amylose, amylopectin, glycogen) do not Some starches can be broken down completely or partially by our enzymes, others cannot (ex: amylopectin) and are called resistant starch These are broken down by bacteria in the gut Fibers are nondigestible carbs that resemble starches (ex: cellulose, pectins, gums, chitin, etc). These are NOT found in animal foods Provide bulk and soften stool for easier passage Fiber in the diet is essential for maintaining motility and intestinal health Dietary Sources of Carbs: fruits, veggies, grain products, milk products

Carbohydrate Metabolism Glucose (aka: Blood sugar) is the main source of energy for ATP generation Galactose is converted to glucose before metabolism in the traditional energy pathway (oxidative cellular respiration) Glycolysis (cytosol) Krebs Cycle (TCA/Citric acid cycle) (mitochondria) electron transport chain (inner membrane mitochondria) ATP produced by F 0 F 1 enzyme C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + 36 ATP Fructose is shunted into the pentose pathway for energy generation before ultimately being broken down for ATP generation Brain tissue and RBCs can ONLY use glucose as fuel where as most other tissue may use either glucose or fats Skeletal muscle tissue prefers fat as fuel during oxidative respiration Glucose metabolism is much faster than fat metabolism Homeo Imbalance: Hyperglycemia and hypoglycemia

Lipids Dietary lipids are mostly Triglycerides We can produce enough of our own cholesterol and phospholipids Come in saturated, monounsaturated, polyunsaturated, and trans-fatty acid form Some are short-chain (

Lipid Metabolism Lipids must be broken down into two-carbon units called acetic acid Acetic acid enters the TCA cycle for oxidation The process is fast but incomplete and side-products; acetoacetic acid and acetone, build up and form ketones which change blood pH to acidic May result in acidosis, or ketoacidosis. Symptoms include fruity, wine-smelling breath Often occurs from no-carb diets (

Proteins Complete proteins provide all essential amino acids (the ones we cant make in our bodies) Essential a.a.s: Tryptophan, Methionine, Valine, Threonine, Phenylalanine, Leucine, Isoleucine, Lysine Aminal sources are best, legumes, nuts, cereals also have protein but are often incomplete Vegetarians must be careful to get amino acid nutrition since non- meat products typically a incomplete a.a profiles

Protein Metabolism After the liver has taken the amino acids it needs, the rest are sent via blood to other tissues Some are used within the cells for protein production (enzymes etc) others are used for export products (hormones, mucus etc) Amino acids are actively transported to ensure adequate supply of all 20+ types May be used to make ATP if they are overabundant or carbs/fat are not available The amino group is removed and forms ammonia while the rest of the molecule enters the TCA cycle Ammonia (toxic) is sent to the liver to join with CO 2 to form urea the major protein component of urine

Liver and Metabolism Functions: Bile production, maintain blood glucose levels, store glycogen, detoxify ammonia, drugs and alcohol, degrade hormones, make vital substances available (cholesterol, blood proteins, clotting proteins, lipoproteins etc), synthesis of non-essential amino acids Second largest organ by mass, we have much more tissue than we need and if damage, it can regenerate easily All nutrients are shunted to the liver via hepatic portal vein before they are circulated to the rest of the tissues

Liver and Glucose Excess blood sugar is stored as glycogen starch in the liver Process is called glycogenesis and is triggered by insulin among other hormones and molecules levels When blood glucose drops, the liver breaks down glycogen during glycogenolysis Can also make new glucose from certain amino acids. Gluconeogenesis is controlled by many hormones such as thyroxine, insulin, glucagon, and cortisol

Cholesterol and Transport Cholesterol is the building blood for many hormones, such as vitamin D, progesterones and androgens Also a major component of cell membranes and prevents membranes from becoming stiff We can make all the cholesterol we need and ~15% comes from our diet Cholesterol is broken down into bile salts, which eventually leave the body in feces Cholesterol must be transported by lipoproteins to and from tissues since it is hydrophobic LDL (bad cholesterol) carries cholesterol to tissues HDL (good cholesterol) carries it to the liver for processing into bile, then out (sort of) Both are necessary and good, but the ratio between them is what may lead to atherosclerotic plaque formation

Body Energy Balance Energy in (calories) = energy out (work, storage, heat) When balanced, body weight remains stable, though body composition may not Carbs: 4 C/g, Protein 4 C/g, Lipids 9 C/g, Alcohol 7 C/g Hunger and satiety is controlled by a variety of not-well- understood factors: Neural input from gut, chemicals in blood, neurotransmitters in the brain, body temperature, and psychological factors Brain receptors associated with hunger include thermoreceptors, chemoreceptors, and receptors for leptin and other peptides

Metabolic Rate Basal Metabolic Rate: the amount of heat produced by the body per unit of time when it is under basal condition (resting) Avg 70 kg male has BMR of ~60 to 72 C/hour Influenced by age, sex, surface area, health status, activity level and other factors including emotional status Thyroxine = major determinant of BMR Hyperthyroidism and hypothyroidism both result in homeostatic imbalance Total Metabolic Rate: the amount of Calories the body must consume to fuel all ongoing activities Muscular work is the major determinant of TMR. Slight increases in activity may cause huge increases in TMR High-level athletes may exercise vigorously for a few minutes and elevate their TMR 15-20 beyond normal. The TMR will remain elevated for several hours afterwards (Build intensity, burn more fuel)

chapter 14. organs of the alimentary canal alimentary canal a.k.a. gastrointestinal tract mouth...

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ai canal slide

food breakdown

mastication of food

mixing food

ai canal esophagus

ai canal sphincters

ai canal organs

canal wall