DIGESTIVE SYSTEM - The tube running from mouth to anus

DIGESTIVE SYSTEM DIGESTIVE SYSTEM A Big Mac is composed of: "Two all beef patties (largely proteins and triglyceride, or neutral fat), special sauce (??), lettuce (cellulose, which you cannot assimilate to any great extent), cheese (again, protein and triglyceride), pickles (cellulose, starch), onions (cellulose, starch) on a sesame seed bun (starch)." • "Exactly what nutients are present in a Big Mac that I can assimilate?" DIGESTIVE SYSTEM • In terms of raw materials, a 219 gram Big Mac consists of roughly: • 25 grams of protein • 30 grams of fat • 47 grams of carbohydrate • Add these all up and you get a total of 560 Calories. DIGESTIVE SYSTEM • tube running from mouth to anus. – Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system DIGESTIVE SYSTEM • The three fundamental processes that take place are: – Secretion: Delivery of enzymes, mucus, ions and the like into the lumen, and hormones into blood. – Absorption: Transport of water, ions and nutrients from the lumen, across the epithelium and into blood. – Motility: Contractions of smooth muscle in the wall of the tube that crush, mix and propel its contents. DIGESTIVE SYSTEM • Mouth: Foodstuffs are broken down mechanically by chewing and saliva is added as a lubricant. • Esophagus: A simple conduit between the mouth and stomach • Stomach: Where the real action begins enzymatic digestion of proteins initiated and foodstuffs reduced to liquid form. DIGESTIVE SYSTEM • Liver: The center of metabolic activity in the body - its major role in the digestive process is to provide bile salts to the small intestine, which are critical for digestion and absorption of fats. • Pancreas: Important roles as both an endocrine and exocrine organ - provides a potent mixture of digestive enzymes to the small intestine which are critical for digestion of fats, carbohydrates and protein. DISGESTIVE SYSTEM • Small Intestine: The most exciting place to be in the entire digestive system - this is where the final stages of chemical enzymatic digestion occur and where almost all nutrients are absorbed. • Large Intestine: water is absorbed, bacterial fermentation takes place and feces are formed. DIGESTIVE SYSTEM • the wall of the digestive tube from the mouth to the anus is composed of four basic layers or tunics. DIGESTIVE SYSTEM • Tunica serosa is the outermost covering of the digestive tube. within the peritoneal cavity, this structure is also referred to as visceral peritoneum. • In the abdominal cavity, the serosa on each side of the tube fuses together to form a suspensory structure called mesentery, which houses vascular and nervous supplies to the digestive tract and is continuous with the lining of the cavity. DIGESTIVE SYSTEM • Tunica muscularis --this tunic consists of two thick layers of smooth muscle. Muscle fibers in the inner layer are aligned circularly, whereas those in the outer layer have a longitudinal orientation. – This combination of circular and longitudinal smooth muscle gives the tube an ability to perform complex movements that squeeze DIGESTIVE SYSTEM • Tunica submucosa lies immediately beneath the mucosa, and is a layer of loose to dense connective tissue containing blood and lymphatic vessels. DIGESTIVE SYSTEM • Tunica mucosa is the innermost layer of the digestive tube and lines the lumen. – several different cell types contribute to the epithelium, including cells dedicated to secretion, absorption or production of hormones. DIGESTIVE SYSTEM • Microbial Life in the Digestive Tract – The gastrointestinal tract contains an immensely complex ecology of microorganisms. A typical person harbors more than 500 distinct species of bacteria, representing dozens of different lifestyles and capabilities. The composition and distribution of this menagerie varies with age, state of health and diet. Microbial Populations in the Digestive Tract of Normal Humans STOMACH JEJUNUM ILEUM COLON VIABLE BACTERIA PER GRAM 0-103 0-104 105-108 1010-1012 pH 3.0 6.0-7.0 7.5 6.8-7.3 CONTROL OF FOOD INTAKE • Why is it important to understand the factors that control food intake? – Obesity is the most prevalent nutritional disease of humans in affluent societies such as ours, exceeding by far the number of nutritional deficiency diseases. – Metabolic demands of people increase with sickness or trauma, often in conjunction with anorexia. Sickness combined with anorexia leads to accelerated starvation. CONTOL OF FOOD INTAKE • FOUR MAIN FACTORS – Role of the central nervous system – Pregastric factors – Gastrointestinal and postabsorptive factors – Long-term controls ROLE OF CNS • Two regions in the hypothalamus that dramatically influence feeding behavior: – Lateral hypothalamus (hunger center): animals with lesions in this area become anorectic and lose weight. – Ventromedial hypothalamus (satiety center): animals with lesions in this area overeat and become obese. PREGASTRIC FACTORS • FACTORS PRIOR TO EATING – Appearance of food: humans like or dislike certain meals based on visual appearance – Taste and/or odor of food: this is extremely important – Learned preferences and aversions: Almost everyone has an aversion to one or more types of foods – Psychologic factors: mental states such as fear, depression and social interactions often affect food intake. • Never eat a live mole! This seagull did, the mole tried to tunnel out and they both died. Gastrointestinal and Postabsorptive Factors • The degree of gastrointestinal fill is the most important signal from the digestive tract per se a full stomach and intestine induce satiety, probably via the vagus nerve relaying that fact back to the hypothalamus. • As nutrients such as glucose and amino acids are absorbed, their concentrations in blood rise, these changes also have been linked to the sensation of hunger or satiety. Long-term Control of Food Intake • Adult animals (including humans)tend to maintain a relatively constant weight known as their "set weight". DIGESTIVE SYSTEM • Three major pairs of salivary glands that differ in the type of secretion they produce: – parotid glands produce a serous, watery secretion – submaxillary (mandibular) glands produce a mixed serous and mucous secretion – sublingual glands secrete a saliva that is predominantly mucous in character DIGESTIVE SYSTEM • Saliva – is produced in and secreted from salivary glands. – The basic secretory units of salivary glands are clusters of cells called an acini. • These cells secrete a fluid that contains water, electrolytes, mucus and enzymes, all of which flow out of the acinus into collecting ducts. What then are the important functions of saliva? • Lubrication and binding: the mucus in saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. • SOLUBILIZES dry food: in order to be tasted, the molecules in food must be solubilized. What then are the important functions of saliva? • Oral hygiene: The oral cavity is almost constantly flushed with saliva, which floats away food debris and keeps the mouth relatively clean. Flow of saliva diminishes considerably during sleep, allow populations of bacteria to build up in the mouth -- the result is dragon breath in the morning. Saliva also contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations. What then are the important functions of saliva? • Initiates starch digestion: begin to digest dietary starch into maltose. • Provides alkaline buffering and fluid: neutralizes acids to prevent tissue damage DIGESTIVE SYSTEM • Mastication, or chewing, is the first step in the breakdown of complex foodstuffs and serves several functions, including: – breaking large pieces into small pieces, resulting in a massive increase in surface area, which is where digestive enzymes work – softening of food and transformation into a size conducive to swallowing – lubrication of food by impregnating it with saliva SWALLOWING • The final step in pregastric digestion is swallowing, also known as deglutition. – This is really a very complex process that can be thought of as occurring in three steps: • First, a bolus of food is pressed backward into the pharynx by the tongue. This is the only step that is voluntary - the remaining steps occur by reflex. SWALLOWING • Once the bolus reaches the pharynx several actions are initiated, which basically involve shunting the bolus into the esophagus • and the epiglottis swings backward to cover the larynx. • http://video.google.com/videoplay?docid=2903453730937795217&q=swallowing+ob ject&hl=en SWALLOWING • Finally, the tongue presses backward and a peristaltic contraction in the pharynx propels the bolus into the esophagus, where the actual act of swallowing takes place. DENTAL ANATOMY OF HUMANS • HUMANS have teeth of different sizes and shapes, a condition known as heterodonty, allowing different teeth to be specialized for different tasks. These specialized teeth include: – – – – Incisors (I) Canine teeth (C) Premolars (P) Molars (M) • HUAMNS also have two sets of teeth: a deciduous set (baby teeth) and a permanent set. DENTAL FORMULA • Dental formula are used to indicate the number of each type of tooth. Because the jaw is bilaterally symetrical, only one half of the jaw is described. – The incisors are indicated first, followed by the canine, promolars and molars. The maxillary arcade or upper jaw is listed over the mandibular arcade or lower jaw. DENTAL FORMULA • UPPER • LOWER ICMP ICMP 3142 3143 TOOTH ANATOMY • The crown is encased in enamel and the root in cementum. – Enamel is the hardest substance in the body – Cementum is calcified connective tissue. – Dentin, a bonelike material, is under the enamel and makes up most of the tooth. – The pulp cavity includes blood vessels, lymphatics and nerves. DIGESTIVE SYSTEM • ESOPHAGUS – least complex section of the digestive tube. – FUNCTION: to convey boluses of food from the pharynx to the stomach. – The esophagus begins as an extension of the pharynx in the back of the oral cavity. It then courses down the neck next to the trachea, through the thoracic cavity, and penetrates the diaphragm to connect with the stomach in the abdominal cavity. DIGESTIVE SYSTEM • STOMACH – FUNCTIONS: – short-term storage reservoir – substantial enzymatic digestion is initiated, particularly of proteins. – Vigorous contractions of gastric smooth muscle mix and grind foodstuffs with gastric secretions, resulting in liquefaction of food – Food is slowly released into the small intestine for further processing. DIGESTIVE SYSTEM • STOMACH – EXTERNAL FEATURES – The right side of the stomach shown above is called the greater curvature and that on the left the lesser curvature. – The most distal and narrow section of the stomach is termed the pylorus - as food is liquefied in the stomach it passes through the pyloric canal into the small intestine. DIGESTIVE SYSTEM • In the empty state, the stomach is contracted and its mucosa and submucosa are thrown up into distinct folds called rugae; when distended with food, the rugae are "ironed out" and flat. DIGESTIVE SYSTEM • Four major types of secretory epithelial cells – Mucous cells: secrete an alkaline mucus that protects the epithelium against shear stress and acid – Parietal cells: secrete hydrochloric acid – Chief cells: secrete pepsin, a proteolytic enzyme – G cells: secrete the hormone gastrin STOMACH • Contractions of gastric smooth muscle serves two basic functions: – ingested food is crushed, ground and mixed, liquefying it to form what is called chyme. – chyme is forced through the pyloric canal into the small intestine, a process called gastric emptying. STOMACH • The upper stomach, composed of the fundus and upper body, shows sustained contractions that are responsible for generating a basal pressure within the stomach. • The lower stomach, composed of the lower body and antrum, develops strong peristaltic waves of contraction that increase in amplitude as they propagate toward the pylorus. – very effective gastric grinder; – occur about 3 times/minute in people "What happens to solids that are indigestible - for example, a rock or a penny? Will it remain forever in the stomach?" • Usually vomited out if too big • Pass into small intestine at later time when food is not present in stomach Dog with swallowed serrated knife Python swallowed electric blanket STOMACH SECRETIONS • four major secretory products of the gastric epithelium, all of which are important either to the digestive process or to control of gastric function: • Mucus: lubricates the gastric surface, and serves an important role in protecting the epithelium from acid and other chemical insults. STOMACH SECRETION • Acid: Hydrochloric acid is secreted from parietal cells into the lumen where it establishes an extremely acidic environment. This acid is important for activation of pepsinogen and inactivation of ingested microorganisms such as bacteria. STOMACH SECRETIONS • Proteases: Pepsinogen is secreted into gastric juice from both mucous cells and chief cells. Once secreted, pepsinogen is activated by stomach acid into the active protease pepsin, which is largely responsible for the stomach's ability to initiate digestion of proteins. • Hormones: The principal hormone secreted from the gastric epithelium is gastrin, a peptide that is important in control of acid secretion and gastric motility. VOMITING • The Act of Vomiting • THREE STAGES: • Nausea is an unpleasant and difficult to describe psychic experience in humans. Physiologically, nausea is typically associated with decreased gastric motility and increased tone in the small intestine. Additionally, there is often reverse peristalsis in the proximal small intestine. VOMITING • Retching ("dry heaves") refers to spasmodic respiratory movements conducted with a closed glottis. While this is occurring, the antrum of the stomach contracts and the fundus and cardia relax. VOMITING • Emesis or vomition is when gastric and often small intestinal contents are propelled up to and out of the mouth. – A deep breath is taken, the glottis is closed and the larynx is raised to open the upper esophageal sphincter. Also, the soft palate is elevated to close off the posterior nares. – The diaphragm is contracted sharply downward to create negative pressure in the thorax, which facilitates opening of the esophagus and distal esophageal sphincter. – Simultaneously with downward movement of the diaphragm, the muscles of the abdominal walls are vigorously contracted, squeezing the stomach and thus elevating intragastric pressure. With the pylorus closed and the esophagus relatively open, the route of exit is clear. TRIGGERS OF VOMITING • VAGUS NERVE – Visceral afferents from the gastrointestinal tract (vagus or sympathetic nerves) - these signals inform the brain of such conditions as gastrointestinal distention (a very potent stimulus for vomition) and mucosal irritation. TRIGGERS • SIGNALS FROM OTHER ORGANS – Visceral afferents from outside the gastrointestinal tract - this includes signals from bile ducts, peritoneum, heart and a variety of other organs. These inputs to the vomition center help explain how, for example, a stone in the common bile duct can result in vomiting. TRIGGERS • BRAIN CUES – Afferents from extramedullary (NOT IN MEDULLA) centers in the brain - it is clear that certain psychic stimuli (odors, fear), vestibular disturbances (motion sickness) and cerebral trauma can result in vomition. FOOD OR LIQUID – HOW FAST? • A large glass of water: The stomach becomes distended, but there are no solids to grind and liquefy, and after the water reaches the small intestine, no further processing is required before absorption - the rate of gastric emptying should be very fast. • A double cheeseburger with fries The stomach is distended and its contents must be liquefied; you would also want the meal to be retained in the stomach long enough for pepsin and acid to get a good shot at digesting the protein. Additionally, the resulting chyme should be allowed to empty in the small intestine slowly so as to not overload that organ, particularly with regard to digestion of fat - the rate of gastric emptying should be slow. • A single chicken nugget ;The stomach will not be distended after this kind of a "meal" and in the absense of distension, there is relatively little stimulus for gastric motility - the rate of gastric emptying should be slow. STOMACH EMPTYING • typical solid meal, there is a lag time of 20 to 30 minutes in which there is minimal gastric emptying. • liquids are generally transported out of the stomach at an exponential rate. – principal determinant of rate of gastric emptying is volume – there is an exponential relationship between volume and rate of emptying - large volumes empty at an exponentially faster rate than small volumes. SOOOO…. HOW LONG? • How long does it take a meal to pass through the digestive tract? SMALL INTESTINE • longest section of the digestive tube; suspended by mesentery membrane • THREE SEGMENTS – Duodenum: a short section that receives secretions from the pancreas and liver via the pancreatic and common bile ducts. – Jejunum: considered to be roughly 40% of the small gut – Ileum empties into the large intestine; considered to be about 60% of the intestine in man Small intestine • its inner surface – absorptive surface area of the small intestine is roughly 250 square meters - the size of a tennis court! • Mucosal folds: circular folds, which not only increase surface area, but aid in mixing the ingesta by acting as baffles. • Villi: the mucosa forms multitudes of projections which protrude into the lumen and are covered with epithelial cells. • Microvilli: the lumenal plasma membrane of absorptive epithelial cells is studded with densely-packed microvilli. • The mucosa of small intestinal mucosa is arranged into two fundamental structures: – Villi are projections into the lumen covered predominantly with mature, absorptive cells, along with occasional mucus-secreting goblet cells. • Goblet cells -- live only for a few days, die and are shed into the lumen to become part of the ingesta to be digested and absorbed. – Crypts (of Lieberkuhn) are moat-like invaginations of the epithelium around the villi, and are lined largely with younger epithelial cells which are involved primarily in secretion. Toward the base of the crypts are stem cells, which continually divide and provide the source of all the epithelial cells in the crypts and on the villi. Small intestine • Following a meal, when the lumen of the small intestine contains chyme, two types of motility predominate: – segmentation contractions chop, mix and roll the chyme – peristalsis slowly propels it toward the large intestine. LARGE INTESTINE • The large intestine is the last attraction in digestive tube and the location of the terminal phases of digestion. – THREE PROCESSES • RECOVERY OF WATER • FORMATION AND STORAGE OF FECES • MICROBIAL FERMENTATION • Recovery of water and electrolytes from ingesta: By the time ingesta reaches the terminal ileum, roughly 90% of its water has been absorbed, but considerable water and electrolytes like sodium and chloride remain and must be recovered by absorption in the large gut. • Formation and storage of feces: As ingesta is moved through the large intestine, it is dehydrated, mixed with bacteria and mucus, and formed into feces. • Microbial fermentation: The large intestine of all species teems with microbial life. Those microbes produce enzymes capable of digesting many of molecules that to vertebrates are indigestible, cellulose being a premier example. LARGE INTESTINE • The large intestine is that part of the digestive tube between the terminal ileum and anus. – three major segments: • vermiform appendix is a blind-ended pouch that in humans carries a worm-like extension • colon constitutes the majority of the length of the large intestine and is subclassified into ascending, transverse and descending segments. • rectum is the short, terminal segment of the digestive tube, continuous with the anal canal. • COLONOSCOPY LIVER • The liver is the largest gland in the body • FUNCTIONS – Vascular functions, including formation of lymph and the hepatic phagocytic system. – Metabolic achievements in control of synthesis and utilization of carbohydrates, lipids and proteins. – Secretory and excretory functions, particularly with respect to the synthesis of secretion of bile. LIVER FUNCTION • Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids, cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine. There are two fundamentally important functions of bile in all species: • Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine. • The gall bladder stores and concentrates bile during the fasting state. Typically, bile is concentrated five-fold in the gall bladder by absorption of water and small electrolytes - virtually all of the organic molecules are retained. • Secretion into bile is a major route for eliminating cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids and lipids like lethicin. Gallstones, most of which are composed predominantly of cholesterol, result from processes that allow cholesterol to precipitate from solution in bile. GALLSTONES PANCREAS • As chyme floods into the small intestine from the stomach, two things must happen: – acid must be quickly and efficiently neutralized to prevent damage to the duodenal mucosa – macromolecular nutrients - proteins, fats and starch - must be broken down much further before they can be absorbed through the mucosa into blood PANCREAS • Pancreatic juice is composed of two secretory products critical to proper digestion: digestive enzymes and bicarbonate. – ENZYMES DIGEST – BICARBONATE NEURTALIZES STOMACH ACID PANCREAS • DIGESTIVE ENZYMES – PROTEASES – BREAK DOWN PROTEIN – PANCREATIC LIPASE – BREAK DOWN LIPIDS – AMYLASE – BREAK DOWN STARCH PANCREAS • In addition to its role as an exocrine organ, the pancreas is also an endocrine organ and the major hormones it secretes insulin and glucagon - play a vital role in carbohydrate and lipid metabolism. They are, for example, absolutely necessary for maintaining normal blood concentrations of glucose. • http://www.vivo.colostate.edu/hbooks/path phys/digestion/pregastric/esophagus.html