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Objectives: Digestive System

Dr York GCC Fall 2012
Know the digestive system anatomy; be able to trace the path of food from the time it enters your mouth until is exits the anus
It first starts at the mouth, where the salivary glands will work on the food and then break down the carbohydrates and send and the tongue will send it down to the esophagus. The food will go down the esophagus and on the way down, the trachea is protected by the epiglottis. After it goes through the esophagus, it goes through the pyloric sphincter into the stomach, and then it goes to the duodenum, and then goes into the small intestine. This is where some of the absorption happens. At this point it is sent down to the large colon where the products are sorted and some are absorbed, and then some of the product is sent to the rectum, and then waiting for defecation.
Explain the general structure of the alimentary canal wall, and know where and how it is modified (specialized)
Alimentary Canal consists of: mouth, pharynx, esophagus, stomach, small intestine, large intestine. The Alimentary canal is a muscular tube 9 meters long, passes through ventral cavity. Structure of wall similar throughout, vary according to specific function of that area. The myenteric and submucosal nerve plexuses make up the entire nervous system, which regulates gastric motility and secretion. Layers of the Alimentary Canal Wall are the tunica mucosa, tunica submucosa, tunica muscularis, tunica serosa.
Describe the general functions of each layer of the alimentary canal wall, including:Mucosa (epithelium, lamina propria, muscularis mucosa)
The inner lining of the digestive tract, called the mucosa or mucous membrane, consists of an inner epithelium, a loose connective tissue layer called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosae. The epithelium is simple columnar in most of the digestive tract, but stratified squamous from the oral cavity through the esophagus and in the lower anal canal, where the tract is subject to more abrasion. The muscularis mucosae tenses the mucosa, creating grooves and ridges that enhance its surface area and contact with food. This improves the efficiency of digestion and nutrient absorption.
Tunica mucosa: Inner most layer; Epithelium attached to connective tissue, protects underlying tissues; Secretion and absorption
Describe the general functions of each layer of the alimentary canal wall, including: Submucosa (w/submucosal plexus)
The submucosa is a thicker layer of loose connective tissue containing blood vessels and lymphatics, a nerve plexus (submucosal plexus), and in some places, glands that secrete lubricating mucus into the lumen.
Describe the general functions of each layer of the alimentary canal wall, including: Muscularis externa
Tunica muscularis externa: Two layers of smooth muscle, circular and longitudinal; propel food
Describe the general functions of each layer of the alimentary canal wall, including: Serosa
The serosa is composed of a thin layer of areolar tissue topped by a simple squamous mesothelium. The serosa begins in the lower 3 to 4 cm of the esophagus and ends just before the rectum. The pharynx, most of the esophagus, and the rectum have no serosa but are surrounded by a fibrous connective tissue layer called the adventitia, which blends into the adjacent connective tissue of other organs.
Tunica submucosa: Loose connective tissue, houses blood an lymph vessels and nerves; Nourishment; Controls the actions of the tunica mucosa per the submucosa plexus
Contrast mechanical and chemical digestion
Mechanical digestion: the physical breakdown of food into smaller particles; achieved by the cutting and grinding of the teeth
Chemical digestion: a series of hydrolysis reactions that break dietary macromolecules into their monomers
Explain how mixing and peristalsis of food occurs through the digestive tract; how do the layers of the muscularis externa coordinate their contraction for these movements to occur?
Tunica muscularis externa: Two layers of smooth muscle, circular and longitudinal; propel food

Peristalsis is moderated partly by a short reflex through the myenteric nerve plexus. The bolus stimulates stretch receptors that feed into the nerve plexus, which transmits signals to the muscularis externa behind and ahead of the bolus. The circular muscle behind the bolus constricts and pushes it downward. Ahead of the bolus, the circular muscle relaxes while the longitudinal muscle contracts. The latter action pulls the wall of the esophagus slightly upward, which makes the esophagus a little shorter and wider and able to receive the descending food.
Understand the structure and function of the mouth and tongue. How do these structures contribute to digestion of food?
The mouth is the first part of the alimentary canal. The mechanical breakdown of food begins in the mouth. Also begins chemical digestion of carbohydrates The mouth functions to receive food and begin mechanical and chemical digestion.
Mastication is the chewing; Breaks food into pieces small enough to be swallowed and exposes more surface to the action of digestive enzymes. It is the first step in mechanical digestion. The first digestive Enzymes are released in the mouth.
Tongue: Covered with taste buds within papillae, which also provide friction to move food around; Attached to floor of mouth by frenulum
Describe the role of teeth in digestion, and describe the gross anatomy of a tooth: Crown, Root, Enamel, Dentine, Gingiva, Periodontal ligament
1. Two sets of teeth develop in sockets within alveolar processes of the maxillary and mandibular bones. They begin mechanical digestion
2. The crown is the portion above the gum
3. The root is the portion below the gum, embedded in alveolar bone
4. Most of a tooth consists of hard yellowish tissue called dentin, covered with enamel in the crown
5. The gum, or gingiva, covers the alveolar bone.
Know where saliva comes from and what it is made of; explain the role of each component of saliva
Salivary Glands: Secrete saliva, a hypotonic liquid which has mucus and enzyme components; used for chemical digestion
Saliva contains:
1. Salivary amylase - begin starch digestion; first enzyme used in chemical digestion
2. Lingual lipase - activated in stomach; not activated till low pH; Breaks down lipids
3. Mucus - aids swallowing
4. Lysozyme - kills bacteria
5. IgA - inhibit bacterial growth
6. Electrolytes - Na+, K+, Cl -, etc...
Explain how the autonomic innervation of salivary glands affect their secretory activity
The extrinsic salivary glands secrete 1.0 to 1.5 L of saliva per day, mainly in response to food in the mouth. Food stimulates the oral taste, tactile, and pressure receptors, which transmit signals to a group of salivatory nuclei in the medulla oblongata and pons. These nuclei integrate this information with input from higher brain centers, so even the aroma, sight, or thought of food stimulates salivation. Irritation of the stomach and esophagus by spicy foods, stomach acid, or toxins also stimulates salivation, perhaps serving to dilute and rinse away the irritants.
Describe the swallowing reflex; what ensures that food doesn't go the wrong way or down the wrong tube?
1. Food forced into pharynx by the tongue
2. Sensory receptors sense food, which triggers swallowing reflex (medulla and pons)
3. Peristalsis transports food through the esophagus to the stomach
Know the anatomy of the stomach and the stomach wall
Stomach: J-shaped muscular organ that receives and mixes food with digestive juices, and propels food to small intestine
Churning: the only mechanical digestion in the stomach
Gross Anatomy
1. Layers of the stomach: Longitudinal muscle, Circular muscle, and Oblique muscle
2. Gastric rugae is the wrinkles on the lumen of the stomach
3. Pyloric sphincter: regulates the entrance of chime into the duodenum of the small intestine
4. Duodenum: first portion of the small intestine; C-shaped
What modifications does the stomach have in order for it to carry out its functions?
Layers of the stomach: Longitudinal muscle, Circular muscle, and Oblique muscle
The Stomach has a third layer for churning, 3 directions instead of 2.
Explain how the stomach protects itself from its own stomach acid
Epithelium of stomach has a high cell turnover rate and the thick bicarbonate mucus covers epithelium.
Mucus and cell turnover, then bicarbonate in small intestine
Explain the organization of gastric pits and gastric glands
The gastric mucosa is pocked with depressions called gastric pits lined with the same columnar epithelium as the surface. Two or three tubular glands open into the bottom of each gastric pit and span the rest of the lamina propria. In the rest of the stomach, they are called gastric glands.
Gastric glands within the mucosa of the stomach open up as gastric pits
Describe the different cells that make up the gastric glands (chief, parietal, and mucous). What substances do they produce, and what are the functions of these substances?
1. Mucous cell: Secrete mucus
2. Chief cell: Secretes zymogens like pepsin (protein digestion), gastric lipase (lipid digestion) Both are not activated til the small intestine
3. Parietal cells: Secretes HCl. Also secrete intrinsic factor, necessary for vitamin B12 absorption in the small intestine
4. Enteroendocrine cells (G cells): Secrete hormones such as histamine and gastrin
Know how gastric secretions are controlled by:
1. CCK (cholecystokinin): released from small intestine; Decrease gastric mobility/secretions; Inhibit stomach productivity
2. Gastrin: A hormone that increases gastric motility and stimulates secretions
3. Secretin: released from small intestine; Decrease gastric mobility/secretions; Inhibit stomach productivity
Describe how parietal cells produce HCl from blood CO2
The parietal cell combines water with CO2 from the blood to form carbonic acid. Carbonic acid breaks down into bicarbonate ion (HCO3−) and hydrogen ion (H+). HCO3− returns to the blood. In exchange, Cl− enters the lumen with H+ and the two form hydrochloric acid.
Define zymogen, and give an example. Understand why the body would want to bother producing a zymogen
Zymogen: a proenzyme that is activated once a piece of it is cleaved.
The body produces zymogens to make sure that they are not activated till the chime reaches the SI for full absorption.
Describe the process of digestion in the stomach after you eat that big mac. What "phases" can stomach digestion be broken into?
1. Cephalic Phase:
Vagus nerve stimulate gastric secretion even before food is swallowed; i.e. the though, smell of food
2. Gastric Phase:
Food stretches the stomach and activates myenteric and vagovogol reflexes
These reflexes stimulate gastric secretion
Histamine and gastrin also stimulate acid and enzyme production
3. Intestinal Phase:
Intestinal gastrin briefly stimulates the stomach but...
Secretin and CCK and enterogastric reflex inhibit gastric secretion and motility while the duodenum processes chime already in it
Sympathetic never fibers suppress gastric activity while vagal stimulation of the stomach is now inhibited
What signals the stomach motility and acid/enzyme secretion, and what inhibits it?
Stimulate the stomach: Gastrin, Histamine, Intestinal gastrin, and Parasympathetic stimulation
Inhibit the Stomach: Secretin, CCK, and Sympathetic stimulation
Starting from the digestion of food, how is pepsin made?
1. Ingested food buffers stomach acid
2. Elevated pH stimulates G cell
3. G cells (enteroendecrine) secrete gastrin
4. Gastrin stimulates chief cells and parietal cells
5. Chief cells secrete pepsinogen
6. Parietal cells secrete HCl
7. HCl helps convert pepsinogen to pepsin
8. Pepsin digests dietary proteins
9. Oligopeptides directly stimulate G cells
10. Oligopeptides and amino acids buffer stomach acid
How Pepsin makes more of itself
Positive feedback loop
Parietal and chief cell secretions together make the enzyme pepsin (for protein digestion)
1. Chief cells secrete pepsinogen: a zymogen
2. Parietal cells secrete HCl to cleave the peptide to make pepsin
3. Pepsin will help cleave peptide to make more of its active form
Describe the structure and function of the pancreas
The pancreas has the exocrine function of secreting pancreatic juice which aids digestion.
Know the secretions released by the pancreas and what they do:
1. Bicarbonate: Duct cells secrete bicarbonate component of pancreatic juice. Bicarbonate serves to neutralize acid chime in the duodendum
2. Pancreatic amylase: digests carbs
3. Pancreatic lipase: digests lipids
4. Nucleases: digests nucleic acids
5. Trypsin, chymotrypsin, carboxypeptidase: digests proteins. Secreted as zymogens, activated in duodenum
Explain how pancreatic secretions are regulated, and where they are produced (eg. enzymes from acinar cells, bicarbonate from the duct cells)
1. Pancreatic juice is Regulated by:
Endocrine and nervous systems
Secretin (hormone): stimulates bicarbonate release from pancreas (duct cell)
Cholecystokinin (CCK) (released from wall of small intestine): stimulate enzyme release from pancreas
2. Acinar cells secrete the enzyme component of pancreatic juice; Makes pancreatic juice. Acinar cells release enzymes in response to CCK
Describe the structure and function of the liver
The liver functions in metabolic activities, storage of nutrients, filtration of blood, and chemical digestion.
Divided into left and right lobes, which consist of lobules containing hepatic cells radiating from a central vein
Functions of the liver:
1. Main role in digestion: Production of bile help in the digestion of lipids
2. Metabolic activities: Synthesis, breakdown, or conversion of carbohydrates, lipids, and proteins AND Detoxification (eg. ammonia urea)
3. Stores: Glycogen, vitamins A, D, B12, iron, and copper
4. Filters the blood, removing damaged RBCs, foreign substances, toxins
5. Secretes bile for digestion
Describe the composition of bile, its function, and where it is stored
1. Bile is a yellow-green liquid that hepatic cells secrete; Bile gets its color from heme breakdown after picked up from liver
2. Contains water, bile salts, bile pigments, cholesterol, and electrolytes; Only bile salts involved in digestion
3. Stored in gall bladder
4. Bile salts emulsify fats and make them easier to digest and absorb
Explain how bile release from the gall bladder is regulated
Regulation of secretion: Release controlled by cholecystokinin (CCK) in response to chime. CCK is released by cells of small intestine wall; targets gallbladder to release bile
Explain the structure and functions of the small intestine; how is the intestine wall well adapted for these functions?
1. Receives secretions from pancreas and liver
3. Transports the remaining residue to the large intestine
4. Major site of absorption: Food, Fuel and Water
5. Consists of duodenum (10"), jejenulum (6'), and ilium (9')
6. Attached to posterior abdominal wall by double layered peritoneum called mesentery
7. Volume absorbed by small intestine per day: 9,000 ml
How are the intestines organized in the abdomen? (mesentery/omentum)
The omentum covers the small intestines like an apron. The small intestines are loosely suspended from it by connective tissue sheets called mesenteries.
Know the different types of cells in the small intestinal mucosa; what are their functions?
A villus is covered with two kinds of epithelial cells: columnar enterocytes (absorptive cells) and mucus-secreting goblet cells.
The duodenum has prominent duodenal (Brunner29) glands in the submucosa. They secrete an abundance of bicarbonate-rich mucus, which neutralizes stomach acid and shields the mucosa from its erosive effects.
Endocrine cells: secretin and cholecystokinin (CCK)
Crypt of LieberkühnL Also called intestinal crypt. They have Mucous and absorptive cells. At base, stem cells
Peyer's patches: Nodes of lymphocytes to fight pathogens
What is the functional advantage of all the folds in the intestinal mucosa? Why the plicae circularis, villi, and microvilli?
Effective digestion and absorption require the small intestine to have a large internal surface area. This is provided by its relatively great length and by three kinds of internal folds or projections: the circular folds, villi, and microvilli. If the mucosa were smooth, like the inside of a hose, it would have a much small surface area. The circular folds increase the surface area by a factor of 3; the villi by a factor of 10; and the microvilli by a factor of 20. A total of 600x.
Explain what "contact digestion" is and know what the "brush border" is
Contact digestion: Several enzymes act on or inside epithelial cells; Surface area of the monomer must be touching the epithelium of the small intestine
Enterocytes have a fuzzy brush border of microvilli. They increase the absorptive surface area of the small intestine and contain brush border enzymes in the plasma membrane. These enzymes carry out some of the final stages of chemical digestion. They are not secreted into the lumen; instead, the chyme must contact the brush border for digestion to occur. This process, called contact digestion, is one reason that it is so important that intestinal contractions churn the chyme and ensure that it all contacts the mucosa.
Be able to describe the digestion of carbohydrates from the time they enter the mouth until they are absorbed through the small intestine wall
1. Initial digestion. Chemical and mechanical digestion begins in the mouth; Salivary amylase preps the carbs for the duodenum.
2. The carb is not broken down in the stomach
3. Secondary digestion. Pancreatic amylase begins the digestion in the lumen of SI (duodenum); Breaks down starch into oligosaccharides;
4. Contact digestion: Enzymes on the brush border will break carbs down further into monosaccharides
5. Glucose and galactose are co-transported with Na+.
6. Fructose slips in the thru facilitated diffusion. This is a passive process; Fructose is going down its pressure gradient but needs a protein channel to help it.
7. All 3 pass to the basal side thru facilitated diffusion
8. Passive pass thru blood capillary
9. Chemical gradient set up by NA/K pump and the drive for Na to come in is coupled with glucose and galactose in co-transport
Be able to describe the digestion of proteins from the time they enter the mouth until they are absorbed through the small intestine wall
1. Mouth: No chemical digestion occurs only mechanical.
2. Stomach: Pepsin hydrolyzes protein to smaller polypeptides
3. Small Intestine: Pancreatic proteases will help break up polypeptides further into oligopeptides
Trypsin and chymotrypsin will hydrolyze some peptide bonds.
Carboxypeptidase cleaves off one AA at a time from the carboxyl end
Aminopeptidase removes an AA from the amino end
Dipeptidase splits dipeptides into single AAs
4. Not an AA until reaches the brush border
5. AAs absorbed via co-transport w/ Na+
6. Dipeptide co-transport with H+
7. Specific carriers for transport of AAs into blood
Facilitated diffusion into blood
Na/K pump helps
Be able to describe the digestion of lipids from the time they enter the mouth until they are absorbed through the small intestine wall
1. Emulsification of Fats: Large gat gloue not able to be digested bc the lipase enymes will not have access to theinterior portion of fat globule; So breaks down into smaller parts; Fat globules broken up and coated by lecithin (lipid) and bile acids
2. Fat hydrolysis: Once in smaller droplets then the pancreatic lipase has an increase in surface area. The pancreatic lipase hydrolyzes the 1st and 3rd fatty acids from the triglycerides. Result is 2 free fatty acids and one monoglyceride
3. Lipid uptake by micelles: The monoglycerides, fatty acids, cholesterol, and fat-soluble vitamins (A, E, K) combine to form micelles. Micelles in the bile pass to the small intestine
4. Chylomicron Formation: Intestinal cells absorb (passive diffusion) lipids from micelles, resynthesize triglycerides, and package triglycerides, cholesterol and phospholipids into protein-coated chylomicrons
5. Chylomicron exocytosis and lymphatic uptake: Golgi apparatus repackages chylomicrons into secretory vesicles. Chylomicrons are released from basal cell membrane by exocytosis and enter the lacteal (lymphatic capillary) of the villus. Enters lymphatic system. Enters subclavian vein the general circulation
Describe the structure and function of the large intestine; how are the intestines organized in the abdomen? (mesentery/omentum)
1. The large intestine absorbs water an forms and stored feces. Doesn't digest or absorb nutrients; Secretes mucus and absorbs water and electrolytes; Contains important intestinal flora which synthesize vitamins (K)
Gross Anatomy
a. The tenia coli contracts from Haustra → pushes down
b. Lesser Omentum - attaches stomach to liver
c. Greater Omentum - covers small intestines like an apron
d. Mesentery of small intestines - holds many blood vessels
e. Mesocolon - anchors the (transverse and sigmoid) colon to the dorsal body wall
Describe the location and role of the appendix
Attached to the argeintestine's lower end is the appendix, a blind tube 2 to 7 cm long. The appendix is densely populated with lymphocytes and is a significant source of immune cells.
Explain what the enteric nervous system is
The esophagus, stomach, and intestines have a nervous network called the enteric nervous system, which regulates digestive tract motility, secretion, and blood flow.
Explain how waste is formed in the large intestine, and how it is eliminated from the body
The large intestine absorbs water an forms and stored feces. Doesn't digest or absorb nutrients; Secretes mucus and absorbs water and electrolytes; Contains important intestinal flora which synthesize vitamins (K)
Haustral Contractions: similar to segmentation
Mass Movement: big contractions of ascending and transverse colon
Internal anal sphincter: smooth muscle; involuntary; responds to pressure in rectum
External anal sphincter: skeletal muscle; under voluntary control
Understand the role of the intestinal flora in our health
We feed them, they help us with Carbohydrate fermentation & absorption, Vit K, biotin, Colonization resistance, Immune system and Even hormones!