Flashcards in Chapter 17 Deck (47)
What does the Digestive System do?
- Breaks down food into smaller components (e.g. amino acids and monosaccharides)
- Processes food, extracts nutrients, and eliminates residue
What is Gastroenterology?
Study of digestive tract and disorders
What are the stages of Digestion?
1) Ingestion --> Selective intake of food
2) Digestion --> Breakdown of food into usable form
3) Absorption --> Uptake of nutrients into digestive tract, then into blood or lymph
4) Compaction --> Absorption of water. Consolidation of indigestible residue into feces
5) Defecation --> Elimination of feces
What is Mechanical Digestion?
The physical breakdown of food into smaller particles achieved by the cutting and grinding action of the teeth and the churning movements of the stomach and small intestine. It exposes more food surface to the action of digestive enzymes.
What Chemical Digestion?
Employs enzymes to break dietary macromolecules into their basic building blocks: Polysaccharides into monosaccharides, proteins into amino acids, fats into monoglycerides and fatty acids, and nucleic acids into nucleotides. All chemical digestion is by the process of enzymatic hydrolysis, breaking covalent bonds by adding water to them.
What nutrients are already present in usable form in the ingested food and are absorbed without being broken down by digestion?
Vitamins, Minerals, Cholesterol, and water
What is Deglutition?
- Involves over 22 muscles in the mouth, Pharynx, and esophagus.
- Coordinated by the swallowing center in the medulla oblongata and occurs in 3 phases.
What are the 3 phases of Swallowing?
1) Oral Phase --> Under voluntary control. Tongue collects food, presses it against the palate to form bolus and pushes it posteriorly. When Bolus reaches a critical size, the epiglottis tips posteriorly and the bolus slides around it through a space on each side, into the laryngopharynx.
2) Pharyngeal Phase --> Involuntary and is initiated when food contacts touch receptors of the laryngopharynx. Soft palate and root of the tongue block food from enter in the nasal cavity or reentering the mouth. The Larynx is pulled up against epiglottis, which prevents food from entering the larynx and trachea. The pharyngeal constrictors contract in order from superior to inferior to drive the bolus downward into esophagus. And Breathing is suspended.
3) Esophageal Phase --> A wave of involuntary contraction ("Peristalsis"). Controlled jointly by the brainstem swallowing center and the enteric nervous system in the esophageal wall. Esophagus constrict ABOVE the Bolus and relaxes BELOW it, propelling the bolus along. As bolus reaches the lower end of esophagus, the lower esophageal sphincter relaxes to let it pass into the stomach.
What is the Stomach?
- Muscular sac in upper left abdominal cavity
- Food storage organ
- Breaks up food particles
- Liquifies food
- Begins chemical digestion of proteins and fat
- Produces semi-digested food -- "Chyme"
The gastric mucosa is pocked with Depressions called _________?
What are the gland types near the esophagus and pylorus?
Cardiac Glands near the esophagus and Pyloric glands at the pylorus. Both secreting mainly mucus.
What are the cells of Gastric Glands in most of the stomach?
1) Mucous Cells --> secrete mucus
2) Regenerative Cells --> Found in base of the put and neck of the gland, divide rapidly and Produce continual supply of new cells.
3) Parietal Cells --> Found mostly in the upper half of the gland and secrete hydrochloric acid and intrinsic factor
4) Chief Cells --> Most numerous and secrete the enzymes "pepsinogen and gastric lipase.
5) Enteroendocrine Cells --> Concentrated especially in the lower end of a gland, secrete hormones and local chemical messengers that regulate digestion. Include the G cells which secretes regulatory hormone "Gastrin."
What solutes compose Gastric Juice?
1) Hydrochloric Acid --> Produced by the parietal cells; activate enzyme pepsin; converts dietary iron to a form the body can absorb and use; breaks up connective tissue and plant cells; and destroys most ingested pathogens
2) Pepsin --> enzyme that digests proteins to shorter peptides. inactive precursor called pepsinogen secreted by chief cells.
3) Gastric Lipase --> secreted by chief cells, it
digests small percentage of dietary fat.
4) Intrinsic Factor --> glycoprotein secreted by parietal cells; needed for absorption of B12 by small intestine; needed to synthesize hemoglobin; only indispensable function of stomach
5) Hormones and Local chemical messengers --> produced by enteroendocrine cells; several also produced in central nervous system (called "gut-brain peptides" and include gastrin, serotonin, histamine, somatostatin, and others)
What happens when you begin to swallow?
Stomach signaled to relax by medulla oblongata. Allows smooth muscle to relax and stretch.
What happens during Peristalsis?
- Contractions churn food and Mix with gastric juice and promote breakup and digestion.
- Contractions governed by pacemaker cells in muscularis externa.
- Waves squeezing shut pyloric sphincter
- Squirts small amount of chyme into duodenum
- Typical meal emptied in 4 hours. less time if more liquid; longer if higher in fat
What is Vomiting?
- The forceful ejection of stomach and intestinal contents from the mouth.
- Involves multiple muscular actions, integrated by the "emetic center" of the medulla oblongata.
How is Vomiting induced?
- overstretched stomach or chemical irritants
- visceral trauma or intense pain
- repugnant sights, smells, or thoughts
What are the dangers of Chronic Vomiting?
- Can cause dangerous imbalances in fluid, electrolyte and acid-base.
- Can erode tooth enamel
- Inhalation of Acid which is destructive to the respiratory tract.
What are the 3 phases of Gastric secretion and mobility?
1) Cephalic Phase --> The stomach responds to the mere sight, smell, taste, or thought of food. The senses converge on the hypothalamus which relays signals to the medulla oblongata. Vagus nerve fibers from the medulla stimulate the enteric nervous system, which, in turn, stimulates gastric activity.
2) Gastric Phase --> Swallowed food activates gastric activity. about 2/3 of gastric secretion occurs during this phase. Ingested food stimulates gastric activity both by stretching the stomach and by raising the pH of its contents.
3) Intestinal Phase --> Occurs as chyme enters the duodenum and triggers local nervous reflexes and the secretion of regulatory hormones. Acids and semi digested fats in the duodenum trigger inhibitory signals, sent to the stomach by way of the enteric nerves system. In addition, signals from the duodenum to the medulla oblongata inhibit vagal stimulation of the stomach and activate sympathetic neurons, which inhibit gastric activity. Furthermore, hormones secretin and cholecystokinin are released which suppress gastric activity.
How is the stomach protected from acids/enzymes?
1) Mucosa coated with thick alkaline mucus which neutralizes HCl and resists the enzymes.
2) Epithelial cells joined by tight junctions that prevent gastric juice from seeping between them and digesting the underlying tissue.
3) Rapid replacement of epithelial cells which live only 3 to 6 days and then sloughed off into the chyme and digested with the food.
What is the Liver?
- Gland immediately inferior to diaphragm
- Fills most of right hypochondriac and epigastric regions
- Body’s largest gland weighing about 3 lbs
- Multiple functions but only one of them, secretion of bile, contributes to digestion.
Describe Bile Flow?
- Secreted into channels between hepatocytes
- Passes to common hepatic duct
- Joined by cystic duct coming from gallbladder thus forming the bile duct which descends toward duodenum.
- Joins duct of pancreas and forms hepatopancreatic ampulla which terminates at major duodenal papilla. Bile and pancreatic juices flow into the duodenum through an opening in the papilla.
What is Bile?
Yellow-green fluid containing minerals, cholesterol, phospholipids, bile acids and pigments. The principle pigment is "Bilirubin" which is derived from decomposition of hemoglobin. further breakdown responsible for colors of feces and urine.
What are Bile Acidds?
steroids aiding in fat digestion and absorption. All other bile components are waste products
Pancreatic juice is excreted by the endocrine portion or the exocrine portion?
Pancreatic juice flows into duodenum by _______?
What are the Pancreatic Juice components?
- Trypsin and chymotrypsin, digest protein
- Pancreatic amylase, digests starch
- Pancreatic lipase, digests fat
- Ribonuclease and deoxyribonuclease digest RNA and DNA, respectively.
What signal molecules activate pancreatic juice and bile secretion?
1) Acetycholine (ACh) --> secreted by vagus and enteric plexus. stimulates pancreatic secretion.
2) Cholecystokinin (CCK) --> hormone secreted by mucosa of duodenum in response to fat. stimulates pancreas; contracts gallbladder. relaxes sphincter of ampulla.
3) Secretin --> hormone produced by duodenum in response to acidity. stimulates liver and pancreas to secrete bicarbonate and buffers stomach acid. The rise in duodenal pH due to this bicarbonate also activates digestive enzymes of the pancreatic juice and intestinal mucosa.
When we say the Small intestine is "small" we are referring to its ___________?
How is the small intestine divided?
Ileum duodenum and jejunum
What are circular folds?
- largest elaborations of the intestinal wall.
- transverse ridges projecting into lumen
- from duodenum to middle of ileum
- causes chyme to spiral along intestine
What is Villi?
small finger - to tongue - shaped projections which give the mucosa a fuzzy texture. They are covered with simple columnar epithelium composed of columnar absorptive cells (enterocytes) and mucus-secreting goblet cells
What are Microvilli?
Form a fuzzy brush border on the surface of each absorptive cell. Greatly increase the absorptive surface area of the small intestine and contain brush border enzymes which carry out the final stages of chemical digestion.
What are Intestinal Crypts?
- On floor of small intestine between villi bases
- Tubular glands in pores on the floor
- Upper half enterocytes and goblet cells
- Lower half dividing epithelial cells which migrate to top of villus where sloughed off.
How much Alkaline Intestinal juice does the small intestine secrete per day?
1-2 Liters. it helps to mix nutrients with digestive enzymes and protect the mucosa from stomach acid.
What are the 2 kinds of contractions which the small intestine exhibits?
SEGMENTATION (most common movement, Ringlike constrictions form along intestine
Relax as new constrictions form, and Churns and kneads contents) and PERISTALSIS (Begins where most nutrients absorbed)
- Both serve to (1) Mix chyme with intestinal and pancreatic juices, bile, (2) Brings chyme in contact with mucosa for digestion (3) Move residue toward large intestine
What is considered the most digestible dietary carbohydrate?
- Broken down by salivary amylase in oral cavity to maltose and other small carbohydrate chains and denatured in stomach
- Pancreatic amylase in small intestine breaks down remaining starch and oligosaccharides to maltose
What are the 3 Brush Border enzymes which act upon the disacharides during carbohydrate digestion?
1) Maltase --> Splits the maltose into glucose molecules.
2) Sucrase --> Splits sucrose into glucose and fructose
3) Lactase --> Digests lactose to glucose and galactose.
What are the monosaccharide products of Carbohydrate Digestion?
Glucose, Fructose and Galactose which are immediately absorbed by carrier-mediated transport across absorptive cell surfaces.
- Within the cell most of the fructose is converted to glucose. Glucose, galactose and any remaining fructose then pass out of the base of the epithelial cells by facilitated diffusion, pass into the blood capillaries of the villi, and are coaried off by the hepatic portal system to the liver for metabolism or storage.
How are Proteins Digested?
- Enzyme Proteases (NONE in the saliva)
- Begins in the stomach where pepsin digests 10% to15% into peptides
- In small intestine 3 proteases (trypsin, Chymotrypsin, and carboxypeptidase) continue the job.
- Brush border enzymes Dipeptidase and aminpeptidase take apart the remaining dipeptides.
What is the problem with Fat Digestion?
Fats are hydrophobic and cannot enter the intestinal blood capillaries in significant quantities.
What are the Fat Soluble Vitamins?
A, D, E, and K
What are the water-soluble vitamins?
B Complex and and Vitamin C
Iron is absorption is stimulated by ________?
Calcium absorption is regulated by what?
Parathyroid Hormone and Calcitrol
Describe Water absorption in the Digestive Tract
- receives about 9 L water/day
- about 6.7 L from secretions
- about 8 L absorbed by small intestine
- about 0.8 L absorbed by large intestine