19. Chapter 21- The Digestive System Flashcards
What is the primary function of the digestive system?
Move nutrients, water and electrolytes from the external environment into the body’s internal environment
Main thing needed for regulation and integration of metabolic processes throughout the body
Proper functioning needed for homeostasis
10% of health care costs are GI system
What is the anatomy of the digestive system?
What is the GI tract?
Extends from mouth to large intestine (15 feet length average)
GI tract: long tube with muscular walls lined by transporting and secretory epithelial (digestion primarily occurs here)
Stomach to anus is the gut!
So it’s mouth to anus
What do the three pairs of salivary glands do with the beginning of digestion in the mouth? (4 things)
- Moisten and lubricate food
- Amylase partially digests polysaccharides
- Dissolve some food molecules (taste)
- Lysozyme kills bacteria
Slide 5 Feb 25
What is the esophagus?
What parts are skeletal and smooth muscle?
Passageway from mouth to stomach
Upper and lower esophageal sphincters
Top 1/3 is skeletal muscle, bottom 2/3 is smooth muscle
Moves food via peristaltic waves
Slide 6 Feb 25
What is the small intestine? (3 parts)
What is the large intestine? (3 parts)
Small intestine- majority of digestion takes place here
First 25 cm is duodenum
Then jejunum
Then ileum
Slide 7 Feb 25
Large intestine- (colon) larger diameter but shorter, made of ascending colon, transverse colon, and descending colon
Water and electrolytes removes to create semisolid feces
Feces entering the terminal section of the large intestine (rectum) trigger a defecation reflex
What are the GI layers? (4 of them)
Slide 9-10 Feb 25!
Slide 9-10 Feb 25 Mucosa Submucosa Muscularis externa Serosa
What is mucosa?
What’s it made of? (3 things)
First layer of GI tract
Epithelium- transporting epithelial cells, endocrine and exocrine secretory cells, shirt life span always making new cells
Lamina propia- contains nerve fibres, small blood vessels and lymph vessels
Muscularis mucosae- thin layer of smooth muscle that can alter surface area available for absorption
Slide 11-12 Feb 25
What is the submucosa?
Seconds part of GI tract below mucosa
Middle distensible layer containing larger vessels (lymph and blood) and the submucosal plexus; one of the major nerve networks in the enteric nervous system
Slide 15 Feb 25
What is the muscularis externa?
What is the serosa?
Muscularis externa: 2 layers of smooth muscle: circular decrease diameter, longitudinal shortens the tube, contains myenteric plexus
Serosa- outer covering of connective tissue that is a continuation of the peritoneal membrane, sheets of mesentary hold intestines in place
Slide 16 Feb 25
What are the 4 steps of the digestive function and processes?
- Digestion- chemical and mechanical breakdown of food into absorbable units
- Secretion- movement of material from cells into lumen or ECF
- Absorption- movement of material from GI lumen to ECF
- Motility- movement of material through the GI tract as a result of muscle contraction
Slide 17 Feb 25
What are the 3 fluid secretions?
Water- ions transported from ECF into the lumen
Digestive enzymes- exocrine glands and epithelial cells in stomach and small intestine
Mucus- viscous glycoprotein (mucins) secretions that protect GI cells and lubricate the contents
What is motility in the digestive system?
- Moves food from mouth to anus
- Mechanically mixing food breaks it into uniformly small particles
This is determined by properties of smooth muscle and modified by chemical input
Slow waves originate in a network of cells known as interstitial cells of cajal (ICC)
Modified smooth muscle cells serve as pacemaker for slow wave activity
What are the 3 basic patterns of contraction that occur in the GI system bringing about different types of movement?
- Migrating motor complex (motilin)
Between meals
Begins in the stomach and slowly passes from section to section, sweeps food remnants and bacteria out of upper GI tract and into the large intestine - Peristaltic contractions- responsible for forward movement
- Segmental contractions- responsible for mixing
Slide 9-10 Feb 27
What are enteric nervous system (ENS) reflexes? (Short and long reflexes)
Short reflexes originate in the enteric nervous system and are carried out entirely within the wall of the gut
Long reflexes are integrated in the CNS, some long reflexes originate outside the GI tract but others originate in the enteric nervous system
What are GI peptides?
Hormones, peptides and cytokines
Can act as hormones or paracrine signals
Excite or inhibit motility and secretion
Can be secreted in to lumen to act on apical membrane receptors or ECF to act on neighbouring cells
What are the 3 hormone families?
Gastrin family- gastrin, cholecystokinin
Secretin family- secretin, vasoactive intestinal peptide, gastric inhibitory peptide
Other- motilin
What are the 3 phases of integrated function within the digestive system?
- Cephalic/oral phase- digestive processes occurring before food enters the stomach
Long reflexes beginning in the brain, increased parasympathetic output from medulla to salivary glands and to the enteric nervous system - Gastric phase- digestive processes in the stomach, 3.5L of food drink and saliva enter the stomach each day, storage digestion and defence are the functions of the stomach
- Intestinal phase- digestive processes in the intestines
How does mechanical digestion start?
Begins with mastication (chewing) of food by teeth
Tongue and lips he’ll manipulate food
Joined by a flood of saliva from the three pies of salivary glands (exocrine secretion) 1.5L/day 99.5% water 0.5% solutes
What is the parotid gland, submandibular gland, and sublingual gland?
Parotid gland- watery solution with amylase
Submandibular- similar to parotid plus some mucus
Sublingual- mainly mucus
Slide 9 Mar 1
What is deglutition? 3 steps
Reflex that pushes a bolus of food or liquid into the esophagus
- Tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex
- Breathing is inhibited as the bolus passes the closed pathway
- Food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity
What is the gastroesophageal reflux disease (heartburn)?
Churning action if stomach contraction can cause backflow
Negative intrapleural pressure during inspiration can cause esophagus to expand drawing gastric acid and pepsin from the stomach
Slide 12 Mar 1
What happens to the stomach upon swallowing food?
Parasympathetic neurons to the ENS cause the fundus if the stomach to relax: receptive relaxation
Distension of stomach enhances motility
Weak peristaltic waves (15-25 seconds) that increase in force proceeding down to the antrum moves chyme toward pylorus and then larger particles are moved back to the body
Slide 14-15 Mar 1
What is the stimuli for acid secretion?
Apical H+/K+ ATPase, Cl- and K+ transporters stored in vesicles
Stimuli cause exocytosis and insertion of apical transporters
Slide 4 Mar 4
What is digestive enzyme secretion?
Stimulated by acid secretion via short reflex
Chief cell
Slide 7 Mar 4
What is paracrine secretion?
ECL- histamine
Activated H2 receptors on parietal cells to stimulate HCl secretion
Parietal cell- intrinsic factor
- forms a complex with Vit B12 so it can be absorbed
D cell- somatostatin
- negative feedback for acid secretions (G cells, parietal cells and ECL cells)
- inhibits pepsinogen release
Slide 8-9 Mar 4
What is mucus secretion?
Mucus and HCO3- secreted from mucous cells
- mucus secretion stimulated by parasympathetic input and irritation
- HCO3- by parasympathetic input and H+
Slide 10 Mar 4
What is a peptic ulcer?
A sore or break in the lining of the stomach or duodenum
Helicobacter pylori
Slide 11 Mar 4
What is the intestinal phase of digestion?
5.5 L of food fluid and secretions enter small intestine and 3.5 L form hepatic pancreatic and intestinal
Combo of segmental and peristaltic contractions mix the chyme with enzymes and expose digested nutrients to mucosal epithelium for absorption
Plicae and villi (secrete mucus) increase surface area
Crypts contain hormone and fluid secretory cells + stem cells
Most nutrients absorbed into capillaries in the villi
Slides 15-17 Mar 4
What does the pancreas do?
Contains endocrine secretory epithelium (islets): insulin and glucagon
Exocrine secretory epithelium: digestive enzymes and NaHCO3
Many pancreatic digestive enzymes released as zymogens
Trypsin activates zymogens
Bicarbonate produced in duct cells neutralizes acid entering stomach
Slide 5-8 Mar 6
What does the liver do?
Slides 9-10 Mar 6
Largest internal organ lying under diaphragm
Bike: a non enzymatic solution secreted from hepatocytes
3 components: bile salts, bile pigments, cholesterol
What is fat digestion?
Fats and related molecules (triacylglycerols, cholesterol, phospholipids, ling chain fatty acids, fat soluble vitamins) are put into fat droplets then a coarse emulsion of large fat droplets in chyme broken to smaller, stable particles by bile salts
Slides 11-12 Mar 6
What is fat absorption? (5 steps)
- Bile salts from liver cost fat cells
- Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles
3a. Monoglycerides and fatty acids move out of micelles and enter cells by diffusion
3b. Cholesterol is transported into cells - Absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons
- Chylomicrons are removed by the lymphatic system
Slide 13 mar 6
What are gallstones?
Hardened deposits likely due to excess cholesterol or bilirubin
Upper right abdominal pain, jaundice
Gallbladder removal
Slide 14 Mar 6
What is carbohydrate digestion and absorption?
Slides 15-16 Mar 6
Digestion: starch and sucrose
Amylase break down glucose polymers to disaccharides, then these are broken down by intestinal brush border enzymes (disaccharidases)
Absorption: glucose galactose fructose
Normally glucose is phosphorylated when entering a cell, but enterocytes primarily use glutamine so glucose 6 phosphate is not formed and free glucose stays high facilitating transport
What is protein digestion?
Slide 17 mar 6 and slide 3 Mar 8
Not all proteins are digestible
Attack peptide bonds in the amino chain forming fragments
Results primarily in free amino acids, dipeptides and tripeptides
What is protein absorption?
Proteins absorbed mostly as free amino acids, a few di and tripeptides are absorbed
Single AA’s on Na cotransporters (apical) and Na exchangers (basolateral)
Di and tripeptides on oligopeptide transporter (H+ transporter)
Slide 4-5 Mar 8
What is vitamin and mineral absorption?
What is ion and water absorption?
Slide 6 Mar 8
Fat soluble vitamins (A, D, E and K) absorbed with fats
Water soluble (C and most B) are absorbed by mediated transport
B12 is absorbed in the ileum after forming a complex with intrinsic factor released from parietal cells
Slide 7 Mar 8
Most water absorbed in the small intestine
This same mechanism exists in the colon
How is the intestinal phase regulated?
Regulated by distension, acidity, and digestive products in the small intestine through short and long reflexes and endocrine signaling
Reduces motility and secretions in the stomach, increases in the intestine
Slide 9 Mar 8
Intestinal motility primarily by ENS neurons
Mo cell-motilin (migrating motor complex)
S cell: secretin
I cells: cholecystokinin
K cells: glucose-dependent insulinoteopic peptide (GIP)
L cells: glucagon like peptide 1 (GLP-1)
What are S cells? What are I cells? What are K cells? What are L cells? Slides 10-13 Mar 6
S cells- secretin, stimulated by acid entering the duodenum from the stomach (primary role is regulation of pH) (stimulates bicarbonate secretion from pancreas and inhibition of gastric acid secretion)
I cells- cholecystokinin (CCK) stimulated by presence of fatty acids and amino acids in chyme. Regulates pancreatic enzyme secretion and regulates bile secretion
K cells- GIP, glucose homeostasis, inhibit gastric acid secretion
L cells- GLP-1, glucose homeostasis, inhibit gastric acid secretion
How is gastric and intestinal phase integrated?
Enterogastric reflex
Acid, enzymes and digested food results in hyperosmotic chyme that inhibits gastric emptying
Which intestine concentrates waste?
The large intestine stores and concentrates fecal manner
1.5 L of chyme proceed through ileocecal valve (valve relaxes as contraction proceeds through ileum, food in stomach causes contraction of ileum and relaxation of ileocecal valve (gastroileal reflex))
Slide 15 Mar 8
What are secretions and digestions?
Secretions- minimal and primarily consists of mucus from goblet cells
Digestion- traditionally believed that zero digestion occurs in the large intestine
Slide 16 Mar 8
