Ch. 18 Digestive System (Day 1) Flashcards
Digestive System: Overview
Processing of ingested food and delivery of nutrients
Largest immune organ
largest single habitat for microflora - bacteria, archaea, fungi; roughly 10^14 organisms (about 10 times more than all the human cells in our body)
What are the 6 functions of the digestive system?
- Motility
- Secretion
- Digestion
- Absorption
- Storage and elimination
- Immune barrier
Motility
Movement of food through the tract
a) Ingestion: taking food into mouth
b) Mastication: chewing and mixing food w/ saliva
c) Deglutination: swallowing
d) Peristalsis: wave-like, one-way movement through tract
e) Segmentation: churning and mixing while moving forward
Secretion
Exocrine: digestive enzymes, HCl acid, mucus, water and bicarbonate
–open tube from one end to the other, secretions put into GI tube
Endocrine: hormones to regulate digestion; secreted into blood stream
Digestion
Break down food into smaller units via both PHYSICAL and CHEMICAL actions
–so nutrients can be liberated and taken up by body
Absorption
Transport of digestion products (nutrients) into blood or lymph
Storage and Elimination
Temporary storage and subsequent elimination of undigested food molecules
Immune Barrier
a) Simple columnar epithelium with tight junctions prevents swallowed pathogens from entering body
b) Immune cells in CT (connective tissue) of tract promote immune responses
What organs are included in the GI tract? (NOT ACCESSORY ORGANS)
Mouth Esophagus Stomach Small intestine Large intestine Rectum, anus
Define Tract
Open at both ends; therefore is continuous w/ environment
- -“outside” the bod
- -undigested materials (e.g. cellulose) never actually enter the body
____-____ transport allows specialization of function along [GI] tract.
One-way
What are the accessory organs of the GI tract?
Salivary glands
Liver/gallbladder
Pancreas
What are the layers of the GI tract (from superficial to deep)?
- Serosa
- Subserosa
- Muscular propria
- Submucosa
- Muscularis Mucosa
- Lamina propria
- Mucosa
What are the 4 MAIN LAYERS of the GI tract (from superficial to deep)?
- Serosa
- Muscularis propria
- Submusoca
- Mucosa
Serosa
Most superficial layer
Outer covering - CT membrane continuous w/ peritoneal membrane
Muscularis Propria
2nd layer (below Serosa)
Inner layer of circular muscle - affects lumen diameter
Outer layer of longitudinal muscle - affects length
Myenteric plexus (Auerbach’s plexus) - lies between muscle layers, controls gut motility via control of contraction/relaxation of the 2 muscle layers
Submucosa
3rd layer (below Muscularis Propria)
CT layer containing larger blood vessels, lymph ducts, and the Submucosal Plexus (Meissner’s Plexus)
Mucosa
Deepest layer; closest to food
Single layer of epithelial cells - enterocytes, endocrine cells, goblet cells
Major functions: secretion, absorption
Muscularis Mucosa
Thin layer of smooth muscle - controls movement of villi, contains efferent nerve endings form submucosal plexus of ENS
Between Submucosa and Mucosa, superficial to Lamina Propria
Lamina Propria
Hydrated, CT matrix - supports epithelium, has capillaries, is collection point for lymph, contains sensory nerves and immune cells
Between Submucosa and Mucosa, deep to Muscularis Mucosa
Regulation of GI Tract
Parasympathetic Division (extrinsic regulation)
- a) stimulates esophagus, stomach, small intestine, pancreas, gallbladder, and first part of large intestine via Vagus Nerve
- b) spinal nerves in sacral region stimulate lower intestine
- c) preganglionic neurons synapse on submucosal and myenteric plexuses
Sympathetic Division (extrinsic regulation)
- a) inhibits peristalsis and secretion
- b) stimulates contraction of sphincters
Hormones - from brain or other digestive organs
Intrinsic regulation
- -> where ENS really comes into its own here
- a) intrinsic sensory neurons in gut wall help in intrinsic regulation via separate Enteric Nervous System (ENS)
- b) paracrine signals
What is meant by “extrinsic regulation”?
Regulation from outside the gut in this case (from brain)
Enteric Nervous System (ENS)
“Second Brain”
Intrinsic nervous system in wall of digestive tract, able to generate reflexes independently of CNS input - if cut neural connections to CNS, gut can still engage in regulated function - GUT IS ONLY ORGAN W/ SUCH A SYSTEM
However, CNS and gut do communicate via afferent/efferent connections to ENS + direction innervation to gut sensory/effector neurons
Mouth to Stomach
Mastication: large pieces of food –> chewing –> smaller pieces and mixes it w/ saliva, which contains mucus, antimicrobial agents, and salivary amylase to start digestion of starch
Deglutition (swallowing):
- 1) Oral: voluntary; muscles of mouth and tongue to mix food w/ saliva to form Bolus
- 2) Pharyngeal: involuntary; initiated by receptors in posterior oral cavity and oropharynx
- ->a) Uvula (soft palate) lifts to cover nasopharynx, and epiglottis covers vocal cords
- ->b) Upper esophageal sphincter (UES) relaxes
- 3) Esophageal: automatic; controlled by swallowing center of brain stem; bolus –> esophagus –> stomach (peristalsis)
Mouth and pharynx and upper esophagus: skeletal muscles innervated by somatic motor neurons
Lower esophagus: smooth muscle controlled by ANS
Lower esophageal sphincter (LES) opens to allow passage into stomach; otherwise stays closed to prevent regurgitation
Stomach
Stores food, breaks it into smaller pieces
Churns food to mix w/ gastric secretions - mixture is now CHYME
Begins protein digestion
Kills bacteria in food
Moves chyme into small intestine
What starts in mouth stops in stomach, but stomach picks it up w/ its own materials
–e.g. secretions in mouth inactivated by stomach acid, secretions in stomach activated by stomach acid
How many layers of muscle does the stomach have? How many layers of muscle do the intestines have?
Stomach has 3 layers, which allows it to churn and mix everything up
Intestines have 2 layers
Stomach: Cardia
Food delivered to cardiac region from esophagus
Stomach: Fundus
Storage; expands to accommodate load (receptive relaxation) limited motility
Stomach: Body
High motility - breaks large particles into small particles, mixes food w/ secreted acid; enzymes and fluid, forming CHYME, continues the limited digestion begun in the mouth
Stomach: Gastric Rugae
Contains gastric glands and pits at base
Stomach: Pylorus
Limits amount emptied, restricts size of emptied particles in chyme
Gastric Pits and Gastric Glands
Gastric pits at base of golds leads to gastric glands that contain several types of secretory cells.
Cells (YOU HAVE TO KNOW FOR EXAM):
- mucous cell
- parietal cell
- chief cell
- enterochromaffin-like cells
- endocrine cells
Mucous Cell (KNOW FOR EXAM)
Secrete mucus (help protect stomach lining from acid)
Parietal Cell (KNOW FOR EXAM)
Secrete HCl and Intrinsic Factor (IF) (required for absorption of vitamin B12)
–if have problems w/ parietal cells, will have decreased B12 absorption (anemia occurs as a result)
Chief Cell (KNOW FOR EXAM)
Secrete pepsinogen (inactive form of proteolytic enzyme)
Enterochromaffin-like Cells (KNOW FOR EXAM)
aka ECL cells
Histamine, serotonin
Endocrine Cells (KNOW FOR EXAM)
Gastrin (G cells)*, somatostatin (D cells)
*Know about gastrin for exam
Secretion of gastric acid by parietal cells follows an ____ ____.
“Alkaline tide”
Stimulation of HCl Secretion
Neurohormonal/paracrine
Gastrin from G cells –> parietal cells via blood; also ECL cells –> secrete histamine
Histamine from ECL cells –> parietal cells via paracrine –> H2 histamine receptors –> stimulate H+ secretion
–examples: Tagamet and Zantac block H2 receptors
Parasympathetic neurons and ACh via vagus: stimulate G, parietal, and ECL cells
Functions of HCl
Creates acid environment in stomach –> pH 1-2
Ingested proteins are denatured (allows enzymes access)
Pepsinogen is converted to active pepsin (digests proteins)
Serves as optimal pH for pepsin activity
Function of Pepsin
Catalyzes hydrolysis of peptide bonds in ingested proteins
Stomach Defenses
Why the stomach doesn’t digest itself
Adherent layer of mucus w/ alkaline bicarbonate
Tight junctions between epithelial cells
Rapid epithelial mitosis (replaces epithelium every 3 days)
–gut is constantly renewing itself all the time
Digestion and Absorption in Stomach
Proteins begin digestion in stomach
Starches begin digestion in mouth, but salivary amylase is not active at pH 2, so this activity stops in stomach
Alcohol and NSAIDs (aspirin) are only common substances absorbed in stomach (due to high lipid solubility)
Small Intestine
Starts at pyloric sphincter
Duodenum: first 10 inches
Jejunum: middle 2/5
Ileum: distal 3/5
Ends at ileocecal valve (cecum picks up here)
Where does most digestion occur?
in Small Intestine
–carried out by pancreatic, intestinal enzymes, aided by other factors from liver via gallbladder - all exocrine secretions enter at DUODENUM
Papilla of Vater
Where bile from gallbladder enters duodenum
Music are folded into ____ while epithelial plasma membranes are folded into ____.
Villi; microvilli
What do villi and microvilli do to surface area (SA) of intestines?
Increase SA for absorption of nutrients
Microvilli is also known as?
“Brush border”
Intestinal Epithelium is Continually Being Renewed
Stem cells niche resides 4-5 cells from bottom of crypt
2 types of stem cells:
- -1) slowly dividing (stem cell renewal)
- -2) rapidly dividing (epithelial replacement)
Continuous proliferation pushes cells up the villus
As they migrate, they differentiate into enterocytes, enteroendocrine cells, goblet cells; transit time to villus tip = 3-5 days
On reaching tip, cells are shed, die (anoikis), and are replaced
–anoikis is specialized type of apoptosis
Another population of proliferating cells moves downward from stem cell niche to bottom of crypt –> paneth cells (live for ~20 days, then phagocytosed and replaced)
Small Intestine Functions
a) complete digestion of CHOs, proteins, and fats
b) absorption of nutrients
- -1) sugars, lipids, AAs, calcium, and iron absorbed in duodenum and jejunum
- -2) bile salts, vitamin B12, water, electrolytes in ileum
- -3) very rapid due to villi and microvilli
Villi
a) capillaries absorb monosaccharides and AAs; lacteals absorb fats
b) intestinal crypts (Crypts of Lieberkuhn) w/ Paneth cells (secrete antibacterial molecules of lysozyme and defense) and mitotic stem cells (divide by mitosis to replenish intestinal cells eery 4-5 days)
- -> crypts constantly renewing everything
Microvilli (brush border)
Folding of apical surface of epithelial cell; site of “brush border enzymes” - stay attached to plasma membrane w/ active site exposed to chyme - hydrolyze disaccharides, peptides, and other substances to simple nutrient molecules
Intestinal Contractions and Motility
Role:
- 1) moves chyme aborally from mouth –> anus
- 2) mixes chyme w/ digestive secretions
- 3) breaks chyme into small particles, increase SA
Smooth muscle contractions occur automatically due to endogenous pacemaker activity w/in ENS
Migrating Myelectric Motor Complex (MMMC): slow wave of peristaltic activity occurring in post absorptive state (e.g. overnight fast) - sweeps gut clear of residue; ceases upon onset of eating
–starts in stomach and progresses all the way to anus
Peristalsis: net aboral (forward) movement by sequential muscular contraction and relaxation behind and ahead of bolus
Segmentation: mixing by simultaneous contractions both behind and ahead of bolus
What are the 2 types of Peristalsis?
- Primary
- -initiated in esophagus by swallowing - Secondary
- -initiated by distension (local ENS reflexes)
Segmentation
Has simultaneous contractions which serve to actually mix (not move); doesn’t effect net movement backward or forward
Large Intestine
Mucosa: columnar epithelial cells with goblet cells, crypts, lymphatic nodules, but NO VILLI
Functions:
- absorption of water, electrolytes, vitamin K, some B vitamins
- production of vitamin K and B vitamins via microbial organisms
- habitat for microflora
- storage/processing of feces
Digestion and Absorption - Overall Strategy
From food, humans must get basic organic molecules to make ATP, build tissues and serve as cofactors and coenzymes
- -digestion breaks polymers (CHOs, fats, and proteins) into monomer building blocks via hydrolysis reactions
- -absorption takes these monomers into bloodstream to be used by cells
Major nutrients are enzymatically split into their component molecules by hydrolysis (cleavage of bonds by H2O)
Digestion and Absorption of CHOs
Most CHO ingested as starch or sugars (e.g. sucrose, lactose)
- a) starch digestion begins in mouth (salivary amylase: polysaccharides –> shorter chains “dextrins”)
- b) no digestion in stomach - too acidic
- c) continuous in intestines w/ pancreatic amylase; short chains –> disaccharides and maltriose
- d) brush border enzymes finish breaking down disaccharides (maltose, sucrose, lactose) to simple sugars, mainly glucose
What’s the difference between pancreatic and salivary amylase?
No difference, they’re the same thing.
Both inhibited in stomach
Maltose is broken down by ____ into?
Maltase into 2 glucoses
Lactose is broken down by ____ into?
Glucose and galactose
Sucrose is broken down by ____ into?
Sucrase into glucose and fructose
Glucose absorbed via secondary active transport
Secondary active transport w/ sodium (Cl- follows)
Facilitated diffusion through GLUT carriers into interstitial fluid and then to capillary blood of villus
Water follows NaCl through paracellular route and is absorbed w/ glucose and NaCl
Digestion and Absorption of Proteins
Begins ins stomach w/ pepsin and HCl acid to produce short-chain polypeptides
Finishes in duodenum and jejunum w/ pancreatic trypsin, chymotrypsin, elastase, and carboxypeptidase, and the brush border enzyme aminopeptidase
Final products: AAs, some dipeptides and tripeptides
Free AAs cotransported w/ Na+
Dipeptides and tripeptides cross vida secondary active transport using a H+ gradient
–hydrolyzed into free AAs w/in cytoplasm of epithelial cells
Free AAs move by facilitated diffusion into interstitial fluid, then to the blood capillaries of villi
