Digestive System Flashcards
What is digestion?
part of the process where large molecules are broken down into smaller components
-mechanical and chemical breakdown
4 layers of tissue in GI tract
- mucosa
- submucosa (dense c.t.)
- muscular layer
- serosa
What is in the mucosa?
-epithelial
-lamina propria
-muscularis mucosa
Regulation of GI Functions
1 - combination of CNS and endocrine system
2- enteric nervous system with intrinsic endocrine paracrine component
Oral Cavity
- aka Buccal
- 2 parts: vestibule and oral
Vestibule of Buccal Cavity
space b/w lips and cheeks, and outer surface of teeth
Oral Cavity of Buccal Cavity
bordered by inner surface of teeth, and hard/soft palates
What is the crown of the tooth covered by?
enamel
What forms the bulk of a tooth?
dentin
What does dentin surround?
pulp cavity that contains blood and nerves
Cementum
-thin
-covers roots of brachyodont teeth and most of the entire tooth superficial to the enamel in hypsodont teeth
Brachyodont teeth
-found in carnivores, humans, pigs, and ruminant incisors
-small crown, well developed roots
*do not grow continually
Hypsodont teeth
-horses incisors and cheek teeth, boar’s tusks, rodents, lagomorphs
-large reserve of crown beneath gingiva
*grow continually
2 categories of Hypsodont teeth
radicular and aradicular
Radicular Hyposodont
-apices of roots remain open for a long time
*apices eventually close and stop growing
-found in cheek teeth of horses
Aradicular Hypsodont
-lack a true root
*grow continuously throughout life of animal
-found in lagomorphs and some rodents
Deciduous Teeth
aka milk teeth/baby teeth
-smaller, whiter
-present in jaw at birth
-erupt through gums at different times depending on species
Tongue
-muscular structure on ventral surface in oral cavity
-papillae on dorsal surface
-pain, temperature, touch, and thermoregulation through panting
Papillae functions
mechanical: grooming and moving food into pharynx
specialized: taste sensations
Amylase
starch digesting enzyme
-dogs, cats, ruminants lack salivary amylase
Salivary bicarbonate
ruminants (very basic) helps neutralize
Functions of Saliva
-lubrication
-antibacterial action
-pH regulation
-thermoregulation
-enzymatic digestion
Saliva is secreted by 3 main paired glands
- parotid
- mandibular
- sublingual
Salivary mucocele
saliva accumulates in the tissue due to damaged salivary gland or duct
Temporomandibular Joint = TMJ
-connections b/w condylar process of mandible and mandibular fossa of temporal bone
-used for extension, flexion, translation (mandible movement laterally and rostrally)
Pharynx
-part of GI tract and respiratory tract
-eustachian tube, tonsils
Esophagus
-muscular tube connecting pharynx to stomach
-lined w/ mucosa
-tunica muscularis: 2 layers (circular and longitudinal) needed to move food
-starts DORSAL to the trachea then goes to the LEFT of it
Deglutition
swallowing
stage 1: voluntary
stage 2: pharyngeal stage- involuntary, epiglottis covers glottis
stage 3: esophageal stage- initiates PERISTALSIS
Salivary secretion is
-regulated by the nervous system
-triggered by conditioned responses
Peristalsis
pattern of muscle contraction that propels food through the GI tract
Serous Membranes found in abdominal cavity:
-visceral peritoneum
-parietal peritoneum
-mesentery
-omentum
What does the Omentum do?
stores fat and insulates abdomen
Functions of the stomach
-storage of ingested food
-mechanical and chemical breakdown of food
-production of intrinsic factor for vitamin B12 absorption in small intestine
Food leaves stomach in semi liquid form called
chyme
-usually hypertonic and has low pH
Monogastric
simple, single stomach
-dogs, cats, horses
Ruminant
complex stomach consisting of 4 chambers
-cows, goats, sheep
Anatomy of the Monogastric Stomach
-c- shaped, behind diaphragm
parts:
cardia, fundus, body, antrum, pylorus
Rugae
transient folds in gastric muscosa
-allows stomach to expand when filled with food
-increase surface area for absorption
Gastric Pits
different glandular cells, each with different secretions
Chief cells
pepsinogen
Parietal cells
HCL/intrinsic factor
Pepsin
-proteolytic enzyme (break apart proteins)
-begins chemical digestions of proteins
-activated in the fundus
-starts as pepsinogen
Pepsinogen is converted to pepsin in the presence of
HCL
Pyloric Gland Region of Stomach
- last glandular portion of stomach
-opens into duodenum through pyloric sphincter
-cells found here: mucous secreting cells, G cells secreting GASTRIN
What 3 substances stimulate secretions by glandular cells?
-acetylcholine
-gastrin
-histamine
Acetylcholine
-from cholinergic neurons
-stimulate chief and parietal cells
Gastrin
-released by G cells
-stimulate mainly parietal cells indirectly which causes ECL to release hist
Histamine
-secreted by ECL cells in gastric mucosa
-stimulate many parietal cells
Acetylcholine and Histamine act on
parietal cells to increase H and Cl production
Gastrin indirectly
works to release histamine
Cephalic phase of secretion
anticipation of eating a meal
*acetylcholine is released and bind to receptors:
-parietal cells secrete H+ and Cl ione
-chief cells secrete pepsinogen into stomach
-G cells secrete gastrin into bloodstream
Gastric phase of secretion
begins when food enters stomach
-glandular cells stimulated by stretching of stomach wall
-peptides formed by protein breakdown
-local enteric reflexes, and vagal reflexes to and from brain
-released acetylcholine stimulates secretions
-stomach can be acidic as ph 2.0
Monogastric Stomach Motility
*muscle contractions of stomach wall help break down food particles
-gastric contents released via pyloric muscle contractions through the pyloric sphincter at controlled rate
What does the fundus do?
expands to accommodate large volumes of food
What does the body of the stomach do?
large mixing chamber
What does the pyloric antrum of the stomach do?
acts like a pump
Peristaltic Contrations
-movements weak in fundus and body
-get stronger as food moves toward pylorus
*larger particles prevented from leaving stomach
-retropulsion: forced backwards to continue grinding
Control of Gastric Motility involves
neurohumoral control = both neurotransmitters and hormones
Fiber of Vagus Nerve X
synapse on cells in gastric myenteric plexus
What does gastrin do?
causes motility to increase but it is more important for the secretion of gastric juices
-stimulated by the presence of food
What is gastrin?
hormone produced by cells in stomach wall; acts upon stomach wall
-causes gastric stomach glands to secrete hydrochloric acid and digestive enzymes
-encourage muscular contractions of stomach wall
What is the rate of gastric emptying controlled by?
-strength of pyloric antral muscle contractions
-(lesser extent) degree of contraction of pyloric sphincter
What hormones released by small intestine can also delay gastric emptying?
-secretin
-cholecystokinin
Secretin
stimulates pancreas to secrete fluid to neutralize acidic chyme
Cholecystokinin
stimulates pancreas to release digestive enzymes
What is essential to increasing amount of food surface area?
mechanical digestion
Chemical digestion
-digestive enzyme responsible
- 2 phases: luminal digestion and membranous chemical digestion (both hydrolysis)
Carbohydrates
made of repeating units of monosaccharides
-starches
-sugars
-cellulose
Chemical digestion of Carbs
-some animals begins with AMYLASE in saliva
(mostly done in the stomach)
-for those who do not have that, begins with PANCREATIC AMYLASE in small intestine
Proteins
repeating amino acid units
-protein bigger than peptide
Lipids
majority of fat found in diet
Protein digestion
-begins in stomach; continues in small intestine
-pepsinogen released by chief cells, activated by HCL to pepsin
What does pepsin do?
breaks down protein into peptide chains
Abomasum
functions the same as monogastric stomach
“true stomach”
Forestomach
(1st 3 chambers)
-reticulum
-rumen
-omasum
Forestomachs are
non glandular
-microorganisms for fermentation in rumen and reticulum
Rumen
“the paunch”
-occupies most of left side of abdominal cavity
-microorganisms ferment carbohydrates
Reticulum
“the honeycomb”
-cranial to rumen
-another area for fermentation
-contents easily enter and exit
Omasum
“many pile” or “book stomach”
-absorption of water and salts here
Esophageal Groove
links esophagus with omasum in young ruminant
-milk from nursing bypasses reticulum and it goes to omasum and abomasum
If esophageal groove did not exist
-bacteria in reticulorumen would ferment milk
-lactic acid produced would inhibit normal microbial action
Abomasum
elongated “true stomach”
-lined with glandular tissue
-functions like simple monogastric stomach BUT IT IS NOT A STORAGE COMPARTMENT
-continuous flow if ingesta
Abomasum in young animals
released renin causes milk protein coagulation
-prolongs time for pepsin to break down proteins
Motility of ruminant stomach
ruminant create a lot of saliva that is very alkaline due to bicarbonate to help neutralize the acids produced by fermentation
Contractions in Reticulorumen
primary = mixing contractions
Rumination
chewing the cud
-ingesta moves from reticulorumen back into oral cavity for additional saliva and chewing
Eructation
belching/secondary contractions
-release of gas
-without = bloat
Reticulorumen Ecosystem
wide variety of bacteria, protozoa, and fungi
-proper balance b/w types acquired after birth
VFA’s
main source of energy for ruminant (in monogastric this is glucose)
Gluconeogenesis
occurs in liver from non-carbohydrate sources
Small intestine
-tube that carries chyme away from the stomach and deposits it into the large intestine
-3 parts: duodenum, jejunum,ileum
Duodenum
receives chyme through pyloric sphincter
Jejunum
-longest part
-most chemical digestion and absorption occur here (in animals that do not rely on fermentation)
Ileum
can not grossly tell the difference b/w jejunum and ileum
-empties into large intestine at cecum (horse), at colon (dog and cat), or at cecum and colon (ruminant and pig)
Adaptations to small intestine to increase surface area
-plications (folds) in mucosal lining
-villi in mucose contain microvilli (brush border)
-intestinal crypts
Lacteals
lymphatic capillaries
-carry absorbed lipids and fat soluble substances to thoracic duct, and into vena cava
Blood capillaries
collect some absorbed nutrients and transport the to the liver
Functions of Cholecystokinin
-inhibits gastric emptying
-causes increased secretion of pancreatic digestive enzymes and of bicarbonate
-triggers gall bladder contraction
-stimulates secretion of enteropeptidase (from duodenum); trypsin to trypsinogen
Functions of Secretin
-decreases HCl production in the stomach
-increases pancreatic and biliary bicarbonate secretions
Pancreas functions
-endocrine: pancreatic islets
-exocrine: groups of acini
Pancreatic Islets
-contain several different cell types
-beta cells secrete insulin
-alpha cells produce glucagon
Groups of Acini
-ducts merge to converge into pancreatic duct
-excretions contain bicarbonate and digestive proenzymes
-anticipation of food causes increase of secretions
-neural and endocrine stimuli increase secretions
What enzymes are vital to digestion?
lipase, amylase, nuclease, protease
Proenzyme (Zymogen)
all proteolytic enzymes secreted in inactive form
-CCK starts the activation process
Trypsinogen into Trypsin is responsible for
activating the other zymogens released by pancreas
Liver
largest digestive gland in the body
Functions of the liver
-secretes substances essential for digestion and absorption of nutrients
-synthesizes nutrients and regulates their release into the bloodstream
-excretes toxic substances
-produces plasma proteins, cholesterol, and blood coagulation factors
Liver cells
hepatocytes
How does the liver receive blood?
hepatic portal vein (blood with all GI nutrients/toxins) and hepatic artery (O2 blood)
Blood leaves liver via
hepatic vein
Gallbladder
concentrates and stores bile until needed
Composition of bile
-bile salts
-phospholipids
-cholesterol
-bile pigments
