The Digestive System Flashcards
Digestive system
Digestive tract
GI tract
Accessory organs
Digestive tract
Oral cavity > pharynx > esophagus > stomach > small intestine > large intestine > anus
Accessory structures
Teeth, tongue, glandular organs, salivary glands, liver, pancreas and gallbladder
Ingestion
Takes place when food enters the oral cavity
Mechanical digestion and propulsion
Involves crushing and shearing of food and then propelling the food along the digestive tract
Chemical digestion
Refers to the chemical breakdown of food into small organic and inorganic molecules suitable for absorption by the digestive epithelium
Secretion
Release of water, acids, enzymes, buggers and salts by the epithelium of the digestive tract, glandular organs and the gallbladder
Absorption
Movement of organic molecules, electrolytes, vitamins, minerals, and water across the digestive epithelium
Defecation
Elimination of wastes from the body
Pharynx
Muscular propulsion of materials into the esophagus
Esophagys
Transport of materials to the stomach
Stomach
Chemical digestion of materials by acid and enzymes; mechanical digestion through muscular contractions
Small intestine
Enzymatic digestion and absorption of water, organic substrates, vitamins and ions
Large intestine
Dehydration and compaction of indigestible materials in preparation for elimination
Salivary glands
Secretion of lubricating fluid containing enzymes that break down carbohydrates
Liver
Secretion of bile, storage of nutrients, many other vital functions
Gallbladder
Storage and concentration of bile
Pancreas
Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones
Peritoneum
Serosa/visceral peritoneum
Pariteal peritoneum
How can the visceral and parietal surfaces slide without friction?
Peritoneal fluid
Ascites
Abdominal swelling caused by the buildup of fluid in the peritoneal cavity
Mesentries
Double sheets of peritoneal membrane that connect with the parietal peritoneum with the visceral peritoneum
Lesser omentum
Stabilises the position of the stomach and provides an access route for blood vessels and other structures entering or leaving the liver
Falciform ligament
Helps stabilise the position of the liver relative to the diaphragm and abdominal wall
Greater omentum
Pouch formed from the enlarged dorsal mesentery
Contains adipose tissue that conform to the shape of the surrounding organs
Mesentery proper
Suspends the small intestine
Thick mesenterial sheet
Peritonitis
Inflammation of the peritoneum
Mesocolon
Mesentery associated with a portion of the large intestine
Layers of the digestive tract
- Mucosa
- Submucosa
- Muscular layer
- Serosa
Layers of the mucosa
- Mucosal epithelium: epithelial cells that differ depending on the place (stratified squamous where there’s mechanical stress, etc.)
- Laminar propria: areolar tissue
- Muscularis mucosae: smooth muscle and elastic fibres
Submucosa
- Dense irregular connective tissue
- Connects mucosa and muscular layer
- Submucosal neural plexus in the margin
Muscular layer
- Smooth muscle cells
- Coordinated by enteric nervous system
- Contains myenteric plexus
Serosa
Covers the muscular layer along portions of the digestive tract enclosed by peritoneal cavity
Adventitia
Fibrous sheath in areas lacking serosa
Peristalsis
Circular muscles contract behind the bolus while circular muscles ahead of the bolus relax
Longitudinal muscles ahead of the bolus then contract, shortening adjacent segments
A wave of contraction in the circular muscles then forces the bolus forward
Segmentation
Cycles of contraction that churn and fragment the bolus, mixing the contents with intestinal secretions
Local factors regulating digestive activities
- Primary stimulus
- Coordinate the responses to changes in the pH of the contents of the lumen, physical distortions of the wall, or the presence of chemicals
Neural control mechanisms
Myenteric reflexes (chemoreceptors, stretch receptors) Long reflexes (higher level control)
Hormonal control mechanisms
Peptides produced by enteroendocrine cells (endocrine cells in the epithelium of the digestive tract)
Functions of the oral cavity
- Sensory analysis
- Mechanical digestion
- Lubrication
- Chemical digestion
What lines the oral cavity?
Oral mucosa
Stratified squamous epithelium
What supports the mucosae of the cheeks?
Buccal fat pads and buccinator muscles
Oral vestibule
Space between the cheeks and the teeth
Gingivae
Gums
Ridges of oral mucosa surrounding the base of each tooth on the alveolar processes of the maxillae and the alveolar part of the mandible
Uvula
Dandling process that helps prevent food from entering the pharynx too soon
Palatoglossal arch
Between the soft palate and the base of the tongue
Fauces
Space between the oral cavity and the pharynx bounded by the soft palate and the base of the tongue
Palatopharyngeal arch
Extend from the soft palate to the pharyngeal wall
Functions of the tongue
- Mechanical digestion
- Manipulation
- Sensory analysis
- Secretion of mucins and lingual lipase
Lingual lipase
Digests lipids
Extrinsic tongue muscles
Large
Perform all gross movements
Intrinsic tongue muscles
Small
Change the shape of the tongue, precise movements
How does dentin differ from bone?
It doesn’t contain cells
Pulp cavity
Receives blood vessels and nerves through root canal
Apical foramen
Opening where blood vessels and nerves enter the root canal
What covers the dentin of the root?
A layer of cement
Gingival sulcus
A shallow groove surrounding the neck of each tooth
What covers the dentin of the crown?
A layer of enamel
Occlusal surface
Biting surface that grinds food against the opposing tooth surface
Formed by enamel
Cusps
Elevations or projections of the occlusal surface
Types of teeth
- Incisor teeth
- Canine teeth
- Premolar teeth
- Molar teeth
Incisor
Front of mouth
Blade shaped
Canine
Conical with a pointed cusp
Lie under the eye
Premolar
Have flattened crowns with two prominent cusps
Molar
Very large flattened crowns with 4-5 prominent rounded cusps
Decidious teeth
- Baby teeth
- Most children have 20
3 pairs of salivary glands
- Parotid glands
- Sublingual glands
- Submandibular glands
Parotid glands
- Contain salivary amylase which breaks down starches
- Drained by parotid ducts
Sublingual glands
- Covered by mucous membrane of the floor of the mouth
- Produce mucous
Submandibular glands
- Secrete a mixture of buffers, mucins and salivary amylase
What gives saliva its lubricating action?
Mucins
Functions of saliva
- Lubricating the mouth
- Lubricating food in the mouth
- Dissolving chemicals
- Beginning digestion
What controls salivary secretions?
Autonomic nervous system
Each salivary gland has sympathetic and parasympathetic innervation
Superior and inferior salivatory nucleus
Where parasympathetic efferents originate
In the medulla oblongata
Pharynx
Throat
Serves as a common passageway for solid food, liquid, and air
Esophagus
Actively moves ingested materials down towards the stomach
Esophageal hiatus
Opening in the diaphragm where the esophagus enters the abdominopelvic cavity
Wall of the esophagus
Mucosa, submucosa and muscular layer
Function of folds in the esophagus
They allow for expansion during the passage of a large bolus
Where are esophageal glands?
Scattered among the submucosa
They produce mucus that reduces friction between bolus and esophageal lining
What innervates the esophagus?
Parasympathetic and sympathetic fibres from the esophageal plexus
Degluition
Swallowing
Phases:
- Buccal
- Pharyngeal
- Esophageal
- Bolus enters stomach
Buccal phase
- Bolus compresses against hard palate
- Tongue retracts, bolus goes into oropharynx
- Soft palate seals off nasopharynx
Pharyngeal phase
- Begins when bolus comes into contact with palatal arches and the posterior pharyngeal wall
- Bolus goes past closed glottis
- Uvula and soft palate block nasopharynx
Esophageal phase
- Begins at contraction of pharyngeal muscles which forces bolus into esophagus
- Peristatic wave pushes bolus into stomach
Bolus enters stomach
Lower esophageal sphincter opens and bolus goes to stomach
Swallowing reflex
Automatic reflex
Begins when tactile receptors on the palatal arches and uvula are stimulated by the passage of the bolus
Swallowing centre
In the medulla oblongata
Primary peristaltic waves
Coordinated by afferent and efferent fibres
Secondary peristaltic waves
Needed if bolus is dry
Functions of the stomach
- Temporarily store ingested food received from the esophagus
- Mechanically digest food through muscular contractions
- Chemically digest food through the action of acid and enzymes
Chyme
Viscous, acidic, soupy mixture of partially digested food
Lesser curvature
Short
Medial surface of stomach
Greater curvature
Long
Lateral surface of stomach
Regions of the stomach
- Cardia: smallest, contains glands
- Fundus
- Body: largest region, mixing tank
- Pyloric: between the body and the duodenum
Pyloric part
Divided into pyloric antrum (connected to the body) and the pyloric canal (empties into the duodenum) and the pylorus which is the muscular tissue surrounding the pyloric orifice
What regulates the release of chyme in the duodenum?
The pyloric sphincter
Rugae
Folds of the stomach
Gastric pits
Shallow depressions that open onto the gastric surface
What secretes most of the acids and enzymes involved in gastric digestion?
Gastric glands in the fundus and body
What secretes mucous and digestive hormones?
Glands in the pyloric part
What two cells dominate the gastric glands?
Parietal cells and chief cells
They secrete gastric juice
Parietal cells
Secrete intrinsic factor, a glycoprotein that helps absorb vitamin B12 across the intestinal lining
Secretion of hydrochloric acid ions
- Hydrogen ions are generated inside parietal cells as carbonic anhydrase converts CO2 and H2O to carbonic acid which then disssociates
- An anion countertransport mechanism ejects the bicarbonate ions into the interstitial fluid and imports chloride ions into the cell
- Chloride ions diffuse across the cell and exit through open chloride channels into the lumen of the gastric gland
- Hydrogen ions are actively transported into the lumen of the gastric gland
Functions of the highly acidic stomach contents
- Kills most of the microorganisms ingested with food
- Denatures proteins and inactivates most of the enzymes in food
- Helps break down plant cell walls and the connective tissue in meat
Chief cells
Most abundant near the base of a gastric gland
Secrete pepsinogen which is converted into pepsin, a protein digesting enzyme
What do the stomach of newborn infants produce?
Rennin and gastric lipase which are important for the digestion of milk
Glands in the pyloric part
Produce mucous secretions
G cells produce gastrin which stimulate secretions of parietal and chief cells and contractions of the gastric wall
D cells release somatostatin, hormone that inhibits release of gastrin
Grehlin, leptin and obestatin
Play a role in hunger and satiety
Chemical digestion
When ingested food reaches stomach, salivary amylase and lingual lipase continue digestion of carbs and lipids
They continue to work until pH falls below 4.5
Pepsin breaks down complex proteins into smaller peptides
Regulation of gastric activit
- Cephalic phase
- Gastric phase
- Intestinal phase
Cephalic phase
Begins when you see, smell, taste, or think of food
Vagus nerves innervate submucosal plexus of the stomach
Postganglionic parasympathetic fibres innervate mucous cells, chief cells, parietal cells, and G cells of the stomach
Production of gastric juice speeds up
Gastric phase
Begins with the arrival of food in the stomach
Local response: histamine released, stimulates acid secretion
Neural response: stimulation of myenteric plexus produces mixing waves
Hormonal response: gastrin secreted, pH of gastric juice reduced
Intestinal phase
Begins when chyme first enters small intestine
Controls rate of gastric emptying
Neural response: enterogastric reflex inhibits gastrin production and contractions
Hormonal response: CCK and gastric inhibitory peptide, secretin and gastrin
Central reflexes
Gastroenteric reflex: stimulates motility and secretion along small intestine
Gastroileal reflex: triggers opening of the ileocecal valve, materials can move from small to large intestine
Emesis
Vomiting
Pyloric sphincter relaxes, contents of the duodenum discharged back into the stomach by peristaltic waves
Gastroesophageal sphincter relaxes and the stomach regurgitates its contents through the esophagus and pharynx and out through the mouth
Bile
Solution stored in the gallbladder for discharge into the small intestine
Contains buffers and bile salts
Pancreas
Lies posterially to the stomach
Retroperitoneal
What surrounds the pancreas?
A connective tissue capsule
What drains the pancreas?
Splenic vein and its branches
Pancreatic duct
Delivers pancreatic secretions of the pancreas to the duodenum
What divides the pancreas into distinct lobules?
Connective tissue
Pancreatic acini
Clusters of cells that produce digestive enzymes and secretions
Pancreatic islets
Perform endocrine function of pancreas
Scattered among pancreatic acini
Secrete insulin and glucagon
Exocrine pancreatic cells
Acinar cells and epithelial cells that line the duct system
Pancreatic juice
Alkaline mixture of digestive enzymes, water and ions secreted by exocrine cells into the small intestine
Secretin
Triggers pancreatic secretion of watery buffer solution that raise pH of chyme
CCK
Stimulates production and secretion of pancreatic enzymes
Pancreatitis
Inflammation of the pancreas
Pancreatic enzymes
Pancreatic alpha-amylase: breaks down starches
Pancreatic lipase: breaks down complex lipids
Nucleases: break down RNA or DNA
Proteolytic enzymes: break apart proteins inc. proteases and peptidases
Proenzymes secreted by the pancreas
Trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase, chymotrypsin, carboxypeptidase, elastase
Liver
Largest visceral organ
Layers of the liver
Tough fibrous capsule and visceral peritoneum
Falciform ligament
Marks the division between the left and right lobes of the liver
Round ligament
Thickening in the posterior margin of the falciform ligament
Hepatocytes
Liver cells
Hepatic veins
Open into the inferior vena cava
Return blood to system circuit
Lobules
Basic functional unit of the liver
Central vein
Within a lobule, sinusoids between adjacent plates empty into the central vein
Stellate macrohages
Line sinusoids of liver
Engulf pathogens, cell debris and damaged blood cells
Portal triads
- There are six
- They contain an interlobular vein, an interlobular artery and an interlobular bile duct
Bile duct system
Bile canaliculi > bile ductules > right and left hepatic ducts > common hepatic duct > bile duct (duodenal ampulla) or cystic duct (gallbladder)
Liver functions
- Metabolic regulation
- Hematological regulation
- Bile production
Bile salts
Assortment of lipids that play a role in digestion of lipids
Emulsification
Bile salts break the droplets apart in a process
Enterohepatic circulation of bile
Cycling of bile salts from the liver to the small intestine and back
Regions of the gallbladder
- Fundus
- Body
- Neck
Duodenal ampulla
Receives buffers and enzymes from the pancreas and bile from the liver and gallbladder
Duodenal papilla
Duodenal ampulla opens into the duodenum at the duodenal papilla
Hepatopancreatic sphincter
Encircles lumen of the bile duct, pancreatic duct and duodenal ampulla
Major function of the gallbladder
Bile storage
Bile modification
When is bile released into the duodenum?
Only under stimulation of CCK
Small intestine
Long muscular tube where chemical digestion is completed and products of digestion are absorbed
What restricts movement of the small intestine during digestion?
Stomach, large intestine, abdominal wall, and pelvic girdle
Segments of the small intestine
Duodenum, jejunum and ileum
Duodenum
Closest to stomach
Mixing bowl, receives chyme from stomach and digestive secretions from pancreas and liver
Ileocecal valve
Ileum ends here
Circular folds
Transverse folds of small intestines
Intestinal villi
Increase SA for absorption
Lacteal
Central lymphatic vessel on each villus
Transport minerals that can’t enter blood capillaries
Goblet cells in intestinal glands
Eject mucins onto the intestinal surfaces
Intestinal glands
Entrance at the base of the of the villi
Stem cells divide at the base of each intestinal glands
Paneth cells
At the base of the intestinal glands have a role in innate immunity and release defensins and lysozome
Duodenal submucosal glands
In intestinal submucosa
Produce copious quantities of mucus when chyme arrives from the stomach
Mucous protects epithelium from acidity of chyme
Functions of intestinal juice
Moistens chyme, helps buffer acids, keeps both the digestive enzymes and the products of digestion in solution
Hormones of the duodenum
Gastrin, secretin, GIP, CCK, VIP and enterocrinin
Gastrin
Trigger: large quantities of incompletely digested proteins
Response: Increased stomach motility and gastric enzymes and secretions
Secretin
Trigger: chyme arriving in the duodenum
Response: increased pH of chyme and secretion of bile
Gastric inhibitory peptide
Trigger: Fats and carbs enter small intestine
Response: inhibition of gastric activity and insulin release
CCK
Trigger: when chyme arrives in duodenum
Response: increases production and secretion of digestive enzymes
Vasoactive inactive peptide
Stimulates secretion of intestinal glands
Enterocrinin
Trigger: chyme entering the duodenum
Response: alkaline mucus production by submucosal glands
Large intestine
Stores digestive waste and reabsorbs water
Cecum
Expanded pouch where material arriving from the ileum first enters
Collects and stores materials from the ileum and begins process of compaction
Appendix
Attached to cecum and ileum by the meso-appendix
Lymphoid nodules dominate the mucosa and submucosa of the appendix
Haustra
Pouches on the wall of the colon that permit colon to expand and elongate
Teniae coli
Bands of smooth muscle on the surface of the colon
Omental appendices
Sacs of fat in the serosa of the colon
Segments of the colon
- Ascending
- Transverse
- Descending
- Sigmoid
Right and left colic flexure
Bends in the colon
Rectum
Expandable organ for temporary storage of feces
Anal column
Last portion of the rectum
Anal columns
Folds in the anal column
Anus
Exit of the anal canal
Internal anal sphincter
Involuntary control
External anal sphincter
Voluntary control
Characteristics of the colon
- No villi
- Abundance of goblet cells
- Intestinal glands
Function of the large intestine
Absorption of water
Absorption of bile salts, vitamins, organic wastes, and toxins
Where do most of the gut bacteria live?
Large intestine
Urobilins and stercobilin
Give feces its colour
Flatus
Intestinal gas produced by metabolic activity of bacteria in the colon
Mass movements
Move material from transverse colon to the large intestine, forcing feces into rectum
Defecation reflex
- Feces move into rectum causing distension, stimulating stretch receptors
- Intrinsic myenteric defecation reflex
- Parasympathetic defecation reflex
- Voluntary relaxation of the external anal sphincter
Intrinsic myenteric defecation reflex
Short feedback loop that triggers a series of peristaltic contractions in the rectum that moves feces toward the anus. Mediated by myenteric plexus in sigmid colon and rectum. Internal anal sphincter relaxes
Parasympathetic defecation reflex
Long feedback loop is a spinal reflex coordinated by the sacral parasympathetic system, stimulating mass movements. Further relaxes the internal anal sphincter
Nutrient use
- Digestive system breaks down physical structure
- Disassembles molecules into smaller fragments
- Cells absorb the fragments into bloodstream and: use them for ATP synthesis or carbs, proteins and lipid synthesis
Hydrolysis
Digestive enzymes break the bonds between the component molecules of carbs, proteins, lipids and nucleic acids
Digestion of carbohydrates
- Salivary amylase and pancreatic alpha-amylase
2. Brush border enzymes break di and trisaccharides into mono
Maltase
Splits bonds between two glucose molecules of the disaccharide maltose
Sucrase
Breaks the disaccharide sucrose into glucose and fructose
Lactase
Hydrolyses the disaccharide lactose into a molecule of glucose and one of galactose
Digestion of lipids
Involves lingual lipase and pancreatic lipase
Lipases
Water soluble enzymes
Micelles
Small lipid-bile salt complexes
Chylomicrons
Triglycerides, absorbed steroids, phospholipids and fat soluble vitamins
Protein digestion
- Mechanical digestion in oral cavity
2. Chemical digestion in the stomach through hydrochloric acid
Vitamins
Organic compounds
Water soluble
Fat soluble: ADEK