Digestive System Flashcards
What is the digestive system
Digestive system processes food, extracts nutrients from it, and eliminates the residue
5 stages
- Ingestion:
• Selective intake of food - Digestion:
• Mechanical and chemical breakdown of food into a form usable by the body - Absorption:
• Uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph - Compaction:
• Absorbing water and consolidating the indigestible residue into feces - Defecation:
• Elimination of feces
Types of digestion- mechanical
Mechanical digestion- the physical breakdown of food into smaller particles
• Cutting/grinding action of the teeth
• Churning in stomach and small intestines
• Exposes more food surface to the action of digestive enzymes
Types of digestion- chemical
Chemical digestion–a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
• Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine
• Polysaccharides into monosaccharides
• Proteins into amino acids
• Fats into monoglycerides and fatty acids
• Nucleic acids into nucleotides
• Vitamins, minerals, free amino acids, cholesterol and water are absorbed without being digested
Anatomy of digestive system - two anatomical subdivisions
- Digestive tract
• Long muscular tube extending from mouth to anus
• Mouth, pharynx, esophagus, stomach, small intestine, and large intestine
Gastrointestinal (Gl) tract is the stomach and intestines - Accessory organs( help aid the digestive track)
• Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Innervation - enteric nervous system
A nervous network in the esophagus, stomach, and intestines
- regulates digestive tract mobility, secretion and blood flow
ENS contains sensory neurons that monitor tension in gut walk and conditions in lumen
Composed of two networks of neurons
1. Submucosal (Meissner) plexus: in submucosa
• Controls glandular secretion of mucosa
• Controls movements of muscularis mucosae
2. Myenteric (Auerbach) plexus: parasympathetic ganglia and nerve fibers between the of the muscularis interna
Regulations of digestive tract
Neural control
• Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus
• Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion
Hormonal
• Chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract
• Gastrin and secretin
Paracrine secretions
• Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
Messnteries
Provide Structure and support for intestine, stop them from twisting and any complications from occurring. They stop abnormal moments
Two mesenteries called OMENTA
- lesser omentum
- greater OMENTA
Mouth - or oral or buccaneers cavity
Function
• Ingestion (food intake)
• Other sensory responses to food: chewing and chemical digestion
• Swallowing, speech, and respiration iod sauamous epithelium
Stratified squamous epithelium lines the mouth
• Keratinized in areas subject to food abrasion: gums and hard palate
• Nonkeratinized in other areas: floor of mouth, soft palate, and insides of cheeks and lips
Tongue
Muscular, bulky, but agile and sensitive organ
• Manipulates food & avoids being bitten
• Extract food particles from the teeth
• Sensitive enough to feel a stray hair
Structure
•Nonkeratinized stratified squamous epithelium covers its surface ingual papillae: bumps and projections on the tongue that are the ites of the taste buds
•Body: anterior two-thirds of the tongue occupies oral cavity
•Root: posterior one-third of the tongue occupies the oropharynx
-Boundary marked by a row of V-shaped vallate papillae
- Behind these a grove called the terminal sulcus
-Intrinsic muscles within the tongue control speech
- Extrinsic (outside) muscles produce the stronger movements of food manipulation
Teeth
Masticate food into smaller pieces
• Makes food easier to swallow
• Exposes more surface area for action of digestive enzymes speeding chemical digestion
Structure
32 adult teeth; 20 deciduous (baby) teeth
• 16 in both mandible and maxilla
• Incisors chisel-like cutting teeth used to bite off a piece of food
• Canine pointed and act to puncture and shred food
•Premolars- broad surface for crushing and grinding
• Molars- even broader surface for crushing and grinding
Mesticatio- chewing
Chewing_-breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes
• First step in mechanical digestion
• Tongue, buccinator, and orbicularis oris manipulate food
• Masseter and temporalis elevate the teeth to crush food
• Medial and lateral pterygoid and masseters produce the side-to-side grinding action
Saliva
Comes form
-Intrinsic salivary glands small glands dispersed € other oral tissues
- Extrinsic salivary glands three pairs connected to oral cavity by ducts
Functions
• Moisten mouth
• Begin starch and fat digestion
• Cleanse teeth
. Inhibit bacterial growth
• Dissolve molecules so they can stimulate the taste buds
• Moisten food and bind it together into bolus to aid in swallowing
• pH: 6.8 to 7.0
Pharynx
A muscular funnel that connects oral cavity to esophagus and allows entrance of air from nasal cavity to larynx
Pharyngeal constrictors–circular muscles that force food downward during swallowing
When not swallowing, the inferior constrictor remains contracted to exclude air from the esophagus
Esophagus
A straight muscular tube 25 to 30 cm
long
•From pharynx to cardiac orifice of stomach
• Lower esophageal sphincter: food pauses at this point because of this constriction
-Prevents stomach contents from regurgitating
- Protects esophageal mucosa from erosive effect of the stomach acid
-Heartburn
Structure
- Nonkeratinized stratified squamous epithelium
-Esophageal glands in submucosa secrete mucus
- Deeply folded into longitudinal ridges when empty
- Skeletal muscle in upper 1/3, mixture in middle 1/3, and only smooth
muscle in the bottom 1/3
Swallowing
Controlled by
• Swallowing centre of medulla (trigeminal, facial, glossopharyngeal & hypoglossal nerves)
Three phases
1. Oral phase: under voluntary control
•Tongue collects food, presses it against the palate forming a bolus, and pushes it posteriorly
• Food accumulates in oropharynx in front of epiglottis
Epiglottis opens and bolus slides to laryngopharynx
2. Phalangeal phase: involuntary
• Soft palate and tongue root stop food/ drink entering nose and mouth to prevent chocking
Breathing paused & larynx pulled up to epiglottis
• Oesophagus widens, phalangeal constrictors contract (superior to inferior) pushing bolus into the
3. Oesophageal phase: is involuntary waves (peristalsis)
• Controlled by brain and myentric plexus
• Stretch receptors stimulated & transmitted to muscularis externa above & below
• Circular muscle above contracts, below relaxes, and longitudinal muscle contracts to pull oesopha shorten
Peristalsis
Wave of muscular contraction that pushes the bolus ahead of it- involuntary reflex
• Entirely involuntary reflex
hen standing or sitting upright, the food and liquid drops through he esophagus by gravity faster than peristalsis can keep up with it
• Ensures you can swallow regardless of body position
• Liquid reaches the stomach in 1-2 sec
• Food bolus in 4-8 sec
• When it reaches lower end of the esophagus, the lower esophageal sphincter relaxes to let food pass into the stomach
Stomach
Primary function
-Mixing compartment
-Storage organ adapts in size depending on food and food can remain 2-4 hours
- secrete gastric juice ( water, mucus
, hydrochloric acid , intrinsic factor and enzyme pepsinogen
Stomach creates chyme
38 О
Gross anatomy of stomach
Divided into four (4) regions
1. Cardiac region (cardia) small area within about 3 cm of the cardial orifice
2. Fundic region (fundus).
dome-shaped portion superior to
esophageal attachment
3. Body (corpus) makes up the greatest part of the stomach - where mixing in the stomach occurs
4. Pyloric region: narrower pouch at the inferior end
• Subdivided into the funnel-like antrum
•Pylorus: narrow passage to duodenum
•Pyloric (gastroduodenal) sphincter regulates the passage of chyme into the duodenum ( valve)
Microscopic anatomy
Wall has four (4) layers = serosa,
muscularis, submucosa, mucosa
1. Mucosa = deepest layer (three layers)
• Epithelium, lamina propria, and muscularis mucosae
2. Submucosa = connective tissue and vascular plexus
• Mucosa and Submucosa, flat when full but wrinkle (rugae) when empty
3. Muscularis - 3 layers
• Outer longitudinal layer
• Middle circular layer
• Inner oblique layer
4. Serosa (visceral peritoneum) - outermost tunic of stomach
• Simple squamous epithelium (outer)
• Connective tissue (inner)
Gastric secretion
Gastric juice
• 2 to 3 L per day produced by the gastric glands
• Mainly a mixture of water,
hydrochloric acid, and pepsin
• High concentration of hydrochloric
(HCI) acid
• pH as low as 0.8
Hydrochloric acid
• Activates enzymes pepsin and lingual lipase
• Breaks up connective
tissue and plant cell walls
Help liquefy food and form chyme
•Converts ingested ferric ions to ferrous ions
• Contributes to nonspecific disease resistance
Pepsin
• Pepsinogen - removal of AA’s
> pepsin
- Digests proteins
Gastric Lipase
• Digests 10-15% of dietary fat
Intrinsic Factor
Glycoprotein
•Absorption of Vit
B12
Gastric motility
As you swallow - swallowing centre of medulla oblongata signals stomach to relax
- Activates receptive-relaxation response of smooth muscle
-Arrival of food stretches stomach
Rhythm of peristaltic contractions
•Gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice
• Becomes a stronger contraction at pyloric region
• After -30 min these contractions become quite strong
• Churn food
• Mix it with gastric juice; and
• Promote its physical breakup and chemical digestion
Regulation of gastric function
- Cephalic Stage (controlled by the brain)
• Stomach responds to the sight, smell, taste or thought of food
• Vagus nerve (from medulla oblongata) stimulates enteric NS
- 40% of juices, in preparation of food - Gastric Phase (controlled by itself
•Period in which swallowed food activates gastric activity
• 2/3 gastric secretion during this stage
• Ingested food stimulates activity by stretching and raising pH - Intestinal Phase (controlled by small intestine)
- Period in which duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes
-Initially stimulates gastric secretion but later inhibits it
Liver
• Mass 1.4 Kg
• Four (4) main functions (but not all)…
1. production of bile, urea, heparin, vitamin
A, plasma proteins, and antibodies
2. Metabolism of fats, amino acids, sugars, and toxins
3. Storage of fats, vitamins A, D, and B12, iron, and glycogen
4. Activates vitamin D
Anatomy of liver
Four (4) lobes-right, left, quadrate, and
caudate
• Falciform ligament separates left and right lobes
• Sheet of mesentery that suspends the liver from the diaphragm
• Gallbladder depression on inferior surface
• Inferior Vena Cava sulcus on posterior aspect
Microscopic Anatomy
Hepatic lobules
- central vein helps with Venus blood drainage
- hepatocytes
- hepatic sinusoids are blood filled channels. Allows transfer of molecules and nutrients. is hepatocytes from blood cells
• Absorb glucose, amino acids, iron, vitamins & nutrients required for metabolism
• Degrade hormones, toxins, bile pigments and drugs
Hepatic triad:
•Branch of hepatic artery supply oxygenated blood to liver
• Branch of hepatic vein carry nutrient rich blood from the same intestine and facilitate drainage of deoxygenated blood and wats substance from liver
• Bile duct helps transport bile
Liver circulation
Liver receives blood from 2 sources: ~70% hepatic portal vein & 30% hepatic arteries
• Hepatic portal vein = blood from stomach, intestines, pancreas, spleen
• All nutrients except lipids
• Hepatic arteries = arterial blood from aorta at the celiac trunk to hepatic artery proper
Meet and drain into sinusoids; mixing venous and arterial
Gallbladder
Stores and concentration of bile
with simple columnar epithelium tissue inside it, which helps with absorption and secretion related to that bio management.
Bile
It contains a mix of minerals, cholesterol, neural neutral fats, phospholipids, bile pigments, and bile acids.
Green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments and bile acids.
Bilirubin - principle pigment comprised of decomposed hemoglobin
Reaches small intestine turns to
Urobilinogen -
• Urobilin responsible for yellowish colour of urine
- Remiaining urobilinogen responsible for brown colour of feces
Bile acids - steroids synthesised from cholesterol
-With lecithin (phospholipid and micelles - aid in fat digestion and absorption
All other components = wastes
• In excess, can lead to gallstones
Bile is collected in the gallbladder with the bile duct reaches full capacity
Reabsorbs water and electrolytes
• Increase concentration of bile 5-20x
Liver secretes about 500 to 1,000 mL of bile daily
80% of bile acids are reabsorbed in the ileum and returned to the liver
20% of the bile acids are excreted in the feces
Bile needs to know
while is green. And it’s a digestive fluid that’s made in the liver stored in the gallbladder and helps digest.It helps with digestion and helps with absorption of fats.
Pancreas
Spongy retroperitoneal gland posterior to the stomach
Measure 12 to 15 cm long, and 2.5 cm thick
Components
• Head (encircled by duodenum)
• Body (mid-portion)
• Tail on the left
Both an endocrine and exocrine gland
• Endocrine portion- -pancreatic islets that secrete insulin and glucagon.
• Exocrine portion- 99% of pancreas that secretes 1,200 to 1,500 mL of pancreatic juice per day.
Pancreatic duct
• Runs lengthwise through the middle of the gland
Accessory pancreatic duct
• Smaller duct that branches from the main pancreatic duct
• Opens independently into the duodenum
• Bypasses the sphincter and allows pancreatic juice to be released into duodenum even when bile is not
Pancreatic Juices
Alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes
Important ones
. Pancreatic amylase - splits glycogen into disaccharides
• Pancreatic lipase - breaks down triglycerides
• Trypsinogen, chymotrypsinogen, and carboxypeptidase (digest proteins)
• Nucleases - digest nucleic acids
• Bicarbonate ions - make pancreatic juice alkaline
Regulation of secretion of pancreas
Three (3) stimuli are responsible for the release of pancreatic juice and bile
- Acetylcholine (Ach)
• From vagus and enteric nerves
•Secrete enzymes during the cephalic phase - Cholecystokinin (CCK)
•Secreted by mucosa of duodenum and small intestine
•In response to fats in small intestine
•Contraction of gallbladder and relaxation of hepatopancreatic sphincter > discharges bile into duodenum - Secretin
•Produced by small intestine in response to acidity of chyme from stomach
•Stimulates liver and pancreas to secrete sodium bicarbonate solution (buffers the hydrochloric acid arriving from the stomach)
Small intestine
Nearly all chemical digestion and nutrient absorption occurs in the small intestine
“Small” intestine refers to the diameter-not length (2.5 cm)
The longest part of the digestive tract
Gross anatomy small intestine
Coiled mass filling most of the abdominal cavity inferior to the stomach and the liver
Divided into three regions
- Duodenum: first 25 cm (10 in.)
• Begins at the pyloric valve
• Major and minor duodenal papilla distal to pyloric valve
• Receives major and minor pancreatic ducts respectively
• Arches around the head of the pancreas
• Ends at a sharp bend called the duodenojejunal flexure
Function:
• Stomach acid neutralised, in the duodenum, fats emulsified by bile acids, pepsin inactivated by pH & pancreatic enzymes take over chemical digestion. - Jejunum: first 40% of small intestine beyond duodenum
• Roughly 1.0 to 1.7 m in a living person
• Large, tall, closely spaced circular folds
• Relatively thick and muscular wall
• Especially rich blood supply which gives it a red color
• Most digestion and nutrient absorption occurs here - Ileum: forms the last 60% of the postduodenal small intestine
• About 1.6 to 2.7 m
• Thinner, less muscular, less vascular, and paler pink color
• Ileocecal valve (sphincter) control’s movement from the ilium to large intestine
Microscopic anatomy small intestine
Tissue layers have modifications for nutrient digestion and absorption
Lumen lined with simple columnar epithelium
Muscularis externa is notable for a thick inner circular layer and a thinner outer longitudinal
Large internal surface area for effective digestion and absorption: by great length and three types of internal folds or projections
Circular folds increase surface area by a factor of 2 to 3
Villi increase surface area by a factor of 10
Microvilli Increase the surface area by a factor of 20
Intestinal secretions - intestinal crypts
Intestinal crypts secrete 1 to 2 L of intestinal juice per day
• In response to acid, hypertonic chyme, an distension of the intestines
• pH of 7.4 to 7.8
• Contains water, mucus, but relatively little enzyme
• Most enzymes that function in the small intestine are found in the brush border
Intestinal motility
Contractions of the small intestine serve three
(3) functions:
To mix chyme with intestinal juice, bile and pancreatic juice allowing these fluids to neutralize acid and digest nutrients more effectively.
Churn chyme and bring it into contact with the mucosa for digestion and absorption
To move residue toward the large intestine
Intestinal Motility (Segmentation)
Segmentation
• Movement in which ringlike constrictions appear at several places alc the intestine
• Most common site is the small intestine
•Allows the kneading/churning which promotes mixing of food, digesti secretions and enhances contact digestion.
Pacemaker cells of the muscularis externa set the rhythm segmentation
•8-9 times per minute in the ileum
• 12 times per minute in the duodenum
Interstitial motility
Intensity of contractions is influenced by nervous and hormonal factors
• When most nutrients have been absorbed, segmentation declines and peristalsis begins
• A peristaltic wave begins in the duodenum and travels 10-70cm, then is followed by another wave.
• The overlapping waves of contraction are called migrating motor complex
•Chyme is milked towards the colon over ~2 hours
•Refilling of the stomach at the next meal supresses peristalsis and
activates segmentation
•Ileocecal valve remains closed until food in stomach triggers the release of gastrin
•Pressure pinches the valve shut
