L1, 2 - The Digestive System - Topic 1 Flashcards
What are the main functions of the digestive system ?
- Take in food
- Break it down into nutrient molecules
- Absorb molecules into the bloodstream across the mucosa
- Rid body of indigestible remains
-
Nutrient production : Synthesis of vitamins by colony of bacteria that lives in large intestine (colon)
– Vitamin K, biotin & other B vitamins - Hormone & hormone-like compound production : Gastrin, ghrelin, cholecystokinin, secretin, VIP, motilin, GIP (gastric inhibitory peptide)
- NT production : Acetylcholine, serotonin, histamine, nitric oxide
How does the body prepare for digestion?
The CNS prepares the body for digestion by anticipating food with the secretion of digestive juices
What are the 2 groups of organs within the digestive system ?
1. Alimentary canal (GI tract)
2. Accessory digestive organs / structures
Name the constitution and function of the alimentary canal / GI tract
- Long pathway made up of a continuous muscular tube, winding body from mouth to anus (terminal openng)
- 9 m in cadaver, shorter in living due to its muscle tone
- Here, food is technically outside the body bc the canal is open at both ends
Function : Digests food and absorbs it through its lining (mucosa) into blood
- To digest = To dissolve
Organs : Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus
Name the constitution and function of the accessory digestive organs / structures
Function : Mechanical & chemical digestion of food
[ in oral cavity ]
- Teeth
- Tongue
[ outside of the GI tract, connect to it via ducts ]
- Gallbladder
- Digestive glands : Produce secretions that help break down foodstuffs
– Salivary glands (amylase – enzyme that begins breakdown of carbohydrates in mouth)
– Liver (contributes bile)
– Pancreas (secretes pancreatic juice – has bicarbonate enzymes that breakdown food to prepare for absorption, neutralize stomach pH acidity)
Describe the Major Digestive Processes
1. Ingestion – Taking food in digestive tract (eating)
2. Mechanical breakdown – Physical force to breakdown food into smaller bits
- Chewing (mouth)
- Churning (stomach – muscle wall pounds)
- Segmentation (rhythmic local constrictions of small intestine)
3. Propulsion
- Swallowing (oropharynx, esophagus – mouth to stomach)
- Peristalsis (esophagus, stomach, small intestine, large intestine)
4. Chemical Digestion (mostly stomach + small intestine)
- Catabolism → Enzymatic breakdown of the food mol into their chemical building blocks
5. Absorption (mostly small intestine) –
Digested end products (plus vitamins, minerals, water) are taken out of GI tract lumen via active OR passive transport through the mucosal cells & transported into blood / lymph
6. Compaction & defecation (large intestine / colon, anus) – Elimination of indigestible substances from the body via anus in form of feces
What is peristalsis ?
Major means of propulsion in GI tract (esophagus, small intestine, some extent in large intestine)
- Adjacent segments of alimentary tract organs alternately contract + relax
- Food is moved forward (distally along the tract)
- Some mixing may occur
What is segmentation ?
Makes absorption more efficient by repeatedly smearing different parts of food mass over the mucosa of the intestinal wall (small & large intestine)
- Rhythmic alternate contractions / relaxations of localized points along GI tract
*Alternately squeezing the middle & end of closed tube of toothpaste
- Food is thoroughly mixed with digestive juices & broken down
- Food is moved forward + backward ; SLOW propulsion
What is the peritoneum ?
- Most extensive serous membranes of the abdominopelvic cavity
Consists of :
- Visceral peritoneum : Membrane covering external surface of most digestive organs, continuous with the …
- Parietal peritoneum : Membrane that lines the body wall
What is the peritoneal cavity ?
- Fluid-filled space between 2 peritoneums
- The fluid is secreted by the serous membranes and works to lubricate mobile organs, allowing them to glide easily across one another / along the body wall as they carry out their activities
What is peritonitis ?
- Inflammation of the peritoneum
- MOST COMMONLY caused by a burst appendix spraying bacteria-containing feces all over the peritoneum
- Here, the peritoneal coverings tend to stick together around the infection site – localizing it to allow time for macrophages to prevent inflammation from spreading
- Can be DANGEROUS & LETHAL if infection spreads within the peritoneal cavity → Must be treated by removing as much infectious debris as possible & administering megadoses of antibiotics
What is the mesentery ?
- Double layer of peritoneum (2 serous membranes) fused together which extends to the digestive organs from the body wall
- Mostly posterior / dorsal
→ Does not suspend all organs within alimentary canal
Functions :
- Provides support for the organs, anchors them in place
- Provides routes + support for blood / lymph vessels & nerves supplying the organs
- Stores fat
Define intraperitoneal
- Organs located within the peritoneal cavity
- These keep their mesentery (e.g. stomach)
Define retroperitoneal
- Organs located outside of / posterior to the peritoneum
- Lost their mesenteries during development
- e.g. Most of pancreas, duodenum, parts of large intestine
Most digestive organs are ______________, and are suspended from the body wall by a ___________ ______________
Intraperitoneal, dorsal mesentery
!! some also suspended by a ventral mesentery …
Most of the length of the alimentary canal / GI tract is made up by the ____________________
Small intestine
All digestive organs within the alimentary canal consist of __ basic tissue layers (tunics), these are the …
4 – Mucosa, submucosa, muscularis externa, serosa
Name the characteristics of the mucosa
Innermost layer
- Moist epithelial membrane (Mucous membrane)
Functions :
- Secretion – Mucus, digestive enzymes, hormones
- Absorption – End products of digestion into blood
- Protection – Against infectious disease
Made up of 3 sublayers
Name and describe the 3 sublayers of the mucosa tunic
1. Epithelium lining – Simple columnar epithelium rich in mucus-secreting (goblet) cells
- Mucus protects digestive organs from enzymes, eases passage of food
- May secrete enzymes & hormones (e.g., in stomach and small intestine)
2. Lamina propria – Overlies epithelium, made up of loose areolar CT
- Has capillaries for nourishment of epithelium lining & to allow absorption of nutrients
- Houses lymphoid follicles (part of MALT) – Provide protection from bacteria / pathogens which have rather free access to our digestive tract
3. Muscularis mucosae – External to lamina propria
- Made up of smooth muscle that produces local movements of mucosa which can enhance absorption + secretion
Name the characteristics of the submucosa
- External to mucosa
- Made up of aerolar CT
- Rich supply of blood & lymphatic vessels, lymphoid follicles
- Nerve fibers, Submucosal nerve plexus → Controls movement (peristalysis / segmentation), innervates glands to trigger to release hormones necessary for chemical digestion of food (catabolism)
- Abundant in elastic fibers → Allows stomach to regain shape after large meal
Name the characteristics of the muscularis externa
- Inner circular & outer longitudinal layers of smooth muscle cells
- Generate movement (segmentation & peristalsis)
- Sphincters in organ-to organ junctions created by thickening of circular SM layer – Act as valves to control food passage from one organ to the next preventing backflow
- Myenteric nerve plexus → In between circular / longitudinal layer, controls movement
Name the characteristics of the serosa
- Visceral peritoneum – Outermost protective covering of intraperitoneal organs
- Areolar CT covered with mesothelium (single layer of squamous epithelium cells)
- Replaced by adventitia in the esophagus (fibrous CT that “binds” the esophagus to surrounding tissues, needed since esophagus is located in thoracic instead of abdominopelvic cavity)
Retroperitoneal organs have both an ____________ & _________
adventitia, serosa
- Serosa on the side facing the peritoneal cavity
- Adventitia on the side against the dorsal body wall
Splanchnic Circulation
- GI tract has an arterial blood supply & venous drainage system
- Includes arteries that branch off the abdominal aorta to serve the digestive organs & hepatic portal circulation
- Celiac trunk branches (hepatic, splenic, left gastric branches) serve spleen, liver, stomach
- Superior / inferior mesenteric arteries serve small & large intestines
Venous return from much of the abdominopelvic region is via the ______________________
Inferior vena cava
Venous return from the digestive viscera is indirect via the hepatic portal circulation, why?
Portal system : Goes through 2 capillary beds before coming back into circulation
→ After digestion, nutrients will diffuse into bloodstream though 1 capillary bed from stomach / small intestine
→ Blood will be oxygen poor BUT nutrient rich from products just asorbed
→ Liver will take blood from GI tract through 2nd capillary bed to process it (takes out glucose for storage - glycogen, pulls out extra fat for storage - lipids, amino acids)
→ Liver will send blood back into venous circulation to the heart
Cardiac Output
{ Volume of blood pumped by heart /min }
= 20 - 25 % of CO diverted to GI tract
- Goes UP after a meal → Because you’re in the process of digestion
- Goes DOWN during exercise → Because blood flow is diverted from digestive organs to skeletal muscles
Enteric (Gut) Nervous System
The NS of the GI tract, “Gut brain” (talks to itself) + Influence from external sympa / para NS
- Made up of enteric neurons that communicate extensively with each other
- Major nerve supply to GI tract wall that controls motility
ENS Innervation
Linked to the CNS via :
- Afferent visceral fibers
- Motor fibers of the ANS (synapses with neurons in the enteric nerve plexuses)
Sympathetic impulses inhibit secretion / motility
Parasympathetic impulses stimulate secretion / motility
Name the 2 interconnecting intrinsic nerve plexuses of the ENS
Nerve plexus : Ganglia connected by unmyelinated fiber tracts
-
Submucosal nerve plexus
- Located in submucosa
- Contains sensory & motor neurons
- Regulates glands (secretion) & smooth
muscle in mucosa (movement) -
Myenteric nerve plexus
- Located between the circular & longitudinal
muscles of muscularis externa
- Provides the major nerve supply to GI tract
- Controls GI tract motility (pacemaker cells & local reflex arcs between enteric neurons – Set rates at which peristalsis / segmentation take place
Short reflexes respond to …
- Stimuli inside the GI tract (internal)
- Control patterns of segmentation & peristalsis
- Mediated entirely by ENS plexuses
Long reflexes respond to …
- Stimuli inside (internal) OR outside (external) the GI tract
- Involves CNS centers + autonomic nerves
How does the CNS trigger the salivation process to prepare the body for digestion ?
CNS (brain / spinal cord) receives / interprets info about sight, smell, taste, thought of food
→ Triggers salivation process
→ Conveys info to local gut brain (ENS) to get it to begin preparing for digestion process / anticipate food
How do receptors detect changes in internal stimuli ?
- Chemoreceptors sense changes in chemical composition & pH
- Osmoreceptors sense changes in osmolarity (hyper/ hypotonicity)
- Mechanoreceptors sense changes in stretch as we eat (stomach stretches as we eat)
Receptors sense internal stimuli within GI tract
→ Send info to local nerve plexus / “gut brain” & CNS
→ Trigger response from smooth muscles / glands to increase / decrease motility, start secretion of hormones, etc
3 Key Concepts of Regulating Digestive Activity
-
Digestive activity is provoked by mechanical & chemical stimuli
- Receptors in walls of GI tract organs respond to stretch, changes in osmolarity & pH, the presence of substrate and end products of digestion -
Effectors of digestive activity are Smooth Muscles & Glands
- Receptors initiate reflexes that stimulate SM to mix & move lumen contents
- Reflexes can also activate / inhibit digestive glands that secrete digestive juices or hormones - Digestive activity is controlled by the Nervous System (intrinsic and extrinsic) & Hormones
NS controls :
– Intrinsic controls involve short reflexes (entirely within ENS)
– Extrinsic controls involve long reflexes (ANS)
Hormonal controls :
– Hormones from cells in stomach & small intestine stimulate target cells in same or different organs to secrete or contract
What are the characteristics of the oral (buccal) cavity ?
- Bounded by lips (anterior), cheeks (lateral), palate (posterior) & tongue (inferior)
- Oral orifice is the anterior opening of the cavity
- Continuous with the oropharynx posteriorly
Constitution : Walls lined with strat squamous epithelia to withstand high abrasion from food
{ Slightly keratinized on gums, hard palate & dorsum of tongue for extra protection }
Function : Beginning of digestion & initiation of swallowing (mastication)
- Food is chewed & mixed with enzyme-containing saliva
Associated organs include :
- Tongue
- Salivary glands
- Teeth
What are the characteristics of the lips & cheeks ?
Core of skeletal muscle + external skin covering
- Help keep food between teeth as we chew
Lips (labia) : Composed of circular fleshy orbicularis oris muscle
Cheeks : Composed of buccinator muscles
- Oral vestibule – Recess bounded externally by lips and cheeks, internally by teeth and gums
- Oral cavity proper – Lies within teeth and gums
- Labial frenulum – Median attachment of each lip to gum, fold of mucous membrane lining oral cavity (anchors in place)
What are the characteristics of the hard palate ?
- Roof of oral cavity
- Underlies the palatine bones & palatine processes of the maxillae
- Slightly corrugated – Overlying bones form a rigid surface to help create friction against which tongue forces food during chewing
What are the characteristics of the soft palate ?
- Most posterior aspect of hard palate
- Fold formed mostly of skeletal muscle
- Closes off the nasopharynx during swallowing (reflex)
- Uvula projects downward from its free edge
What are the characteristics and functions of the tongue ?
accessory organ
- Interlacing bundles of skeletal muscle → Willful control (you can fold / twist / curve)
Functions :
- Repositioning + mixing food (with saliva) during chewing
- Formation of the bolus
- Initiation of swallowing (pushes bolus posteriorly into pharynx), speech (consonants), and taste
Intrinsic muscles (not attached to bones) change the __________ of the tongue
shape ( thickness / length )
Extrinsic muscles change the __________ of the tongue
position ( side to side, protrude / retract )
What is the functional anatomy of the tongue ?
Lingual frenulum : Attachment to the floor of the mouth, anchors tongue to underlying structures, helps to limit movement of tongue posteriorly (prevents slide back)
Midline septum divides into symmetrical halves
Terminal sulcus marks the division between :
Body – Anterior 2/3 residing in the oral cavity
Root – Posterior third residing in the oropharynx
What is ankyloglossia ?
- Congenital condition in which children are born with an extremely short lingual frenulum
- “Tongue-tied” or “fused tongue”
- Restricts range of motion, compromises speech (impediment), fixed by surgery
What are surface papillae ?
- Projections of underlying mucosa (lamina propria covered with epithelium) dotting the dorsum of the tongue
- BIGGER at the back
Function : Help increase friction, house taste buds or touch receptors
- Foliate — Lateral aspects of the posterior tongue (taste)
- Filiform — Whitish, give the tongue roughness + provide friction (no taste buds, only touch receptors)
- Fungiform — Reddish, scattered over the tongue, has a vascular core (taste)
- Circumvallate (vallate) — V-shaped row in back of tongue, HUGE, each houses between 100 - 300 taste buds (taste)
What is saliva ?
Fluid secreted by salivary glands
Functions :
- Cleanses mouth (antimicrobial agents, antibodies)
- Dissolves food chemicals for taste
- Moistens food (watery-based secretion) & helps compact it into a bolus to be swallowed
- Begins breakdown of starch with enzyme amylase, fat digested by lipase
- Protects against microorganisms ( IgA antibodies, lysozyme, defensins )
What is the composition of saliva ?
- Hypoosmotic (mostly water)
- Electrolytes
- Digestive enzymes (amylase, lipase)
- Proteins (mucin, lysozyme, IgA)
- Metabolic wastes (urea, uric acid)
Salivary glands are composed of 2 different types of secretory cells :
Serous cells — Produce watery secretion, enzymes, ions, tiny bit of mucin
Mucous cells — Produce mucus
What are intrinsic (buccal) glands ?
- Scattered in the oral mucosa
- Constantly produce low-level (SMALL AMOUNT) of salivation to keep mouth, tongue & lips moist
- Help with speech
What are extrinsic glands ?
- Lie outside the mouth
- Exocrine – Secretion released into ducts that empty into the oral cavity
Parotid Gland — Large, triangular, mainly serous cells, 25 % of saliva
- drains into mouth from upper teeth
Submandibular Gland — Walnut size, serous & mucous cells, 70 % of saliva
- drains into mouth from under tongue
Sublingual Gland — Small, almond-shaped, mostly mucous & serous cells, 5 % of saliva
- drains into floor of mouth
What is mumps ?
- Viral infection of parotid gland, caused by paramoxial virus
- Causes swelling / fever / malaise
- Vaccine developed 1967
Control of Salivation
1500 ml/day can be produced
- Salivation controlled primarily by parasympa division of ANS
Major salivary glands are activated by parasympathetic NS when ingested food stimulates chemo & mechanoreceptors in mouth, sending signals to :
- Salivatory nuclei in brain stem which stimulates parasympathetic impulses along fibers in cranial nerves VII and IX to glands
→ Dramatically increases output of watery (serous), enzyme-rich saliva
- Sympa NS counteracts effects of para, can trigger release of a thick, mucin-rich saliva or almost completely inhibits saliva release
Other stimuli :
- Swallowing irritating foods
- Nausea
- Smell/sight of food
- Upset GI
Thought – smell of food → Triggers salivation via hypothalamus that sends messages to salivary centers → ANS
What happens during dehydration ?
- Mouth gets dry bc body is trying to conserve water by decreasing salivation
- May cause palitosis – bad breath, bacteria cause odors
- Low BV reduces filtration pressure at capillary beds
What are the characteristics of teeth ?
- Lie in sockets in gum-covered margins of mandible & maxilla
- Made of calcified protein (primarily dentin – contributes firmness), anchored in place by CT ( periodontal ligament )
Tooth structure :
- Crown – Exposed part above gingiva (gums)
- Root – Portion embedded in jawbone, counts as anything below gum, connected to the crown by neck
Enamel – Outer portion of teeth, hard, calcium + mineralized salts (cells degenerate, doesn’t repair after being chipped)
Pulp cavity – Middle space, enlarged towards crown / narrows into root canal, contains blood vessels + nerves + lymphatic vessels that enter through apical formen (opening)
Teeth Classification
Classified according to shape
- Incisors : Chisel shaped for cutting (anterior)
- Canines : Fanglike teeth that tear or pierce
- Premolars (bicuspid) : Broad crowns with rounded cusps used to grind or crush
- Molars : Broad crowns, rounded cusps: best grinders (more than 1 root), located at the back
Digestive Processes of the Mouth
Mechanical Breakdown
- Mastication – Chewing, production of bolus (food mechanically crushed, mixed with saliva becomes a pasty lump to make it easy to swallow)
- Closed lips and cheeks
- Teeth
- Tongue (mixes, pushes posteriorly to pass bolus into pharynx)
Chemical Digestion (Enzymatic)
- Breakdown of starch (carbs) by salivary amylase
- Breaking of fats by lingual lipase (moreso in the stomach)
What are the transport structures of the GI tract ?
pharynx & esophagus
What are the characteristics of the pharynx ?
- Allows passage of food, fluids, and air
- Food passes from mouth into oropharynx + then into laryngopharynx
Anatomy :
- SSE lining with mucus-producing glands
- External skeletal muscle layer consists of :
– Inner layer of muscles runs longitudinally
– Outer pharyngeal constrictors encircle wall of pharynx
( contractions of these muscles aid in persitalsis )
What are the characteristics of the esophagus ?
- Flat muscular tube (~25 cm) that runs from laryngopharynx to stomach
- Collapsed when not involved in food propulsion
- Pierces diaphragm at esophageal hiatus
- Joins stomach at cardial orifice
-
Gastroesophageal / lower esophageal sphincter surrounds cardial orifice
– Smooth muscle “valves”, contract to seal off lumen to keeps orifice closed when food is not being swallowed
– Mucus cells on both sides of sphincter help protect esophagus from acid reflux
if sphincter doesn’t work, regurgitation of contents from stomach may occur (heartburn / acid reflux)
Describe the histology of the esophagus
All 4 alimentary canal layers
- Esophageal mucosa contains SSE that changes to simple columnar epithelium at the stomach
- Esophageal glands in submucosa (CT + glands) secrete mucus to aid in bolus movement
- Muscularis externa : Skeletal muscle superiorly - upper part; mixture in middle; smooth muscle inferiorly - lower part [ this is why part of swallowing is voluntary (skel), part involuntary (smooth) ]
Adventitia instead of serosa
Process of Swallowing / Deglutition
- Pharynx & esophagus : Passage of food from mouth to stomach –> Propulsion that starts with deglutition (swallowing)
Involves the tongue, soft palate, pharynx, esophagus, 22 muscle groups & 2 phases :
Buccal phase :
- Voluntary contraction of the tongue (decide to swallow)
Pharyngeal - esophageal phase :
- Hits point of no return, becomes involuntary
- Controlled by swallowing center in the medulla & lower pons
The passage of food is regulated by …
- 2 sphincters – Upper & lower esophageal sphincters
- Peristalsis – Involuntary muscle movements controlled by medulla oblongata & facilitated by mucus produced by the submucosal glands
What are the STEPS of swallowing …
[ watch video ]
