Digestive System Concepts

Question 1

Order the 6 GI tract structures from proximal to distal, and group the 6 accessory organs into 3 superior (and the part of the tract they associate with) and 3 inferior (and the part of the tract they associate with).

Answer 1

GI tract structures:
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine

Accessory organs:

superior:
Salivary glands
Teeth
Tongue
- They associate with the mouth

inferior:
Liver
Gall bladder
pancreas
- They associate with the small intestine

Question 2

Name and give a brief description of the 3 parts of the mucosa of the wall of GI tract structures.

Answer 2

Deep to superficial:
Epithelium= in direct contact with the lumen of the GI tract
Lamina propria= loose areolar C.T. contains traditional basement membrane
Muscularis mucosae= thin layer of smooth muscle that churns food in the stomach

Question 3

Group the 6 parts of the GI tract into conductive or absorptive, and name the epithelia used in each.

Answer 3

Stomach = absorption
Mouth = conductive
Small intestine = absorption
Large intestine = absorption
Esophagus = conductive
Pharynx = conductive

• Nonkeratinized stratified squamous epithelium in the mouth, pharynx, and esophagus
• Simple columnar in stomach, and small and large intestine

Question 4

Name and briefly describe various parts of the submucosa of the wall of GI tract structures. Include the role of the submucosal plexus of the ENS.

Answer 4

• Loose areolar C.T. full of blood and lymphatic vessels. These vessels receive tons of absorbed food molecules
• Lots of glands empty contents into the lumen

The submucosal plexus of the ENS senses the chemical composition of the lumen and stretch of walls

Question 5

Name and briefly describe various parts of the muscularis of the wall of GI tract structures. Include the variance in the musculature of the stomach wall, the role and location of the myenteric plexus of the ENS, and the mechanisms and role of peristalsis.

Answer 5

Skeletal= if referring to the mouth, pharynx, and superior parts of the esophagus

Smooth= if referring to the rest of the GI tract

The inner sheet is circular, the outer sheet is longitudinal
The myenteric plexus innervates the muscularis.

Question 6

Name and briefly describe the two possible choices for the most superficial layer of the wall of GI tract structures. Know when to use each, and how that relates to position in the peritoneal cavity

Answer 6

Serosa= if referring to the part of the GI tract suspended in the abdominopelvic cavity (stomach, most of the small & large intestine)

Adventitia= if referring to part of the GI tract NOT suspended in the abdominopelvic cavity (esophagus)

Question 7

Describe the neural connections between the two plexuses of the ENS – focusing on sensing, integrating, and responding.

Answer 7

Sensation = submucosal plexus senses the chemical composition of the lumen and stretch of luminal walls

Integration = happens in both plexus, but only the myenteric plexus can drive the submucosa, but not vice versa.

Motor response = glandular secretion (submucosal plexus) or peristalsis (myenteric plexus, contraction)

Question 8

Contrast the effect on the GI tract of parasympathetic vs. sympathetic innervation.

Answer 8

Parasympathetic innervation increases secretion and motility to increase digestive function

Sympathetic shuts down secretion and motility during “fight or flight” situations

Question 9

Briefly name and describe the anterior, lateral, superior, and inferior borders of the oral cavity.

Answer 9

superior= hard palate (the palatine process of the maxilla and horizontal plate of the palatine bone)

inferior= tongue and associated skeletal muscles that move the tongue

lateral= cheeks and teeth

anterior= teeth and lips

Question 10

Describe the general role of the frenula and be able to name and locate at least one in the oral cavity

Answer 10

Frenula restricts the movement of the lips and tongue

• The lingual frenulum is located right below the tongue and restricts the posterior movement of the tongue

Question 11

Describe the role of the uvula in deglutition

Answer 11

When swallowing, it pushes posteriorly and superiorly to block off the pharynx and wall of the oropharynx from the nasal pharynx so water and food don’t enter the nasal cavity

Question 12

Know the role of the palatine tonsils and their location relative to the two arches of the oral cavity

Answer 12

Palatine tonsils lie between the two arches to act as the first defense against pathogens and help stimulate an immune response to fight off infection

Question 13

Describe briefly some contents of saliva and an overview of its role in chemical digestion.

Answer 13

Water & ions
Dissolved gases
urea/uric acid
Mucous
IgA
Lysozyme
Salivary amylase

Role:
- Saliva dissolves food in an aqueous medium
- Chemically digest food via salivary amylase

Question 14

Name and locate the 3 primary salivary glands of the mouth. Describe their innervation

Answer 14

Sublingual= CN VII (facial nerve)
Submandibular CN VII (facial nerve)
Parotid= CN IX (glossopharyngeal)

Question 15

Name the 2 key enzymes of chemical digestion found in saliva and briefly describe the work they make

Answer 15

Salivary amylase breaks starches into dimers, trimers, and shorter polymers

Lingual lipase breaks triglycerides into diglycerides

Question 16

Contrast, briefly, the role of the external and internal muscles of the tongue

Answer 16

External muscles move the tongue laterally, anteriorly, and posteriorly to shape food into a round mash and move it positively for swallowing

Internal muscles alter the shape and size of the tongue to assist in swallowing and speech

Question 17

Describe the role of the apical foramina in the roots of teeth

Answer 17

The apical foramen is at the base of the root canal where nerves, lymphatic, and blood vessels enter and exit the tooth

Question 18

Briefly describe the 3 phases of deglutition, indicating which are voluntary

Answer 18

Voluntary phase:
The initial phase of deglutition, occurs in the oral cavity when the bolus is pushed to the posterior aspect of the oral cavity and into the oropharynx by elevation and retraction of the tongue via contraction of the extrinsic skeletal muscles of the tongue

Pharyngeal (involuntary stage):
The first involuntary phase of delutition. When the bolus enters the oropharynx it triggers receptors that connect to the delegation center in the brain stem to stimulate the contraction of skeletal and smooth muscle to push the bolus. In addition, in this phase, the uvula and soft palate move superiorly to block passage to the nasopharynx, and the epiglottis closes off the opening to the larynx. Once the upper esophageal sphincter relaxes, the bolus passes into the esophagus

Esophageal stage (involuntary stage):
A series of coordinated muscular contractions called peristalsis that moves the bolus down the esophagus. The circular rings of smooth muscle superior to the bolus contract to constrict the esophagus and force the bolus inferiorly, while inferiorly, the longitudinal smooth muscle sheets contract to dilate the esophagus to make room for the bolus

Question 19

Trace the route of the esophagus inferiorly

Answer 19

Runs inferiorly from the laryngopharynx, anterior to the vertebral column, and through the mediastinum
From the thoracic cavity, it punctures the diaphragm ( via the esophageal hiatus) and runs inferiorly within the abdominopelvic cavity, and terminates at the stomach

Question 20

Describe the 4 anatomical layers of the esophagus’s wall

Answer 20

Mucosa:
Mucous secreted into the esophagus lubricates the passage of the bolus through the esophagus

Submucosa:
It contains some mucous glands

Muscularis:
- The superior 3rd is skeletal muscle, the inferior 3rd is smooth, and the middle section is a mix of both

• At both ends of the esophagus, the muscularis thickens to form 2 sphincters: upper and lower esophageal sphincters

Adventitia:
Doesn’t hang within the abdominopelvic cavity so its adventitia

Question 21

Name the 2 esophageal sphincters, their locations, and roles

Answer 21

At both ends of the esophagus, the muscularis thickens to form 2 sphincters: upper (controls movement of the bolus into the esophagus) and lower (controls movement of the bolus into the stomach) esophageal sphincters

Question 22

Describe the role of the stomach and the nature of its pH.

Answer 22

• The overall goal is to convert the bolus (solid to semi-solid) to a liquid (chyme)
• pH of 2 to digest food
• Digestion of starches and lipids continues - digestion of proteins begins

Question 23

Discuss the enzymes of chemical digestion of the stomach and what they’re working on.

Answer 23

Chief cells
- Pepsinogen (inactive form) is secreted into the lumen and chops up proteins
- Gastric lipase splits triglycerides into monoglycerides (lipids)

Lingual lipase was secreted in the oral cavity, inactively, until the high acid gastric juice activated it to cut up triglycerides into diglycerides (lipids)

Question 24

Describe the classic 4 anatomical layers of the stomach. Include the role of rugae and gastric pits

Answer 24

Deep to superficial
Mucosa:
- Wrapped into multiple folds (rugae)
- Simple columnar epithelium
- Gastric pits are the deep depressions between the rugae

Submucosa:
- Loose areolar C.T. with lymphatic and blood vessels

Muscularis:
- The oblique layer then circular and longitudinal
- The oblique layer is mostly found in the body of the stomach
- Standard myenteric plexus of the ENS between the circular and longitudinal layers

Serosa:
- Since the stomach is suspended in the abdominopelvic cavity
- Serous membrane over loose areolar C.T.
- Visceral peritoneum wrapped around the stomach

Question 25

Describe how peristalsis works, how it’s controlled, and what it’s trying to accomplish in the stomach.

Answer 25

Peristalsis by the 3 muscular layers created “mixing waves” every 20 seconds or so. Waves mix the bolus with gastric juice and turn it into soupy chyme. The waves are directed distally

Question 26

Contrast pepsinogen and pepsin, and relate the two to trypsinogen/trypsin, chymotrypsinogen/chymotrypsin…

Answer 26

• Pepsinogen is secreted into the lumen by chief cells
• The high acidity of the stomach converts pepsinogen to pepsin, which hydrolyzes peptide bonds to convert protein chains to amino acids

-Pepsin does the initial breakdown of proteins in the stomach; but as the resultant smaller polypeptides and peptides pass into the small intestine via the duodenum, they are exposed to trypsin and chymotrypsin for further breakdown to their amino acid components to be absorbed

Question 27

Be able to VERY generally (not detailed!) describe the secretion of HCl by parietal cells of the stomach, and describe the role of the acid secretion and how your gastric mucosa (hopefully!) is protected.

Answer 27

In parietal cells, protein pumps swap K+ for H+ pumped out into the lumen

Also, non-electronically, K+ diffuses back out to the lumen, along with Cl- ions (both move by facilitated diffusion)

Surface mucous cells and mucous neck cells bomb the lumen of the stomach with a 1-3mm thick layer of mucous to protect the mucosa from acid

Question 28

Name the one parasympathetic and one hormonal way to control HCl secretion by parietal cells

Answer 28

ACh from parasympathetic neurons

Gastrin secreted by G cells of the mucosa

Question 29

Discuss retroperitoneal, focusing on the duodenum and pancreas.

Answer 29

The pancreas and duodenum are retroperitoneal, meaning the organ sits behind the “balloon”

The “balloon” lining the peritoneal organs is the visceral peritoneum, so the pancreas and duodenum sit behind the visceral peritoneum

Question 30

Give the structural and functional details of the pancreas, including composition (key enzymes of chemical digestion) and role of pancreatic juice

Answer 30

The head is might rightward and tucks into the curve of the duodenum

The body and tail extend leftward and tuck up against the spleen

1% of pancreatic cells are alpha, beta, delta, and pancreatic polypeptide cells of the endocrine system

The other 99% of cells are exocrine and produce and secrete pancreatic juice, a mix of water, salts, sodium bicarb, and digestive enzymes

Question 31

Describe the anatomical features of the liver, focusing on the hepatic acinus as the functional unit, and its form and function.

Answer 31

Sits just inferior to the diaphragm in the upper right portion of the abdominal pelvic cavity

2 lobes, larger right, and smaller left

The functional unit of the liver is the hepatic acinus (“berry”)

Question 32

Contrast hepatic sinusoids and bile canaliculi, the direction of flow of each relative to hepatic laminae, and the role of each.

Answer 32

Hepatic sinusoids
- allow the liver to filter blood
- Runs right to left

Bile canaliculi
- Bile is dumped in bile canaliculi
- Runs left to right

Question 33

Name the components of the hepatic portal triad and the roles of all 3 structures.

Answer 33

The hepatic artery, carries afferent blood

The hepatic portal vein, carries deoxygenated, but nutrient-laden blood from the intestines

The bile duct, carrying bile efferently

Question 34

Contrast Zone 1 and Zone 3 hepatocytes of a hepatic acinus

Answer 34

Zone 1 cells will be least impacted by hypoxia and have the largest role in carbohydrate metabolism - glycogenesis in times of surplus and glycogenolysis in times of hypoglycemia and most mitotically active

Zone 3 cells are the inverse

Question 35

Give general functions of the liver

Answer 35

Carbohydrate, lipid, and protein metabolism

Processing of drugs and hormones

Bilirubin excretion

Bile salt synthesis

Storage

Phagocytosis

Vitamin D activation

Question 36

Describe the role of the gall bladder, and connect the cystic bile duct to the hepatic & common bile ducts. Describe bile in general, its components, and its function in chemical digestion.

Answer 36

The role is to store bile

The gallbladder body and neck are superior, opening onto the Cystic duct and eventually joining the common hepatic duct to form the bile duct

Bile is made of water, bile salts, cholesterol, lecithin, bile pigments (bilirubin), and ions

Bile primarily emulsifies large globules into suspensions of many smaller lipid globules to increase the surface area for pancreatic lipase to access more triglycerides to digest quicker and more efficiently

Question 37

Define the role of the Sphincter of Oddi in bile secretion/storage

Answer 37

If the sphincter of Oddi is closed (between meals) and bile is still produced in hepatocytes and moved into the bile duct the bile will move back into the gallbladder for storage

Question 38

Contrast the 3 sections of the small intestine – location, length, primary function & peritoneal vs. retro

Answer 38

Duodenum = Begins with a short (25cm), curved, retroperitoneal tube, where pancreatic juice and bile enter.

Jejunum = 40x longer than the duodenum - at 1m long! So much absorption, peritoneal

Ileum = 80x longer than the duodenum - at 2m long! So much absorption, peritoneal

Question 39

Discuss the 3 adaptations to increase the surface area of the small intestine.

Answer 39

Circular folds = larger folds of mucosa and submucosa

Vili = finger-like projections of the epithelium

Microvilli = On each absorptive cell, the apical membrane has tiny projections into the intestinal lumen

Question 40

Describe the classic 4 anatomical layers – name an exocrine and an endocrine cell type of the intestinal glands.

Answer 40

Mucosa:
- Simple columnar epithelium (absorptive cells), with lots of accessory cells
- Goblet cells
- Paneth cells (secrete lysozyme and can act as fixed macrophages)
- Enteroendocrine cells secrete digestive and absorptive hormones

Submucosa:
- Loose areolar C.T. containing the usual, plus duodenal glands for an alkaline mucous to neutralize the highly acidic stomach pH

Muscularis:
- Circular and longitudinal layer

Serosa:
- Wrapping around all of the peritoneal small intestine except for the retroperitoneal duodenum which is wrapped occasionally with visceral peritoneum

Question 41

Describe the structure and function of microvilli and connect them to brush border enzymes, as well as to increased surface area for secretion and absorption

Answer 41

Because the microvilli are so tiny, they look like a fuzzy “brush” microscopically, so it’s called “brush border.” With so much real estate available across the surface of the microvilli, it becomes another place for digestion. In addition to the lumen, digestion by brush border enzymes can also take place on the brush border.

Question 42

Contrast segmentation and migrating motility complexes

Answer 42

Segmentation = appositonal-like contractions of the intestinal mucularis. They move only in a superficial to deep direction, about once every 5 seconds alternating compressing different sections of the intestine. The movement sloshes the chyme in and out of the circular folds, facilitating chemical digestion and absorption

Migration motility complexes = essentially peristalsis. Alternating contraction of circular smooth muscle rings proximally to the chyme, and contraction of longitudinal sheets distal the the chyme, with the overall result of moving the chyme distally down the SI

Question 43

Give a brief overview of luminal and/or brush border digestion of starches, proteins, and lipids from polymer to monomers, key enzymes involved, and locations of each enzyme

Answer 43

Starches:
- Salivary amylase was shut down in the stomach, so not much went on before the intestines
- Starches are cut into: Maltose, Maltotriose, and A-dextrins
- Pancreatic amylase will continue this work
- BB enzymes get to work and cut down until only monosaccharides remain: fructose, glucose, and galactose

Proteins:
- Pepsin was a one-horse show in the stomach. Now, from pancreatic juice secreted into the duodenum, there are reinforcements: Trypsin, Chymotrypsin, Carboxypeptidase, and Elastase
- A couple of BB enzymes (aminopeptidase and dipeptidase) also pitch in

Lipids:
- The goal is splitting triglycerides into component fatty acids
- Lingual and gastric lipase have been working away, now pancreatic lipase helps
- Triglycerides come in huge water-hating globules. So amphipathic bile salts emulsify the globules into smaller globules, which gives pancreatic lipase way better access to the triglyceride inside

Question 44

Give a brief overview of small intestinal absorption of starches, proteins, and lipids, focusing on mechanisms of transport and the monomers (and dimers?) that are transported

Answer 44

Starches:
- Fructose enters intestinal absorptive cells via facilitated diffusion
Glucose and galactose have to use secondary active transport via Na+-K+ ATPase activity and Na symport due to concentration gradients

Proteins:
- Singular amino acids are absorbed via active transport. Mostly in the duodenum and jejunum
- Na+ symport also transports some amino acids, and an H+ symporter brings in dipeptides and tripeptides

Lipids:
-Short-chain FAs are easily absorbed as they move directly through the apical membrane by simple diffusion
- Long chains are tricky
- Bile salts surround the hydrophobic FAs and make tiny spheres called micelles that are small enough to diffuse directly into the cell by simple diffusion
- Once in the cell, FAs and monoglycerides diffuse out of the micelles and the micelle cycles back out into the lumen to repeat
- Inside the cell, FAs and monoglycerides combine to form triglycerides, which aggregate into globules and get coated with proteins in larger spheres
- These chylomicrons are bigger than micelles and won’t diffuse through capillary fenestrations so they enter lacteals, then into lymphatic vessels, and ducts, and eventually back into the systemic circulation

Question 45

Name the parts of the large intestine, proximally to distally, the locations of each, and whether each is peritoneal or retroperitoneal.

Answer 45

Cecum = a small, inferior pouch right where the SI meets the LI.

Colon = ascending, transverse, descending, sigmoid

Rectum = inferior, short, and broad tube, located medially and distal to the sigmoid colon

Anal canal = short terminal section of the rectum, housing the internal (involuntary) anal sphincter

Question 46

Describe colic flexures, tenia coli, and haustra.

Answer 46

Colic flexures = 90-degree (nearly right angle) turn at the lateral aspects of the transverse colon, that mark the transition from ascending to transverse, and transverse to descending

Tenia coli = Thick bands of longitudinal muscle running lengthwise along the sections of the colon. Tonic (sustained) contraction bunches the colon into a distinct series of pouches called haustra

Haustra = Bunches segments of the colon that participate in haustral churning to move fecal matter distally along the colon

Question 47

Give the relative locations and roles of the internal anal sphincter, external anal sphincter, and anus

Answer 47

The internal and external anal sphincter is located in the anal canal. The anus is the most distal part of the anal canal. The internal anal sphincter is involuntary and the external sphincter is voluntary.

Question 48

Discuss the classic 4 anatomical layers of the wall of the large intestine

Answer 48

Mucosa:
- Has intestinal glands
- Absoprrtive and mucous (goblet) cells line the intestinal glands as they dip down towards the submucosa
- Simple columnar
- Lamina propria has lots of MALT
- No mucosal circular fold or vili
- Absorptive cells of the LI have apical microvilli

Submucosa:
- Loose areolar C.T. with lymphatic and blood vessels

Muscularis:
- “Standard” in cross-section, except for thick bands called teniae coli line the 4 colonic segments
- Tonic (constant) contraction of these smooth muscle bands gathers the LI into haustral sections which churn in the form of peristalsis

Serosa:
- Same as always for transverse and sigmoidal colon
- The ascending and descending are retroperitoneal, so they have adventitia most superficially
- Omental appendices are pouch-like sacs of the visceral peritoneum (serosa) filled with adipose tissue and attached to the Teniae coli

Question 49

Describe the role of gastrin in filling the cecum with chyme and the process of haustral churning

Answer 49

Gastrin (secreted by G cells of the stomach) relaxes the ileocecal sphincter which allows chyme to enter the cecum

Hautral churning = each haustra fills, distends, and squeezes contents into the next haustra

Question 50

Give an overview of absorption in the large intestine, as well as the production of feces, and general components of feces.

Answer 50

The only molecules absorbed at this point are bacterially produced vitamins (some B’s and K’s)

Water absorption via osmosis is key as the liquid chyme is dehydrated by water absorption to become solid feces

Feces is: a bit of residual water, inorganic salts, sloughed cells, bacteria and their metabolic products, undigested material, and unabsorbed digested material

Question 51

Describe, briefly, the defecation reflex and the role of the external anal sphincter

Answer 51

Decreasing the overall volume of the rectum increases pressure against the internal walls, which forces open the internal anal sphincter

Voluntary relaxation of the external sphincter allows for the elimination of feces

Question 52

Name and describe the 3 phases of digestion.

Answer 52

1. Cephalic = taking place in the head - sight, smell, and even the thought of food triggers this phase of primary salivation and some preparation and secretion of gastric juice in advance of food moving into the stomach
2. Gastric = Triggered once food reaches the stomach. The goal is to increase gastric digestion and motility - and gastric emptying. Has neuronal (ENS and parasympathetic ANS) and hormonal (gastrin) component
3. Intestinal = Triggered once food reaches the intestine. The goal is to slow down gastric motility and gastric emptying to keep the duodenum from being overloaded. Has neuronal (inhibited parasympathetic and stimulated sympathetic ANS and hormonal (CCK and secretin) component

Question 53

Contrast the relationship of the gastric and intestinal phases to gastric emptying.

Answer 53

In the Gastric phase, Gastrin is released hormonally to stimulate gastric juice secretion, tighten the lower esophageal sphincter, increase motility, and relax the pyloric sphincter to get gastric emptying going.

In opposition to the gastric phase, the intestinal phase is inhibitory to slow the exit of chyme from the stomach and keep the duodenum from being overloaded with chyme

Question 54

Compare and contrast neural vs hormonal control of the gastric phase of digestion

Answer 54

Neural regulation:
Chyme entering the duodenum triggers stretch receptors to signal the medulla - which inhibits parasympathetic activity and stimulates sympathetic ANS activity which inhibits gastric motility

Hormonal regulation:
CCK inhibits gastric motility as well as stimulates the secretion of pancreatic juice and bile, and relation of the sphincter of Oddi to allow pancreatic juice into the duodenum

Question 55

Name the 2 hormones that control the intestinal phase of digestion, and which cells secrete them

Answer 55

• Cholecystokinin (CKK) from CCK cells in intestinal crypts
• CCK