BioMed GI Lectures I & II

The GI system is a what

tube from the mouth to the anus- technically external to our body

the mouth is composed of

tongue, teeth, oropharynx (back of mouth), laryngopharynx, major salivary glands (parotid, submandibular, sublingual), minor salivary glands, hard palate, soft palate, uvula

the tongue is a what muscle 

long voluntary muscle 

superior surface of tongue

stratified sqamous epithelium and numerous papillae (3 for taste, 1 is not)

many of the papillae on the tongue contain

sensory receptors (specilaized nerve endings) for the sense of taste in the taste buds

bloody supply of the tongue

lingual branch of external carotid artery

venous drainage of the tongue

lingual vein which joins the internal jugular vein

the taste buds are involved in detecting the 5 elements of taste perception which are

saltiness, sourness, bitterness, sweetness, and savoriness(umami)

which papillae do not contain taste buds

filiform papillae

what nerves supply the tongues voluntary muscle

the hypoglossal nerves (12th cranial)

which nerves are the nerves responsible for somatic sensation (pain, temp, touch)

lingual branch of mandibular nerves 

what are the nerves of taste

the facial and glossopharyngeal nerves

what are the functions of the tongue

chewing (mastecation), swallowing (deglutition), speech, and taste

the teeth are embedded where 

in the alveoli or sockets of the alveolar ridges of the mandible and the maxilla

babies are born with how many sets of teeth

2

functions of the teeth

incisors and canine- cutting teeth

premolar and molar- grinding and chewing

blood supply of teeth

maxillary arteries 

veins that dump into internal jugular

upper teeth nerve supply

maxillary nerves 

lower teeth nerves

mandibular nerves.

salvia contains what?

enzymes that begin the process of breaking down carbohydrates in the mouth

which enzymes does the salvia contain

amylase (ptyalin) which begins digestion of carbs and lingual lipase that starts breakdown of fats 

also contains lysozyme that has antimicrobial properties to help protect against infection by killing bacteria

salivia has 3 other main functions: 

• lubrication: moistens the mouth which is necessary for comfortable speech and forming food into a mass that is easy to swallow

• oral hygeine: helps wash away food particles and bacteria, neutralizing acids and reducing the risk of tooth decay and gum disease 

• protection: contains minerals to strengthen teeth and antibacterial agents to protect the mouth from infection

how many L of saliva do we produce a day 

1.5 L

what controls saliva production

the autonomic nervous system- specifically the parasympathetic

eating triggers

parastalsis

the GI tract has 4 distinct layers 

• mucosa: epithelia, lamina propria, and loose muscle 

• submucosa: glands, ducts, neurons, lymphatics

• muscularis: circular and longitudinal muscle, lympathics and neurons 

  • serosa: smooth outer layer and its associated connective tissues 

the mucosa layer consists of 3 layers of tissue:

• Mucous membrane formed by columnar epithelium (innermost layer- protection, secretion, absoprtion)

• lamina propria consisting of loose connective tissue that supports the blood vessels that nourish the inner epithelial layer and varying amounts of lymphoid tissue that protects against microbial invaders

• muscularis mucosa a thin outer layer of smooth muscle that provides involutions of the mucosal layer

the mucosa contains many folds that

increase surface area for optimal nutrient absorption

the submucosa layer is 

a layer that consists of. loose areolar connective tissue containing collagen and some elastic fibers, which binds the muscle to the mucosa 

within the submucosa are

blood vessels, nerves, and lymph vessels

nerve plexus of submucosal layer

submucosal plexus that contains sympathetic and parasympathetic nerves

muscular layer consists of 

2 layers of smooth (involuntary) muscle but the stomach contains a 3rd

how are the muscle fibers arranged

longitudinally in outer layer and inner layer encircle the wall of the tube

between the two muscle layers is what

the myenteric plexus- sympathetic and parasympathetic nerves that control the rhythm and force of muscle contraction

how does contraction and relaxation in the gut work?

in waves which push the contents of the tract onwards - called peristalsis 

serosa is the outer most layer that apart of what

the mesentary

in the abdomen the serosa is a serous membrane that covers the organs… whats it called

the peritoneum

the peritoneum is richly supplied with

blood and and lymph vessels, contains many lymph nodes 

the peritoneum consists of 2 layers

the parietal peritneum which lines the abdominal wall

the visceral peritoneum which covers the organs

this is a physical barrier which locals the spread of infection

the two layers of the peritoneum are in close contact and friction between them is prevented by.

the presence of serous fluid i

ingestion- what can go wrong?

difficulty eating, swallowing, loss of appetite/anorexia, hyperphagia 

propulsion- what can go wrong?

motility disorders Hirschsprung’s disease, anti-cholinergics, GERD, “retroperistalsis,” constipation, diarrheas 

mechanical digestion- what can go wrong?

motility disorders, diverticular diseases, constipation, diarrheas, 

chemical digestion- what can go wrong?

many things- gastritis, chrons, celiac, lactose. intolerance,

absorption - what can go wrong?

constipation, diarrheas

defecation- what can go wrong?

constipation, hemmorhoids, anal diseases 

types of glands/hormone secreting cells

exocrine- act locally, secrete into spaces via ducts

endocrine- secrete into a bloodstream, act globally

paracrines- secreted into blood or interstitial fluid but act locally

autocrines- released into interstitial fluid and act on releasing cell

types of secreted products

simple solutions, complex solutions, enzymes, hormones

the stomach mucosas epithelial lining consists of what

surface mucus cells that secrete a protective coat of alkaline mucus 

what dots the surface of the epithelium giving it the appearance of a well-used pin cushion

gastric pits

each pit marks the entry to what

to each gastric gland which secretes a complex digestive fluid referred to as a gastric juice

the gastric gland contains what cells and what are their purpose 

parietal cells- produce both HCL and intrinsic factor. HCl is responsible for the high acidity of the stomach needed to activate the protein-digesting enzyme, pepsin 

Chief cells- secrete pepsinogen the inactive form of proenxyme pepsin. HCl is necesary for that conversion 

the glands of the body and fundus produce most of the

gastric juices needed for digestion

Gastrin- where is it secreted? what stimulates its secretion? whats its function?

G-cells, food stimulates and low pH inhibits, induces HCl release

Histamine- where is it secreted? what stimulates its secretion? whats its function?

ECL cells, food stimulates, induces HCl release 

HCl- where is it secreted? what stimulates its secretion? whats its function?

secreted by parietal cells, food stimulates its secretion, it degrades food, kills microbes, cleaves pepsinogen to make pepsin

Pepsinogen- where is it secreted? what stimulates its secretion? whats its function?

secreted by chief cells, stimulated by food, breaks down proteins 

Intrinsic factor- where is it secreted? what stimulates its secretion? whats its function?

secreted by the parietal cells, stimlated by food, absorbs vitamin B12 in ileum, loss of Intrinsic factor results in pernicious anemia

somatostatin - where is it secreted? what stimulates its secretion? what is its function?

secreted from D cells, secreted bc of low pH, it is a paracrine hormone in the stomach and inhibits release of other products 

mucous- where is it secreted? what stimulates its secretion? what is its function?

secreted by the mucous cells.. aka the goblet cells, ongoing secretion, protects from HCl and pepsin

types of non-ductal secretions

• autocrine: released into interstitial fluid and act on the releasing cell

• paracrine: released into interstitial dfluid but act locally on other cells

  • endocrine: secrete into blood stream via capillaries

what happens in GERD

• lower espohageal sphincter weakens and allows acidic sromach contents back into the esophagus

• heartburn, acid reflux

• becomes a disorder when it occurs chronically

• causes anorexia or weight loss

• can lead to barrets metaplasia which is a risk factor for esophageal cancer

• causes: body habitus, medications, diet, hiatal hernia, smoking and alcohol

there is always a small quanitity of gastric juice present in the stomach, even when it contains no food.. what is this called 

gastric juice 

secretion reaches its maximum level about

1 hour after a meal and then declines to fasting level after about 4 hours

phases of gastric secretion

1. cephalic phase

2. gastric phase

3. intestinal phase

cephalic phase 

before it reaches the stomach, flow of juice due to reflex stimulation of the vagus (parasym.) nerves initiated by the sight, smell or taste of food 

gastric phase stimulated by

presence of food and the enteroendocrine cells in the pylorus and duodenum secrete the hormone gastrin which passes directly into the circulating blood.

what happens in gastric phase

• gastrin, circulating in the blood which supplies the stomach, stimulates the gastric glands to produce more gastric juice (HCl)

• in this way secretion of digestive juice is continued after completion of a meal and the end of the cephalic phase

  • gastrin secretion is suppressed when the pH in the pylorus falls to about pH

intestinal phase is. when 

partially digested contents of the stomach reach the small intestine, two hormones, secretin and cholecystokinin, are produced by endocrine cells in the intestinal mucosa 

this phase slows things down 

production of HCl

1. CO2 from the blood and H2O from parietal cells from carbonic acid (H2CO3). this is then catalyzed by carbonic anhydrase

2. this immediately disassociates into H+ and bicarb

3. the H+ proton is pumped into the stomach

4. chloride is moved from the blood into the cell and then pumped into the lumen. to combine with H+ to form HCl.

once pepsin is activated by HCl from pepsinogen, pepsin begins to 

cleave peptide bonds within dietary proteins, breaking them down into smaller peptide fragments 

the small intestine is continuous with the. stomach at

the pyloric sphincter q

length of small intestine

5 meters long and 2.5 cm in diameter

in the small intestine, the chemical digestion of food is completed and 

absorption of most nutrients takes place 

secretions from the gall bladder and pancreas merge in a common structure-

heptaopancreatic ampulla and enter the duodenum at the duodenal papilla

the duodenal papilla is guarded by

a ring of smooth muscle, the hepatopancreatic sphincter.

the ileum is the terminal section of the small intestine and ends at the 

ileocecal valve which controls flow of material from ileum to cecum and prevents backflow str

structure of the small intestine:

1. serosa: peritoneum mesentary

2. muscle layer: made up from longitudinal and circular muscle layer

3. submucosal layer: made up from areolar tissue

4. mucosa: SA is greatly increased by circular folds, villi, and microvilli

   1. goblet cells that secrete mucus that are present between the enterocytes

cholecystokinin (CCK)- where is this secreted? what stimulates this secretion? what does this secretion do?

secreted in duodenum, stimulated by chyme, coordinates gall bladder and pancreatic secretions, maintain the gallbladder and pancreas, prolongs gastric emptying 

secretin- where is this secreted? what stimulates this secretion? what does this secretion do?

secreted in the duodenum, secretions stimulated but H+ and fatty acids, stimulates pancreas to release HCO3 and inhibits gastrin release

gastric inhibitory peptide (GIP) - where is this secreted? what stimulates this secretion? what does this secretion do?

secreted in duodenum, stimulated by glucose/fatty acids/peptide, stimulates pancreatic release of insulin, inhibits HCl release by parietal cells

enzymes- where is this secreted? what stimulates this secretion? what does this secretion do?

secreted by pancreas, stimulated by CCk, proteases/lipases/amylases/nucleases

HCO3- where is this secreted? what stimulates this secretion? what does this secretion do?

secreted by pancreas, stimulated by CCK, neutralizes chyme pH

bile - where is this secreted? what stimulates this secretion? what does this secretion do?

secreted by gall bladder, CCK, emulsify fats 

insulin and glucagon- where is this secreted? what stimulates this secretion? what does this secretion do?

secreted by pancreas, stimulated by blood glucose, manages blood glucose

where does food absorption take place

enterocytes

the entire epithelium is replaced every 

3-5 days 

digestion of proteins in small intestine

proteins broken down to amino acids

• trypsinogen and chymotrypsinogen are inactive enzyme precursors activated by enterokinase, an enzyme in the microvilli which converts them into the active proteolytic enzymes trypsin and chromotrypin

• these enzymes convert polypeptides to tripeptides to dipeptides to amino acids

  • these needs to be produced as inactive precursors and are activated only upon their arrival in the duodenum otherwise they would digest the pancreas

digestion of carbohydrates in the small intestine

• begins in the mouth

• once in the stomach salivary amylase activity is halted due to an acidic environment

• further digestion resumes in the small intestine with the action of pancreatic amylase, which breaks down polysaccharides into disaccharides like maltose, sucrose, and lactose

• dissacharides are located in the brush border of the small intestines then they hydrolyze into monosaccharides absorbed by intestinal cells and transported into blood stream

water soluble vitamins are absorbed how

C and B- dircetly into the bloodstream from the small intestine

fat soluble vitamins are absorbed how

A, D, E, and K - absorbed along with dietary fats and are transported via chylomicrons

minerals are absorbed

through active transport or passive diffusion in the small intestine and transported to various tissues for utilization

pernicious anemia

a type of anemia caused by the body’s inability to absorb vitamin B12 from the small intestine

• autoimmune disorder: body produces antibodies that attack the cells in the stomach that produce intrinsic factor, a protein needed to absorb vitamin B12

• lack of dietary intake: a strict vegan diet or severe malnutrition can lead to vitamin B12 deficiency

  • S+S: fatigue, pale skin, weakness, shortness of breath, dizziness, paresthesias, loss of appetite, sore tongue

• surgery: removal of part or all of the stomach or small intestine can interfere with vitamin B12 absorption

lactose intolerance 

• occurs when absorptive cells of small intestine do not produce enough lactase which is the enzyme that breaks down lactose 

• bacteria in large intestine ferment the undigested lactose, a process that produces gas 

  • S+S: cramps, bloating, diarrhea

bile is produced in the _____ and stored in the ______ and released into the _______

bile is produced in the liver, stored in the. gall bladder, and released into the duodenum via the bile duct 

bile does what

emulsifies dietary lipids preparing them for digestion and absorption

• makes lipids water soluble

what is bile 

• various forms of cholic acids forming salts with Na or K 

• synthesized in the liver from cholesterol 

• can be made by gut microbes 

• bile acids are amphipathic and help to organize dietary lipids aiding in their digestion

chylomicrons

large lipoproteins particles that transport dietary fats, including triglycerides and cholesterol, from the intestine to other tissues like the liver and peripheral tissues

VLDL

primarily composes of triglycerides (over 50%) with smaller amounts of cholesterol and protein. Produced in the liver and released into the bloodstream. it travels to tissues where triglycerides are extracted, leaving behind a smaller particle called IDL (intermediate-density lipoprotein)

LDL

primarily composed of cholesterol (over 50%) with smaller amounts of triglycerides and protein. travels in the blood stream to tissues where cholesterol is deposited