Week 10 - the digestive system

Function of digestive system

• System made of multiple organs that function together to extract nutrients from the food that we ingest - absorb as much nutrients as possible

• Focus on the processing of the macronutrients: proteins, carbs, and lipids.  

What is another function of the digestive tract other an absorption?

role in protecting the body from harmful pathogens/toxins: along with the vomiting reflex and increased intestinal motility causing diarrhea, there are protective secretions to help protect internal environment.  

Which digestive organs directly contact food, and what are accessory organs which make secretions that support digestion?

• The oral cavity, esophagus, stomach, small and large intestine are all organs that directly contact ingested food.  

• Salivary glands are near the oral cavity but do not directly contact good so considered an accessory to digestive tract. Same as liver, gallbladder and pancreas. 

What is pathway of food from eating through digestive tract?

first enters through the oral cavity through the esophagus to the stomach where it is held before entering into the small and then large intestine.

Think of digestive system as a long tube in which food travels down the middle. The food encounters epithelial cells in the tube which have different jobs (secretion/absorption) 

What are the 4 processes of the digestive system?

digestion, secretions, motility, and absorption

What is digestion

mechanical (chewing – making smaller physically) and chemical (enzymes – making food smaller physically and chemically for absorption). 

What are secretions in digestion tract (3 types)

secretions: to chemically digest epithelial cells need to secrete –  

• Exocrine = Enzymes/acid/water (from cell into lumen),  

• Endocrine = hormones secreted into the blood so it cannot mix with food in lumen. 

• Paracrine = signals between epithelial cells can change how other epithelial cells function. 

What is motility?

moves food throughout the digestive tract – purpose is to move food but can also help with mechanical movement like chewing/stomach digestion.

What is absorption?

movement of food particles from the lumen, through the epithelial cells of the digestive tract and into the blood. 

What are the 4 layers of the digestive tract (in stomach and small/large intestine)

Each tissue layer has a function and other than the mucosa are similar between the three digestive organs - mucosa, submucosa, muscularis externae, and serosa

Mucosa location/structure/function - what is the muscularis mucosae

faces the inside of the lumen of the gastrointestinal tract and therefore comes into direct contact with the lumen contents. The mucous membrane is a single layer of epithelial cells that are a component of the mucosa layer. These epithelial cells are variable between different organs (make different secretions) 

Has a layer of supporting tissue under epithelial cells and then a thin layer of smooth muscle that sits at the bottom of the mucosal layer which changes the shape when necessary called the muscularis mucosae. 

submucosa location/structure/function

serves as a connecting tissue to adhere the mucosa to the muscularis externa layer. Within this is a plexus of the enteric nervous system called the submucosal plexus. 

Muscularis externae location/structure/function

usually made up of two layers of smooth muscle that are circumferential (circular) or longitudinal in orientation. Within this tissue layer is the second plexus of enteric neurons called the myenteric plexus

• Patterns of contractions/relaxations are coordinated so we get motility. 

• In the stomach we have an oblique layer – muscle in a diagonal orientation 

Serosa location/structure/function

outer protective layer which is continuous with the mesentery (connective tissue layer) that lines the abdominal cavity of the body. 

How does the shape of the digestion tube change

tube varies along the length of the tract; the stomach is a J shaped sac whereas intestines are more circular. 

What is on these tubes that promotes absorption?

Throughout the tubes there are also always blood vessels (to take up the food being absorbed and take the hormones being secreted), and lymph nodes (part of immune system and for absorption of lipids). 

What is the enteric NS and where is it found?

Neuron bodies of the enteric NS are found in two layers of the gastrointestinal tract, the submucosal and muscularis externa layer - submucosal and myenteric plexus.

essential for the ability to co-ordinate multiple organs that are spread through the length of the abdominal cavity as well as being able to efficiently signal through the thick layers of the gastrointestinal tract.  

What is the ENS regulated by?

Both the parasympathetic and sympathetic NS innervate and regulate the ENS, however the ENS is autonomous and can function without these inputs from the CNS.  

Function of myenteric plexus

helps coordinates relaxation/contraction of muscles which will cause different motility patterns to move food along tract (stimulate/inhibit smooth muscle). 

Function of submucosal plexus

• neurons in the submucosal layer project to the epithelial cells which trigger them to make secretions (enzymes/water/mucus) which go into the lumen.  

• Submucosal plexus neurons can also serve as sensory neurons detecting the composition of the lumen (is it liquid/solid/calories/ lipid/ stretching etc). 

What is the start of the digestive tract?

Mouth is start of digestion of food – almost no absorption of macronutrients occurs here but the mechanical break up of food does start. 

What is saliva made of and what secretes it?

• solution secreted within the mouth and is made of mostly water but also proteins, ions, and digestive enzymes – salivary amylase and lingual lipase. 

• Most of the cells that line the mouth secrete saliva but most is secreted by three pairs of exocrine glands.  

What is the structure of the salivary glands?

converging duct system similar to pancreas; starts with a single gland cell that secretes into its own duct, multiple ducts flow into a smaller number of large ducts until meeting at a main duct.  

What are the three salivary glands and what do they secrete?

• 1) Parotid glands: secretes a watery liquid that contains salivary amylase and lingual lipase 

• 2) Submandibular gland: secretes a thicker liquid that contains more mucus and less salivary amylase and lingual lipase. 

• 3) sublingual gland: secretes more mucus and almost no enzymes 

Functions of saliva

lubricates food, helps us taste food, aids with speech, protects mouth and teeth (acid) 

At what different situations are different salivary glands active (eating versus talking)

eating food = more parotid stimulation, more talking = sublingual activation 

What is mastication?

(chewing) is the mechanical manipulation of food, resulting in the breakdown of whole food particles into smaller pieces.  

What is a bolus?

Along with the salivary secretions, food is converted into this softened mass

How does chemical digestion occur in the oral cavity?

through the actions of salivary amylase and lingual lipase:

• Salivary amylase chemically digests the complex carbohydrate (amylose), while lingual lipase digests lipids. 

• Lingual lipase is not active in the oral cavity, it must be brought to the stomach where the pH is lower (where acid is being secreted) 

What are the three motility aspects in the oral cavity?

mastication (chewing), swallowing, and peristalsis (movement of food from top of esophagus to bottom) which is distinct from chewing/swallowing reflex.

What is the voluntary stage of swallowing?

number of times to voluntarily chew is up to the individual even if not thoughtfully considered. The production of a bolus is pushed to the back of the oral cavity also under voluntary control of the tongue.  

What is the pharyngeal stage of swallowing?

the sensation of the bolus at the pharynx (back of oral cavity) will trigger the pharyngeal stage. Sensory neurons will activate the neural reflexes required to close off the nasopharynx.  

• The epiglottis is bent back over the glottis to prevent food/fluid from entering into the airway (trachea).  

• The upper esophageal sphincter relaxes to allow the bolus down the esophagus. 

What is the esophageal stage of swallowing?

the bolus is pushed down the esophagus through coordinated muscular contractions and relaxations in a motility pattern called peristalsis. The lower esophageal sphincter is also coordinated to relax to allow for the bolus to enter the top part of the stomach. 

What is secondary peristalsis (within esophageal stage)

initiated if a bolus does not successfully make it to the stomach -  a more forceful set of muscular contractions of the esophagus to push the bolus unidirectionally down into the stomach. 

What is peristalsis?

motility pattern of the esophagus - Gravity is not involved in peristalsis – movement is due to smooth muscle contraction. 

How much absorption occurs in the oral cavity/ what can be absorbed?

Minimal absorption occurs in the oral cavity; there is some sublingual absorption that can occur for small non-polar molecules which is considered in the design for some medications (under the tongue) 

Glucose can be absorbed in the oral cavity because there is some expression of glucose carriers in the epithelial cells of the mouth. 

What is the structure/function of the stomach?

acts as a resivoir for food before it enters the intestines to be absorbed. Considered a tube but shaped so that one side extends out more than the other – called the greater curvature on the left side of the organ, and the lesser curvature on the right side.  

What is the fundas?

The upper portion of the stomach that first gets the bolus

What is receptive relaxation?

The fundas must stretch out in response to the bolus since the stomach is no opened at rest

What happens when the bolus is pushed into the antrum regions of the stomach?

it starts to become liquified by coordinated muscular contractions for mechanical breakdown and mixture with gastric juices.  

What are rugae

folds of the mucosa layer of the stomach which can help expanding the stomach volume by stretching out when a meal is larger.  

What is chyme and where does it exit?

As the bolus is mixing with the secretions is becomes a new solution (liquified) which can now enter into the first part of small intestine when the pyloric sphincter opens up (mostly closed at rest and opening is regulated).

What are the unique things about which layers of the stomach

secretions of the mucosa layer - and the addition of the oblique layer of the muscularis externae

How much gastric juice is secreted into the stomach in one day and function?

Approximately 2-3 liters of gastric juices are secreted into the stomach in a day to aid in both mechanical and chemical digestion.  

What are gastric glands/where

located of mucosa layer of the stomach contain both exocrine and endocrine cells - glands in the fundas and body of the stomach contain epithelial cells that secrete hydrochloric acid (HCI) and various enzymes.  

What are mucus neck cells located/function

located on the gastric glands and secrete mucus to protect epithelial cells against the acid they produce - sits on top of mucosa layer.

What are parietal cells

acid secreting cells of the gastric gland which also secrete an intrinsic factor - protein that aids in vitamin b12 absorption in the intestine as well as HCL causing the low pH of the stomach.

What are chief cells?

secrete the enzyme gastric lipase and the zymogen, pepsinogen (active form = pepsin). 

What are zymogens?

inactive precursor proteins which when acitvated function as enzymes.

What is different about glands in the antrum - and what hormones are secreted there?

they contain more endocrine cells which secrete hormones

• Gastrin is a hormone secreted by G cells and is involved in controlling gastric motility and stimulating acid secretion from parietal cells.  

• Also within the antrum of the stomach mucosa layer are D cells which secrete somatostatin, a hormone that inhibits gastrin release from G cells.  

• Different stimuli will either stimulate G or D cells. 

How do acidic gastric juices affect salivary amylase and lingual lipase?

Acidic gastric juice secreted in the stomach inactivates salivary amylase and activated lingual lipase - due to difference in pH

What is the role of pepsinogen and when is it activated?

released by chief cells is a zymogen and activated by an acidic environment - responsible for starting protein digestion in the stomach.

How else does stomach acid help digest proteins?

aids in protein chemical digestion through helping to denature (unfolds) the ingested proteins, so that primary and secondary strctures are availaible for enzymes digestion.  

To reduce bolus to chyme what motility patterns occur in the stomach? Describe process

• As the exocrine cells increase their secretions, the bolus is mixed with the secretions in the lower regions of the stomach. The sphincter at the bottom of the stomach (pyloric sphincter) remains closed while the mixing begins.  

• Gentle mixing waves are generated every 15-25 seconds to mix the food with the secretions of the gastric glands within a few minutes of food entering the stomach, termed propulsion – smooth muscle contraction to push contents from upper to lower region. 

• More vigorous mixing occurs as mechanical digestion proceeds, starting at the body of the stomach, which intensifies towards the pylorus called grinding – this liquifies bolus into chyme. 

• Retropulsion is used to describe the movement of chyme back into the body of the stomach where more mixing can occur.  

When is the pyloric sphincter stimulated to open - and what is this movement called

During grinding and retropulsion, the pyloric sphincter can be stimulated to open briefly, allowing a small amount of chyme to empty into the small intestine.

empty into the small intestine onces it has been thoroughly digested; movement is called gastric emptying. 

When is the lower esophageal sphincter triggered to open?

only triggered to open to permit the entry of the bolus into the stomach and after that should remain closed to prevent the reflux of gastric contents into the esophagus. 

What can be absorbed in the stomach/ how much absorption occurs here and why

Minimal to no nutrient absorption – there is a thick mucus in place to protect epithelial cells from acid which does not allow for nutrient movement. Exception = small, lipophilic drugs and alcohol. Stomachs main job is mechanical and chemical digestion.

Where does the most chemical digestion and absorption occur?

Small intestine

What are the three functional segments of the small intestine?

The duodenum, jejunum, and the iluem

What is the duodenum (location/function)

first portion of small intestine where the secretions of the pancreas (many enzymes from here support chemical digestion) and liver first interact with the chyme that has just exited the stomach. 

short segment of the small intestine,but is the region where most of the chemical digestion is completed. 

mucosa wall has villi to increase surface area to improve chemical digestion but also to aid in some nutrient absorption.  

What is the jejunum (location/function)

next segment which has many villi to aid in maximal nutrient absorption Many absorptive epithelial cells are found, as the apical membrane of these cells have the necessary transporters to move the contents of the lumen across the absorptive epithelia. 

• This is where most of the nutrient absorption occurs.

What is the ileum (location/function)

longest segment of the small intestine and although it has villi for increased surface area, it is an extra length of the lumen with enterocytes to assist in any nutrient absorption that may not have normally occurred in the jejunum (backup). Some contents (vitamin b12 and bile salts) are mostly absorbed here.

What are villi?

finger-like projections that reach into the lumen of the small intestine to increase surface area (helps for nutrient absorption and chemical digestion). Found on the mucosa layer of the small intestine. 

What is an enterocyte?

absortive epithelial cells in the mucosa layer

What are goblet cells

found in epithelial layer that secrete mucus to aid in lubricating the small intestine. 

What are intestinal gland cells

secrete a watery mucus solution that also contains bicarbonate to aid in the neutralization of the acidic chyme coming from the stomach - protects the mucosa layer but is not as thick as in the stomach to still allow for nutrient absorption (bicarbonate secretion is best protection)

What are the endocrine cells of the small intestine mucosa and what do they secrete?

• S cells – secrete hormone secretin which stimulates bicarbonate secretions from pancreas and liver.  

• I cells – release the hormone cholecystokinin (CCK) which regulates the digestive tract by stimulating enzyme secretions from the pancreas and causes the gallbladder to contract releasing bile solution into the small intestine. 

• K cells – secrete hormone glucose dependent insulinotropic peptide (GIP) which stimulates the release of another hormone insulin. 

What are the big folds of the small intestine called - and those found on the individual epithelial cells

• The bigger folds are called plicae. 

• There is even more increased surface area on each individual epithelial cells called microvilli (folds x3). 

What is enterocyte structure?

• transporters on apical membrane, and microvilli. 

• Under microscope looks blurry – called brushed border. 

What are brush border enzymes and function?

Embedded within the apical membranes of enterocytes are several digestive enzymes called brush border enzymes. 

digestion of some macromolecules is achieved by the presence of these enzymes on the plasma membrane, on the microvilli.  

How do brush boarder enzymes cause absorption?

• these enzymes are not secreted but are part of enterocyte membranes.  

• These enzymes cause the production of the absorbable nutrient units on the same cells that will absorb the nutrients (breaks down and then absorbs)

What are the three brush border enzymes that break down disscharides and into what?

• 1) Lactase: also called disaccharidase because it chemically digests the disaccharide lactose into two monosaccharides glucose and galactose. 

• 2) Sucrase: a disaccharidase that chemically digests sucrose into two monosaccharides glucose and fructose 

• 3) Maltase: Another disaccharidase that chemically digests maltose into two units of glucose 

What are the other 4 brush boarder enzymes and what do they chemically digest into/absorb?

• 4) Alpha-limit dextrinase: an enzyme that chemically digests branched polymers of glucose (complex carbohydrates like amylopectin) into linear glucose polymers 

• 5) Dipeptidase: a protein digesting enzyme also called a protease that chemically digests a dipeptide into two single amino acids.  

• 6) Aminopeptidase: a protease that chemically digests a peptide or protein, by removing a single amino acid from the amino-terminus of the protein strand. 

• 7) Enterokinase: also called enteropeptidase, a protease that can chemically digest proteins/peptides, including an important zymogen called trypsinogen into the enzyme trypsin. 

What are segmentations - and where are differences in speed

small intestine motility in which localized mixing contractions function to increase the interactions of the food particles in the chyme with the absorptive cells in the intestinal mucosa - requires the presence of chyme

Segmentations are more rapid in the earlier sections of the small intestine (duodenum) as compared to the more distal segment (ileum) - but depends on the composition of the intestine (don’t need much for water)

In the absence of chyme, what is the motility pattern of the small intestine?

the migrating motility complex (MMC) which is first initiated in the stomach and waves through the small intestine. MMC pushes out any particles of food that remain in the stomach or in the small intestine. This is automatic and not stimulated by presence of chyme.

In what instance would the migrating motility complex (MMC) be helpful?

• chyme is usually moved out of the stomach efficiently during regular digestion, the pyloric sphincter only permits particles that are 1-2mm in size.  

• Larger fragments that are not digestible can be moved out of the stomach via the MCC since the pyloric sphincter fully relaxes in this state to permit passage of these larger items.  

What are the three phases of MMC

gentle, moderate, and vigorous

What stops MMC waves?

MC waves move from the stomach to the large intestine until disrupted by the presence of food – if you ingest a meal while the MMC is moving throught lower segments of tract, it will automatically stop and be ready for the typical motility patterns (propulsion, grinding, retropulsion; small intestine; segmentations, peristalsis) 

What percentage of nutrients are absorbed in the small intestine?

almost 100% of all ingested carbohydrates are absorbed and 95-98% of all proteins and lipids are absorbed.  

What do carbs, proteins, and lipids have to be broken down into so they can be absorbed?

• Carbohydrates must be broken down into monosaccharides to be absorbed 

• Proteins must be broken down into individual amino acids to be absorbed (can also absorb domes di and tri-peptides) 

• Lipids (triglycerides) must be broken down into free fatty acids, monoglycerides, or cholesterol to be absorbed. 

What is the function of the large intestine/what is absorbed and created here?

finish the process of absorption - nutrients absorbed here are generally not those that were absorbed in the small intestine (monosaccharides/amino acids/fatty acids) but the other contents of the lumen. 

production of certain vitamins and the formation of feces that will be expelled. 

Where does chyme enter the large intestine?

Through the ileocecal valve which relaxes each time a peristaltic wave reaches it.  

What path does the chyme travel through the large intestine?

chyme solution is moved up the ascending colon, across the transverse colon, down the descending colon, through the sigmoid colon towards the rectum.  

What are the haustra?

An interesting feature of the mucosa layer - pocket like structures that force the lumen contents to churn as they propel forwards flipping the contents over to expose the chyme contents to the surface of the mucosa. 

What is the microbiome (bacteria)

resides in the large intestine: Enzymes from bacteria help digest difficult items like cellulose and fibers. The nutrients released from this are utilized by bacteria to help them grow/stay healthy. Some of these nutrients are shared by the bacteria into circulation. can cause sickness depending on population.

Found along the entire digestive tract but mostly in large intestine

What is absorbed in great quantity in the large intestine and what does the leftovers create?

• Water is absorbed in the large intestine, although some was absorbed before this a lot is left here for absorption. 

• As the water is removed from the cyme and any remaining nutrients it becomes less liquid are more solid – goes from chyme to fecal matter where it is stored in the rectum until expulsion

What is the main secretion of the large intestine?

Main secretion of the large intestine is mucus from goblet cells that are part of the epithelial cells in the mucosa. 

Some electrolytes like potassium and chloride are also secreted but there are no other digestive enzymes secreted by the large intestine.  

Any remaining chemical digestion occurs in this organ because of the enzymes expressed by the bacteria

What is the speed of movement of chyme within the large intestine?

motility through the small intestine moves contents from the pyloric sphincter to the ileocecal valve in about 4 hours, forward movement of chyme in the large intestine is much slower around 18-24 hours.  (peristalsis is slower)

What triggers the opening of the ileocecal valve?

one reason it will open is due to the presence of a new bolus in the stomach.

What is the gastroileal reflex?

• A neural reflex is initiated that starts with the sensation of the bolus in the stomach, opening the ileocecal valve called the gastroileal reflex. 

• This reflex will stimulate a faster emptying of the small intestine contents into the large intestine to make room in the small intestine for a new meal.  

What are the benefits of bacteria

salvage undigested nutrients (like lactose and cellulose), they also produce significant amounts of absorbable vitamins – especially vitamin K. 

Intestinal gases called flatus, like hydrogen sulfide which are a byproduct produced by some species of bacteria, escape from the digestive tract – these gas produces can contribute to normal well-being. 

What and how much is left by the end of the small intestine versus end of the large intestine

By the end of the ileum, only about 1.5 liters of unabsorbed chyme remain. The large intestine absorbs most of this volume so that normally only about 0.1 liters of water is lost daily in the fecal matter that is expelled from the body.