PNB 2265 SI slides exam 3

5 functions of digestive system

• Ingestion of nutrients

• Secretion

• Mechanical and chemical processing

• Excretion

• Absorption

ingestion of nutrients

• Food and liquids enter the body through the mouth

• Begins the digestive process

• Allows the body to take in nutrients needed for energy and health

• Digestion of food begins in the mouth

mechanical and chemical processing

• Chewing and muscle contractions to break food into smaller pieces

• Prepares food for easier digestion

secretion

• Digestive organs release substances into the GI tract

• Includes enzymes, acids, bile and mucus

• Helps break down food and protect digestive tissues

absorption

• The movement of nutrients from digested food into the blood or lymph

• Happens mostly in the small intestine

• Allows the body to use nutrients for energy, growth, and repaire

excretion

• Undigested material becomes waste

• Waste moves to the large intestine because water is absorbed

• Feces are eliminated from the body

digestive gross anatomy

• Mouth

• Esophagus

• Stomach

• Small intestines

• Large intestines

• Rectum

mouth

• Entry point for food into the digestive tract

• Teeth chew food to break it into smaller pieces (mechanical digestion)

• Saliva begins chemical digestion

esophagus

• Muscular tube that connects the mouth to the stomach

• Peristalsis (wave like muscle contractions) move food downward

• Transports the bolus to the stomach for further digestion

small intestine

• Main site of digestion and nutrient absorption

• Enzymes and bile break down food further (chemical digestion)

• Nutrients pass into the blood or lymph for the body to use

large intestine

• absorbs water and electrolytes from indigestible food

• Forms and stores feces until elimination

• Houses bacteria that help break down some remaining nutrients

rectum

• Stores feces before they are eliminated

• Signal the body when it's time to have a bowel movement

• Ends the digestive tract for excretion

4 accessory organs of the GI tract

• Accessory organs > aids digestion but food does not pass through it

• Salivary glands

• Liver

• Pancreas

• Gallbladder

salivary glands

• Produces saliva containing:

• Water and mucus to lubricates food

• Enzymes (ex: amylase) which begins starch digestion

• Helps form the bolus for easy swallowing

• Supports oral hygiene by washing away food and bacteria

liver

• Produces bile, which emulsifies fats for digestion

• Processes nutrients from the hepatic portal vein

• Performs metabolism: carbohydrates, amino acids, lipids and detoxification

pancreas

• Produces digestive enzymes (amylase, lipase, proteases) for the small intestine

• Secretes bicarbonate to neutralize stomach acid in the small intestine

• Supports chemical digestion of carbs, fats, and proteins

gallbladder

• Stores and concentrates bile produced by the liver

• Release bile into the small intestine when fats are present

• Helps emulsify fats to increase digestion efficiency

4 layers of the GI tract

• Serosa

• Muscularis

• Submucosa

• Mucosa

Serosa

• Outermost layer of the digestive tract

• Made of connective tissue and a layer of epithelial cells

• Protect organs and reduces friction as they move in the abdominal cavity

muscularis

• Made of muscle and nervous tissue

• Muscle: moves and mixes food through peristalsis and segmentation

• Nervous tissue > coordinates contractions via the myenteric plexus

• Important > essential for motility and proper digestion

myenteric plexus

• Network of nerves located between the circular and longitudinal muscle layers of the muscularis

• Control muscle contraction for peristalsis and segmentation

• Essential for coordinating motility throughout the digestive tract

submucosa

• Made of connective, nervous, and lymphatic tissue

• Connective tissue > supports the mucosa and binds it to the muscularis

• Nervous tissue (submucosal plexus) > regulates secretion and local blood flow

• Lymphatic tissue > helps defend against pathogens in the gut

mucosa

• Made of epithelial, muscle, nervous, and lymphatic tissue

• Epithelial tissue > lines the lumen and absorbs nutrients

• Muscle tissue > creates fold and movement to mix food

• Nervous and lymphatic tissue > coordinate secretion, absorption, and immune defense

muscularis externa

• Smooth muscle layer of muscularis that creates GI motility

• Inner circular layer > squeezes the lumen to mix and push food

• Outer longitudinal layer > shortens the tube to move food along

• Myenteric plexus > major site of nerve control in the GI tract, mostly parasympathetic

• Surrounds the lumen, allowing controlled movements based on radius and length

mucosa: epithelium

• Top of the mucosa, lines the lumen

• Simple or stratified columnar cells

• Cell types:

• Enterocytes > absorb nutrients

• Enteroendocrine cells > secrete hormones

• Mucin- producing cells > secrete mucus for protection

mucosa: lamina propria

• Connective tissue layer beneath the epithelium

• Supports small blood vessels and lymph nodes

• Provides nutrient supply and immune defense

mucosa: muscularis mucosae

• Thin layer of smooth muscle

• Shapes fold (villi) and microvilli to increase surface area

• Aids movement of mucosa for better absorption and secretion

motility: peristalsis

• Found in: esophagus, stomach, small intestine, large intestine

• wave like contractions of smooth muscle

• Primary function > moves food forward through the GI tract

motility: segmentation

• Back and forth movements that mix food with digestive juice

• Found in > small intestine

• Primary function > mixes chyme to enhance digestion and absorption

motility: tonic contractions

• Sustained, long contractions of smooth muscle

• Found in sphincter muscles

• Function > prevent backflow of food

motility: phasic contractions

• Short, quick contractions of smooth muscle

• Present throughout GI tract

• Function > produce forward and mixing movements of food

macro motility

•  large coordinated movements that move food along the GI tract

• Examples > peristalsis (forward movement) and segmentation (mixing in small intestine)

micro motility

•  small, local contractions that control movement and mixing within a section

• Examples: tonic contractions (sphincters) and phasic contractions (short, rhythmic contractions)

important molecules for motility

• Smooth muscle contraction is controlled at the cellular level

• Important molecules:

• Calcium (Ca 2+) > triggers contraction

• Calmodulin > binds calcium to activate MLCK

• Myosin light chain kinase (MLCK) > phosphorylates myosin to enable contraction

• Regulation for contraction:

• Cyclic nucleotides > modulate contraction/ relaxation

• Protein kinase A (PKA) > can inhibit contraction via phosphorylation

• Myosin light chain phosphatase (MLCP) > removes phosphate, causing relaxation

calmodulin

binds calcium to activate MLCK

Myosin light chain kinase (MLCK)

 phosphorylates myosin to enable contraction

Cyclic nucleotides

modulate contraction/ relaxation

protein kinase A (PKA)

can inhibit contraction via phosphorylation

Myosin light chain phosphatase (MLCP)

 removes phosphate, causing relaxation

apical surface

faces the lumen (inside the GI tract, "external")

• Function depends on epithelial specializations in the mucosa

basolateral surface

faces the blood (inside the body, "internal")

• Function depends on epithelial specializations in the mucosa

secretion and absorption cellular layout

• Mucosal cell line the inside of the GI tract and separate lumen (food inside the gut) from the blood

• ICF (intracellular fluid) / cytosol is the inside of the cell, where nutrients and molecules move through before reaching the blood

• This setup allows selective movement of molecules:

• From the lumen > cytosol > blood (absorption)

• From the blood > cytosol > lumen (secretion)

• Think of the mucosal cell as the gatekeeper, controlling what enters and leaves the body

From the lumen > cytosol > blood

absorption

From the blood > cytosol > lumen

secretion

overview of secretion and absorption

• Secretion > substances move from blood/ cytosol > lumen

• Absorption > substances move4 from lumen > cytosol/blood

• Together, they allow digestion, nutrient uptake, and protection

mechanical digestion

• Physically breaks down food into smaller pieces

• Examples > chewing and stomach churning

• Prepares food for easier chemical digestion

chemical digestion

• Enzymes break down macromolecules into absorbable forms

• Starches > simple sugars

• Polypeptides > amino acids

• Triglycerides > free fatty acids and monoglycerides

• Makes nutrients small enough to be absorbed by the small intestine

carbohydrate digestion

• Starches > simple sugars (monosaccharides)

• Enzymes like amylase break down complex carbs

• Only simple sugars can be absorbed into the blood

protein digestion

• Polypeptides > amino acids

• Broken down by

• Endopeptidases > cut inside the chain

• Exopeptidases > remove amino acids from chain ends

• Absorbed as amino acids, dipeptides or tripeptides

fat digestion

• Triglycerides > free fatty acids and monoglycerides

• Bile emulsifies fats > small droplets

• Lipases break down triglycerides

• Form micelles for absorption into the small intestine

digestion > complex to simple

• Meal (large, complex): contains starches, polypeptides, triglycerides

• Broken down (smaller, complex): partially digested starches, proteins, fats

• Components absorbed (small, simple)

• Simple sugars

• Amino acids

• Free fatty acids and monoglycerides

GI enzymes

• Small intestine > main site of digestion and absorption

• Enzymes usually working pairs for efficiency

• Break down carbs, proteins, and fats

carbohydrate digestion

• only simple sugars (monosaccharides) like glucose, fructose, and galactose can be absorbed into the blood

• Complex carbohydrates (starches, disaccharides) must be broken down by enzymes before absorption

• Digestion starts in the mouth and continues in the small intestine with pancreatic and brush border enzymes

steps of carbohydrate digestion

• Mouth:

• Chewing + salivary amylase begins starch breakdown

• Small intestine:

• Pancreatic amylase continues starch digestion

• Brush border enzymes (ex: maltase, lactase, and sucrase)

• Located on the microvilli of enterocytes

• Finish breaking down disaccharides into monosaccharides (glucose, fructose, galactose)

• Monosaccharides are ready for absorption

isocaloric vs isometabolic

Key difference > calories vs metabolic effect

isocaloric

same number of calories in a meal or diet

isometabolic

produces the same metabolic response (energy use, blood sugar, etc.

absorption of carbohydrates: apical surface

• Happens in the small intestine at the surface of intestinal cells

• SGLT1:

• Moves glucose and galactose into the cell

• Uses sodium (Na +) to help pull sugar in, even if their levels are higher inside the cell

• GLUT5:

• Moves fructose into the cell

• Works passively, letting fructose flow from high > low concentration

• Simple summary:

• SGLT1 > uses energy (Na+) to bring glucose and galactose

• GLUT5 > passive, brings in fructose

SGLT 1

• Moves glucose and galactose into the cell

• Uses sodium (Na +) to help pull sugar in, even if their levels are higher inside the cell

• apical surface

GLUT 5

• Moves fructose into the cell

• Works passively, letting fructose flow from high > low concentration

• apical surface

absorption of carbohydrates: basolateral surface

• GLUT2 > transports all monosaccharides from the cell into the blood

• Ensures nutrients move efficiently from the lumen to circulation

digestion: protein endopeptidases

• Endopeptidases > enzymes that cut peptide bonds within the protein chain (inside the molecule)

• Starts in the stomach and continues to the small intestine

• Examples:

• Stomach > pepsin

• Pancreas > trypsin

• Small intestine peptidases

• Function > break large proteins into smaller fragments

digestion: proteins exopeptidases

• Exopeptidases > enzymes that remove amino acids from the ends of peptide chains

• Start in the small intestine, mainly from pancreatic secretions

• Example: carboxypeptidase

• Proteins absorbed in multiple forms, but amino acids are preferred

• Breakdown pathway > proteins > smaller proteins > tri/dipeptides > amino acids

absorption of proteins

• Occurs in the small intestine

• Transport mechanisms:

• Amino acids > absorbed through Na+ symport

• Di and tripeptides > absorbed through H+ symport

• Large peptides > absorbed via transcytosis (endocytosis)

di and tripeptides

absorbed through H+ symport

amino acids

absorbed through Na+ symport

large peptides

absorbed via transcytosis (endocytosis)

fat digestion mechanical and emulsification

• Bile from the liver breaks large fat droplets into smaller ones (emulsification)

• Prepares fat for efficient enzymatic digestion

fat digestion: chemical

• Pancreatic lipases break triglycerides into

• Monoglycerides

• Free fatty acids

• Converts fats into forms small enough for absorption

fat digestion : micelles and diffusion

• Micelles carry monoglycerides and fatty acids to the intestinal lining

• Absorption occurs mostly in the small intestine

• Fat molecules enter enterocytes through diffusion

absorption: fats

• Occurs in the small intestine

• Chylomicron formation > packs fats, proteins, and cholesterol into transport particles

• Exocytosis > chylomicrons enter the lymphatic system (lacteals) for transport to the body

hepatic portal system

• Venous blood from the gut flows into the portal vein

• Blood passes through the liver before entering the hepatic vein > vena cava

• Liver metabolism includes

• Carbohydrates > regulation and storage

• Amino acids > processing and protein synthesis

• Lipids > metabolism and packaging into lipoproteins

• Detoxification > removal of toxins and drugs

alcohol metabolism and liver

• Alcohol dehydrogenase (ADH) > converts ethanol (EtOH) > acetaldehyde

• Acetaldehyde > toxic, causes headaches, vomiting and is carcinogenic

• Acetaldehyde dehydrogenase (ALDH) and Glutathione > break down acetaldehyde into less harmful compounds

Acetaldehyde dehydrogenase (ALDH) and Glutathione

break down acetaldehyde into less harmful compounds

Alcohol dehydrogenase (ADH)

converts ethanol (EtOH) > acetaldehyde

Acetaldehyde

 toxic, causes headaches, vomiting and is carcinogenic

phases of integrated meal response

• Cephalic phase

• Gastric phase

• Intestinal phase

• Colonic phase

cephalic phase

• Region: head and esophagus

• First step: chewing (mechanical digestion)

saliva (structure)

• Produced by parotid, sublingual, and submandibular glands (autonomic regulation)

• Made by acinar cells, which connect to ducts that carry saliva into the mouth

• Composition:

• Mostly water and ions, plus important chemicals:

• Fluoride > strengthens enamel

• Bicarbonate > neutralizes acids

• Lysozymes > antibacterial

saliva (function)

• Begins chemical digestion of carbs (salivary amylase)

• Small amount of lipid digestion (salivary lipase, high in infants)

• Lubricates food to prevent choking

bolus formation and transportation

• Chewed food and saliva > bolus

• Tongue moves bolus back of mouth

• Peristalsis moves bolus through esophagus to stomach

swallowing steps

• Oral phase > tongue pushes bolus against the soft palate, triggering the swallow reflex

• Pharyngeal phase > upper esophageal sphincter relaxes while the epiglottis closes to keep food out of airway

• Esophageal phase > food moves down the esophagus through peristalsis

esophagus: structure and swallowing

• Contains two sphincters

• Upper esophageal sphincter (UES) > skeletal muscle, voluntary control

• Lower esophageal sphincter  (LES) > smooth muscle, involuntary control

• Swallowing > mostly involuntary (also called swallowing reflex)

• Relaxation of UES allows bolus to enter esophagus

• Food moves down via peristalsis (phasic smooth muscle contraction) + gravity (assists but not required)

pressure gradients and function

• Swallowing reflex requires coordinated contraction and relaxation of smooth muscle, which creates pressure gradients

• Pressure gradients are critical for moving bolus

• Lower esophageal sphincter (LES) must:

• Open to allow food into the stomach when the pressure in the stomach is higher

• Stay closed at other times to prevent reflux

• Increased BMI causes increased IAP and IGP

• Increased IGP and GEPG increases risk of HH

• IEP decreases with age, increasing GEPG

achalasia

• Disorder where high pressure in the LES (lower esophagus sphincter) makes it difficult to move the bolus

• Esophageal peristalsis is impaired > food may get stuck

• Can cause regulation, chest discomfort, or difficulty swallowing solids and liquids

• Treatment > sometimes requires surgical opening or dilation of LES

GERD

• Gastroesophageal reflux disease

• Caused by LES failure > stomach contents reflux into esophagus

• Contributing factors: caffeine, alcohol, cigarettes, chocolate

• Consequences:

• Indigestion

• Heartburn

• Esophageal irritation from the stomach

gastric phase

Events in the stomach when food enters

anatomical regions of the stomach

• Cardia

• Fundus

• Body

• Antrum

• Pyloric sphincter

functional divisions of the stomach

• Proximal stomach (reservoir)

• Cardiac stomach, fundus, and body

• Stores food

• Distal stomach (pump/grinder)

• Antrum, pyloric sphincter

• Mixes, grinds, pumps

stomach structure and motility (proximal)

• Rugae > gastric folds > increase surface area and allows stomach to stretch for food volume

• Proximal stomach > low peristalsis, mainly accommodates food

• Empty regulation > stretch receptors, vasovagal reflex, small intestine feedback

stomach structure and motility

• Distal stomach motility > strong peristalsis due to oblique muscle layer > mechanical digestion

• Mixing location > antrum

• Distal stomach function > high motility, grinds food, prepares chyme for small intestine

stomach regions and stomach

• Cardiac stomach

• Secretes mucus and bicarbonate > protects tissue, control food entry, helps burping

• Fundus and body

• Secretes acid (H+), intrinsic factor, mucus, bicarbonate, pepsinogen, lipase

• Functions as reservoir and maintains gentle pressure to move food

• Antrum and pylorus

• Secretes mucus and bicarbonate

• Functions > mixing, grinding, sieving, controlling emptying

gastric secretory cells

• Parietal cells

•  produce stomach acid (HCL) and intrinsic factor

• Chief cells

• Produce pepsinogen (protein digestion) and gastric lipase (fat digestion)

• G cells

• Release gastrin > hormone that stimulates acid production

• D cells

• Release somatostatin > hormone that slows acid production

• ECL cells

• Release histamine > stimulates acid production

• Mucous neck cells

• produce mucus and bicarbonate protect stomach lining

regulation of gastric acid secretion

• H+/K+/ATPase (proton pump)

• Located in parietal cells

• Secretes H+ into stomach lumen

• Rate limiting step of acid secretion

activators of acid secretion

• Three main stimulators

• Histamine > increase cAMP > PKA > increase acid

• Gastrin > increase Ca 2+ > PKC > increase acid

• Acetylcholine (Ach) > increase Ca 2+ > increase PKC > increase acid

• Works synergistically for maximal acid secretion

inhibition

• Somatostatin (from D cells)

• Decrease cAMP

• Inhibits proton pump

• Decrease acid production

negative feedback

• Self regulation

• D cells sense increase H+ (low pH)

• Release somatostatin

• Result:

• Prevents over-acidification

big picture

• Gastric acid is controlled by a balance of stimulatory and inhibitory signals

• The proton pump (H+/K+/ATPase) in parietal cells is the final step where all signals act

• Stimulatory inputs

• Histamine increases cAMP and stimulates the proton pump

• Gastrin and acetylcholine increases intracellular calcium and enhance acid secretion

• These signals work together to maximize acid production during a meal

• Inhibitory control

• Somatostatin is released from D cells when acidity is high

• It inhibits the proton pump and decreases gastrin and histamine release

• Negative feedback

• Increased acid (low pH) triggers somatostatin release

• This reduces further acid secretion and prevents excess acidity

• Overall, acid production when needed but limited to protect stomach lining

vagus and neural control

• Parasympathetic NS: controls motility, secretion, relaxation (rest and digest)

• Receptive relaxation > esophagus stretch > inhibits proximal stomach smooth muscle

• Adaptive relaxation > stomach stretch > inhibits smooth muscle to accommodate food

vasovagal reflex

• Triggered by stomach distention and chemical stimuli (peptides, amino acids)

• Mediated by the vagus nerve (afferent signals to brainstem, efferent signals back to stomach)

• Functions:

• Coordinates gastric motility (mixing, peristalsis)

• Stimulates gastric secretion (acid, enzymes)

• Promotes receptive relaxation of the fundus

• Overall role:

• Integrates neural control during the gastric phase to optimize digestion