Digestive System Flashcards
What is the digestive system
Digestive system processes food, extracts nutrients from it, and eliminates the residue
5 stages
- Ingestion:
• Selective intake of food - Digestion:
• Mechanical and chemical breakdown of food into a form usable by the body - Absorption:
• Uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph - Compaction:
• Absorbing water and consolidating the indigestible residue into feces - Defecation:
• Elimination of feces
Types of digestion- mechanical
Mechanical digestion- the physical breakdown of food into smaller particles
• Cutting/grinding action of the teeth
• Churning in stomach and small intestines
• Exposes more food surface to the action of digestive enzymes
Types of digestion- chemical
Chemical digestion–a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues)
• Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine
• Polysaccharides into monosaccharides
• Proteins into amino acids
• Fats into monoglycerides and fatty acids
• Nucleic acids into nucleotides
• Vitamins, minerals, free amino acids, cholesterol and water are absorbed without being digested
Anatomy of digestive system - two anatomical subdivisions
- Digestive tract
• Long muscular tube extending from mouth to anus
• Mouth, pharynx, esophagus, stomach, small intestine, and large intestine
Gastrointestinal (Gl) tract is the stomach and intestines - Accessory organs( help aid the digestive track)
• Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Innervation - enteric nervous system
A nervous network in the esophagus, stomach, and intestines
- regulates digestive tract mobility, secretion and blood flow
ENS contains sensory neurons that monitor tension in gut walk and conditions in lumen
Composed of two networks of neurons
1. Submucosal (Meissner) plexus: in submucosa
• Controls glandular secretion of mucosa
• Controls movements of muscularis mucosae
2. Myenteric (Auerbach) plexus: parasympathetic ganglia and nerve fibers between the of the muscularis interna
Regulations of digestive tract
Neural control
• Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus
• Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion
Hormonal
• Chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract
• Gastrin and secretin
Paracrine secretions
• Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
Messnteries
Provide Structure and support for intestine, stop them from twisting and any complications from occurring. They stop abnormal moments
Two mesenteries called OMENTA
- lesser omentum
- greater OMENTA
Mouth - or oral or buccaneers cavity
Function
• Ingestion (food intake)
• Other sensory responses to food: chewing and chemical digestion
• Swallowing, speech, and respiration iod sauamous epithelium
Stratified squamous epithelium lines the mouth
• Keratinized in areas subject to food abrasion: gums and hard palate
• Nonkeratinized in other areas: floor of mouth, soft palate, and insides of cheeks and lips
Tongue
Muscular, bulky, but agile and sensitive organ
• Manipulates food & avoids being bitten
• Extract food particles from the teeth
• Sensitive enough to feel a stray hair
Structure
•Nonkeratinized stratified squamous epithelium covers its surface ingual papillae: bumps and projections on the tongue that are the ites of the taste buds
•Body: anterior two-thirds of the tongue occupies oral cavity
•Root: posterior one-third of the tongue occupies the oropharynx
-Boundary marked by a row of V-shaped vallate papillae
- Behind these a grove called the terminal sulcus
-Intrinsic muscles within the tongue control speech
- Extrinsic (outside) muscles produce the stronger movements of food manipulation
Teeth
Masticate food into smaller pieces
• Makes food easier to swallow
• Exposes more surface area for action of digestive enzymes speeding chemical digestion
Structure
32 adult teeth; 20 deciduous (baby) teeth
• 16 in both mandible and maxilla
• Incisors chisel-like cutting teeth used to bite off a piece of food
• Canine pointed and act to puncture and shred food
•Premolars- broad surface for crushing and grinding
• Molars- even broader surface for crushing and grinding
Mesticatio- chewing
Chewing_-breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes
• First step in mechanical digestion
• Tongue, buccinator, and orbicularis oris manipulate food
• Masseter and temporalis elevate the teeth to crush food
• Medial and lateral pterygoid and masseters produce the side-to-side grinding action
Saliva
Comes form
-Intrinsic salivary glands small glands dispersed € other oral tissues
- Extrinsic salivary glands three pairs connected to oral cavity by ducts
Functions
• Moisten mouth
• Begin starch and fat digestion
• Cleanse teeth
. Inhibit bacterial growth
• Dissolve molecules so they can stimulate the taste buds
• Moisten food and bind it together into bolus to aid in swallowing
• pH: 6.8 to 7.0
Pharynx
A muscular funnel that connects oral cavity to esophagus and allows entrance of air from nasal cavity to larynx
Pharyngeal constrictors–circular muscles that force food downward during swallowing
When not swallowing, the inferior constrictor remains contracted to exclude air from the esophagus
Esophagus
A straight muscular tube 25 to 30 cm
long
•From pharynx to cardiac orifice of stomach
• Lower esophageal sphincter: food pauses at this point because of this constriction
-Prevents stomach contents from regurgitating
- Protects esophageal mucosa from erosive effect of the stomach acid
-Heartburn
Structure
- Nonkeratinized stratified squamous epithelium
-Esophageal glands in submucosa secrete mucus
- Deeply folded into longitudinal ridges when empty
- Skeletal muscle in upper 1/3, mixture in middle 1/3, and only smooth
muscle in the bottom 1/3
Swallowing
Controlled by
• Swallowing centre of medulla (trigeminal, facial, glossopharyngeal & hypoglossal nerves)
Three phases
1. Oral phase: under voluntary control
•Tongue collects food, presses it against the palate forming a bolus, and pushes it posteriorly
• Food accumulates in oropharynx in front of epiglottis
Epiglottis opens and bolus slides to laryngopharynx
2. Phalangeal phase: involuntary
• Soft palate and tongue root stop food/ drink entering nose and mouth to prevent chocking
Breathing paused & larynx pulled up to epiglottis
• Oesophagus widens, phalangeal constrictors contract (superior to inferior) pushing bolus into the
3. Oesophageal phase: is involuntary waves (peristalsis)
• Controlled by brain and myentric plexus
• Stretch receptors stimulated & transmitted to muscularis externa above & below
• Circular muscle above contracts, below relaxes, and longitudinal muscle contracts to pull oesopha shorten
Peristalsis
Wave of muscular contraction that pushes the bolus ahead of it- involuntary reflex
• Entirely involuntary reflex
hen standing or sitting upright, the food and liquid drops through he esophagus by gravity faster than peristalsis can keep up with it
• Ensures you can swallow regardless of body position
• Liquid reaches the stomach in 1-2 sec
• Food bolus in 4-8 sec
• When it reaches lower end of the esophagus, the lower esophageal sphincter relaxes to let food pass into the stomach
Stomach
Primary function
-Mixing compartment
-Storage organ adapts in size depending on food and food can remain 2-4 hours
- secrete gastric juice ( water, mucus
, hydrochloric acid , intrinsic factor and enzyme pepsinogen
Stomach creates chyme
38 О
Gross anatomy of stomach
Divided into four (4) regions
1. Cardiac region (cardia) small area within about 3 cm of the cardial orifice
2. Fundic region (fundus).
dome-shaped portion superior to
esophageal attachment
3. Body (corpus) makes up the greatest part of the stomach - where mixing in the stomach occurs
4. Pyloric region: narrower pouch at the inferior end
• Subdivided into the funnel-like antrum
•Pylorus: narrow passage to duodenum
•Pyloric (gastroduodenal) sphincter regulates the passage of chyme into the duodenum ( valve)
Microscopic anatomy
Wall has four (4) layers = serosa,
muscularis, submucosa, mucosa
1. Mucosa = deepest layer (three layers)
• Epithelium, lamina propria, and muscularis mucosae
2. Submucosa = connective tissue and vascular plexus
• Mucosa and Submucosa, flat when full but wrinkle (rugae) when empty
3. Muscularis - 3 layers
• Outer longitudinal layer
• Middle circular layer
• Inner oblique layer
4. Serosa (visceral peritoneum) - outermost tunic of stomach
• Simple squamous epithelium (outer)
• Connective tissue (inner)
Gastric secretion
Gastric juice
• 2 to 3 L per day produced by the gastric glands
• Mainly a mixture of water,
hydrochloric acid, and pepsin
• High concentration of hydrochloric
(HCI) acid
• pH as low as 0.8
Hydrochloric acid
• Activates enzymes pepsin and lingual lipase
• Breaks up connective
tissue and plant cell walls
Help liquefy food and form chyme
•Converts ingested ferric ions to ferrous ions
• Contributes to nonspecific disease resistance
Pepsin
• Pepsinogen - removal of AA’s
> pepsin
- Digests proteins
Gastric Lipase
• Digests 10-15% of dietary fat
Intrinsic Factor
Glycoprotein
•Absorption of Vit
B12
Gastric motility
As you swallow - swallowing centre of medulla oblongata signals stomach to relax
- Activates receptive-relaxation response of smooth muscle
-Arrival of food stretches stomach
Rhythm of peristaltic contractions
•Gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice
• Becomes a stronger contraction at pyloric region
• After -30 min these contractions become quite strong
• Churn food
• Mix it with gastric juice; and
• Promote its physical breakup and chemical digestion
Regulation of gastric function
- Cephalic Stage (controlled by the brain)
• Stomach responds to the sight, smell, taste or thought of food
• Vagus nerve (from medulla oblongata) stimulates enteric NS
- 40% of juices, in preparation of food - Gastric Phase (controlled by itself
•Period in which swallowed food activates gastric activity
• 2/3 gastric secretion during this stage
• Ingested food stimulates activity by stretching and raising pH - Intestinal Phase (controlled by small intestine)
- Period in which duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes
-Initially stimulates gastric secretion but later inhibits it
Liver
• Mass 1.4 Kg
• Four (4) main functions (but not all)…
1. production of bile, urea, heparin, vitamin
A, plasma proteins, and antibodies
2. Metabolism of fats, amino acids, sugars, and toxins
3. Storage of fats, vitamins A, D, and B12, iron, and glycogen
4. Activates vitamin D
Anatomy of liver
Four (4) lobes-right, left, quadrate, and
caudate
• Falciform ligament separates left and right lobes
• Sheet of mesentery that suspends the liver from the diaphragm
• Gallbladder depression on inferior surface
• Inferior Vena Cava sulcus on posterior aspect
Microscopic Anatomy
Hepatic lobules
- central vein helps with Venus blood drainage
- hepatocytes
- hepatic sinusoids are blood filled channels. Allows transfer of molecules and nutrients. is hepatocytes from blood cells
• Absorb glucose, amino acids, iron, vitamins & nutrients required for metabolism
• Degrade hormones, toxins, bile pigments and drugs
Hepatic triad:
•Branch of hepatic artery supply oxygenated blood to liver
• Branch of hepatic vein carry nutrient rich blood from the same intestine and facilitate drainage of deoxygenated blood and wats substance from liver
• Bile duct helps transport bile
Liver circulation
Liver receives blood from 2 sources: ~70% hepatic portal vein & 30% hepatic arteries
• Hepatic portal vein = blood from stomach, intestines, pancreas, spleen
• All nutrients except lipids
• Hepatic arteries = arterial blood from aorta at the celiac trunk to hepatic artery proper
Meet and drain into sinusoids; mixing venous and arterial
Gallbladder
Stores and concentration of bile
with simple columnar epithelium tissue inside it, which helps with absorption and secretion related to that bio management.
Bile
It contains a mix of minerals, cholesterol, neural neutral fats, phospholipids, bile pigments, and bile acids.
Green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments and bile acids.
Bilirubin - principle pigment comprised of decomposed hemoglobin
Reaches small intestine turns to
Urobilinogen -
• Urobilin responsible for yellowish colour of urine
- Remiaining urobilinogen responsible for brown colour of feces
Bile acids - steroids synthesised from cholesterol
-With lecithin (phospholipid and micelles - aid in fat digestion and absorption
All other components = wastes
• In excess, can lead to gallstones
Bile is collected in the gallbladder with the bile duct reaches full capacity
Reabsorbs water and electrolytes
• Increase concentration of bile 5-20x
Liver secretes about 500 to 1,000 mL of bile daily
80% of bile acids are reabsorbed in the ileum and returned to the liver
20% of the bile acids are excreted in the feces
Bile needs to know
while is green. And it’s a digestive fluid that’s made in the liver stored in the gallbladder and helps digest.It helps with digestion and helps with absorption of fats.
Pancreas
Spongy retroperitoneal gland posterior to the stomach
Measure 12 to 15 cm long, and 2.5 cm thick
Components
• Head (encircled by duodenum)
• Body (mid-portion)
• Tail on the left
Both an endocrine and exocrine gland
• Endocrine portion- -pancreatic islets that secrete insulin and glucagon.
• Exocrine portion- 99% of pancreas that secretes 1,200 to 1,500 mL of pancreatic juice per day.
Pancreatic duct
• Runs lengthwise through the middle of the gland
Accessory pancreatic duct
• Smaller duct that branches from the main pancreatic duct
• Opens independently into the duodenum
• Bypasses the sphincter and allows pancreatic juice to be released into duodenum even when bile is not
Pancreatic Juices
Alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes
Important ones
. Pancreatic amylase - splits glycogen into disaccharides
• Pancreatic lipase - breaks down triglycerides
• Trypsinogen, chymotrypsinogen, and carboxypeptidase (digest proteins)
• Nucleases - digest nucleic acids
• Bicarbonate ions - make pancreatic juice alkaline
Regulation of secretion of pancreas
Three (3) stimuli are responsible for the release of pancreatic juice and bile
- Acetylcholine (Ach)
• From vagus and enteric nerves
•Secrete enzymes during the cephalic phase - Cholecystokinin (CCK)
•Secreted by mucosa of duodenum and small intestine
•In response to fats in small intestine
•Contraction of gallbladder and relaxation of hepatopancreatic sphincter > discharges bile into duodenum - Secretin
•Produced by small intestine in response to acidity of chyme from stomach
•Stimulates liver and pancreas to secrete sodium bicarbonate solution (buffers the hydrochloric acid arriving from the stomach)
Small intestine
Nearly all chemical digestion and nutrient absorption occurs in the small intestine
“Small” intestine refers to the diameter-not length (2.5 cm)
The longest part of the digestive tract
Gross anatomy small intestine
Coiled mass filling most of the abdominal cavity inferior to the stomach and the liver
Divided into three regions
- Duodenum: first 25 cm (10 in.)
• Begins at the pyloric valve
• Major and minor duodenal papilla distal to pyloric valve
• Receives major and minor pancreatic ducts respectively
• Arches around the head of the pancreas
• Ends at a sharp bend called the duodenojejunal flexure
Function:
• Stomach acid neutralised, in the duodenum, fats emulsified by bile acids, pepsin inactivated by pH & pancreatic enzymes take over chemical digestion. - Jejunum: first 40% of small intestine beyond duodenum
• Roughly 1.0 to 1.7 m in a living person
• Large, tall, closely spaced circular folds
• Relatively thick and muscular wall
• Especially rich blood supply which gives it a red color
• Most digestion and nutrient absorption occurs here - Ileum: forms the last 60% of the postduodenal small intestine
• About 1.6 to 2.7 m
• Thinner, less muscular, less vascular, and paler pink color
• Ileocecal valve (sphincter) control’s movement from the ilium to large intestine
Microscopic anatomy small intestine
Tissue layers have modifications for nutrient digestion and absorption
Lumen lined with simple columnar epithelium
Muscularis externa is notable for a thick inner circular layer and a thinner outer longitudinal
Large internal surface area for effective digestion and absorption: by great length and three types of internal folds or projections
Circular folds increase surface area by a factor of 2 to 3
Villi increase surface area by a factor of 10
Microvilli Increase the surface area by a factor of 20
Intestinal secretions - intestinal crypts
Intestinal crypts secrete 1 to 2 L of intestinal juice per day
• In response to acid, hypertonic chyme, an distension of the intestines
• pH of 7.4 to 7.8
• Contains water, mucus, but relatively little enzyme
• Most enzymes that function in the small intestine are found in the brush border
Intestinal motility
Contractions of the small intestine serve three
(3) functions:
To mix chyme with intestinal juice, bile and pancreatic juice allowing these fluids to neutralize acid and digest nutrients more effectively.
Churn chyme and bring it into contact with the mucosa for digestion and absorption
To move residue toward the large intestine
Intestinal Motility (Segmentation)
Segmentation
• Movement in which ringlike constrictions appear at several places alc the intestine
• Most common site is the small intestine
•Allows the kneading/churning which promotes mixing of food, digesti secretions and enhances contact digestion.
Pacemaker cells of the muscularis externa set the rhythm segmentation
•8-9 times per minute in the ileum
• 12 times per minute in the duodenum
Interstitial motility
Intensity of contractions is influenced by nervous and hormonal factors
• When most nutrients have been absorbed, segmentation declines and peristalsis begins
• A peristaltic wave begins in the duodenum and travels 10-70cm, then is followed by another wave.
• The overlapping waves of contraction are called migrating motor complex
•Chyme is milked towards the colon over ~2 hours
•Refilling of the stomach at the next meal supresses peristalsis and
activates segmentation
•Ileocecal valve remains closed until food in stomach triggers the release of gastrin
•Pressure pinches the valve shut
What are the parts of the GI?
The Gl tract is a long, muscular tube that extends from the mouth to the anus and is responsible for the digestion and absorption of food. It can be divided into several distinct regions:
1. Oral Cavity (Mouth):
2. Pharynx:
3. Oesophagus:
4. Stomach:
5. Small Intestine:
6. Large Intestine (Colon):
7. Rectum:
8. Anus:
Purpose of GI system
Digest
Absorb
Transport the nutrients
Oral cavity (mouth)
- Oral Cavity (Mouth):
- The process of digestion begins in the mouth.
- Food is broken down mechanically by chewing, and enzymes in saliva start breaking down carbohydrates.
Function of the Oral cavity
1. Enzyme Site: While the mauth is not a primary site for enzyme secretion, it does contain enzymes, such as salivary amulase.
2. Enzyme Name: Salivary amylase (also known as plyalin)
3. Role: Salivary amylase is responsible for initiating the digestion af carbohydrates (starches) in the mouth. it breaks down complex carbohydrates into simpler sugars, primarily maltose.
This initial breakdown of carbohydrates is a crucial step in digestion.
Salivary amylase
First enzyme that helps break down carbohydrates
Bolus
The substrate of food. When food goes in mouth and turns to mush it’s called bolus
Salivary Amylase
1.Salivary Amylase (Ptyalin):
- Source: Salivary amylase is secreted by the salivary glands, primarily the parotid glands, submandibular glands, and sublingual glands.
- Role: Salivary amylase begins the digestion of complex carbohydrates, such as starches and glycogen, into simpler sugars. It catalyses the hydrolysis of starch molecules into maltose (a disaccharide composed of two glucose molecules) and other smaller sugar molecules. This process is the initial step in carbohydrate digestion and takes place in the mouth when you start chewing and mixing food saliva.
- Optimal pH: Salivary amylase works best in a slightly alkaline or neutral pH.
Apart from salivary amylase, the saliva in the oral cavity also contains other components like mucin, lysozyme, and immunoglobulins that help with lubrication, protection against pathogens, and maintaining oral health.
Oesophagus
- Oesophagus:
- The esophagus is a muscular tube carrying food from the pharynx to the stomach through peristalsis.
- There are sphincters at both ends of the esophagus to prevent backflow of stomach contents.
Stomach
- Stomach:
- The stomach is a muscular organ that stores, mixes, and partially digests food.
- It secretes gastric juices, including hydrochloric acid and enzymes, to break dow proteins.
Role of the stomach - enzymes
Role of the Stomach:
- Enzymes:
a. Pepsin: Pepsin is the most significant enzyme produced 1 the stomach. It is secreted in an inactive form known as epsinogen by the chief cells of the gastric glands. When it omes into contact with hydrochloric acid (HCI), also roduced in the stomach, pepsinogen is converted into its ctive form, pepsin. Pepsin is responsible for breaking down roteins into smaller peptide fragments.
b. Gastric Lipase: This enzyme is secreted by the stomach’s chief cells. It plays a minor role in digesting fats, primarily breaking down dietary triglycerides into fatty acids and monoglycerides.
- Function: The stomach’s primary function is to:
a. Mechanical Digestion:.
b. Chemical Digestion
c. Storage
Small intestine
- Small Intestine:
- The small intestine is where most digestion and nutrient absorption occur.
- It consists of three sections: the
a) duodenum,
b) jejunum, and
c) ileum.
- Enzymes from the pancreas and bile from the liver help digest food, and nutrients are absorbed into the bloodstream through the intestinal walls.
Enzymes of the small intestines
Enzymes of the Small Intestines?
Digestive Enzymes:
- Pancreatic Enzymes:
The pancreas secretes several digestive enzymes into the small intestine to break do macronutrients. - Pancreatic Amylase:
This enzyme breaks down carbohydrates (starches) into simpler sugars like matterea glucose. - Trypsin and Chymotrypsin:
These enzymes break down proteins into smaller peptides and amino acids, - Pancreatic Lipase:
It digests fats (triglycerides) into fatty acids and monoglycerides, making them more absorbable. - Intestinal Enzymes:
The small intestine’s lining also produces enzymes.
Intestinal Lipase:
This enzyme continues the digestion of fats, particularly in the final stages of fat absorption.
- Sucrase, Lactase, and Maltase:
These enzymes break down disaccharides (sucrose, lactose, and maltose) into their constituent monosaccharides (glucose, fructose, and galactose), facilitating carbohydrat absorption.
Bile function
Bile Function:
- Bile is produced by the liver and stored in the gallbladder before being released into the small intestine. While bile is not an enzyme, it plays a crucial role in digestion.
- Bile emulsifies fats, breaking them into smaller droplets.
This emulsification process increases the surface area of fat particles, making it easier for lipases (enzymes) to digest and absorb dietary fats.
Large intestine
Large Intestine (Colon):
- The large intestine absorbs water and electrolytes, forms faces from undigested food, and houses beneficial gut bacteria.
- Fermentation
- It comprises the cecum, colon, rectum, and anal canal.
Key aspect of large intestine
Water absorption
Rectum
- Rectum:
- The rectum is the last part of the large intestine before the anus.
- It stores faces until they are ready to be eliminated.
Anus
- Anus:
- The anus is the external opening through which feces are expelled from the body.
Oral cavity
Function: food goes though chemicals break down. Includes teeth, tongue and glands
Enzymes: salivary amylase
Process: mastication and carbohydrate breakdown
Stomach
Function: mix, stage two digestion and absorption
Enzymes: pepsin, HCL, lipase
Process:
Small intestine
Function: Absorption and digestion
Enzymes: pancreas, bile and sucrease
Process:
Large intestine
Function: water absorb , absorb - feaces, fermentation
Enzymes:
Process:
Anus
Function: expell faeces
Enzymes:
Process::
Large intestine
Absorbs water and salts and eliminates feces by defecation
Gross anatomy large intestine
Haustra
• Muscle tone of tenia coli contracts the colon lengthwise and causes its wall to form haustra
• Haustra are absent in the rectum and anal canal
Sphincter is large intestine
Anus is regulated by two sphincters:
• Internal and external sphincters
Microscopic Anatomy large intestine
Mucosa-simple columnar epithelium through entire large intestine
Anal canal has nonkeratinized stratified squamous epithelium in its lower half
• Provides abrasion resistance
No circular folds or villi to increase surface area in large intestine
Have a greater density of mucus-secreting goblet cells
Lamina propria and submucosal layers have large amount of lymphatic tissue
Bacterial flora
Bacterial flora populate large intestine
About 800 species of bacteria vary depending on many factors diet, age
Play a digestive role
Digest cellulose and other undigested carbohydrates
Body absorbs resulting sugars
Help in synthesis of vitamins B and K
Absorption and Motility on large intestine
Large intestine takes about 36 to 48 hr to reduce the residue of a meal to feces
• Reabsorbs water and electrolytes
Feces consist of:
• 75% water; and
• 25% solids
• 30% is bacteria,
• 30% undigested fiber
• 10% to 20% fat
• Small amount of mucus, and sloughed epithelial cells
Haustral contractions occur every 30 minutes
• This kind of colonic motility is a form of segmentation
• Distension of a haustrum stimulates it to contract
Mass movements occur 1-3x a day
• Triggered by gastrocolic and duodenocolic reflexes
• Filling of the stomach and duodenum stimulates motility of the colon
• Moves residue for several centimeters with each contraction
Defecation
Stretching of the rectum stimulates defecation
It invokes tow involuntary reflexes
1. Intrinsic defecation reflex
• Operates within the myenteric nerve plexus
• Stretch signals travel through the plexus to the descending and sigmoid colon and the rectum
• A peristaltic waves drives the faces out
2. Parasympathic defecation reflex
• A spinal reflex: stretch signals are detected by th spinal cord, signals the pelvic nerves to intensify peristalsis
(4) Impulses from the brain prevent untimely defecation by keeping the external anal sphincter contracted.
Defecation occurs only if this sphincter also relaxes.
Chemical digestion
Involves the digestive enzymes breaking the bonds of the food macromolecules
Macromolecules or
macronutrients from our diet include:Carbohydrates
Fats or lipids
Protein
Carbohydrates
Starch - the most digestible carbohydrate
• First digested to:
• 1. Oligosaccharides that are up to eight glucose residues long
• 2. Then into the disaccharide maltose
• 3. Finally to glucose which is absorbed by the small intestine
Process begins in the mouth
• Salivary amylase hydrolyzes starch into oligosaccharides
• Amylase works best at pH of 6.8 to 7.0 of oral cavity
• Amylase quickly denatured on contact with stomach acid and digested by pepsin
After the mouth
• In the small intestine, pancreatic amylase converts starch - oligosaccharides and maltose within 10 min.
• Chyme contact brush border enzymes:
• Dextrinase, glucoamylase:
oligosaccharides
• Maltase: maltose
- Sucrase: sucrose -› glucose + fructose
- Lactase: lactose -› glucose + galactose
Carbohydrates
80% of absorbed sugar is glucose
- Taken up by sodium-glucose transport (SGLT)
proteins - Glucose transported out of absorptive cell into
ECF by facilitated diffusion - Sugar entering CF increases its osmolarity
- Draws water osmotically from lumen of intestine, through now leaky tight junctions between epithelial cells
- Water carries more glucose and other nutrients by solvent drag
- SGLT absorbs galactose, fructose is absorbed by facilitated diffusion
- Glucose, galactose, and any remaining fructose are transported out of the base of the cell by facilitated diffusion
- Absorbed by blood capillaries in the villus
- Hepatic portal system delivers them to the liver
Proteins
AAs absorbed by the small intestine come from 3 sources
Dietary proteins
Digestive enzymes digested by each other
Sloughed epithelial cells digested by enzymes
Endogenous As from last two sources - 30 g/day
Exogenous AAs from our diet - 44 to 60 g/day
Proteases (peptidases)-enzymes that digest proteins
Begin in the stomach in optimum pH of 1.5 to 3.5
Pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine
•Pepsin digests PRO into shorter peptides and some free AAs
Proteins: digestion in the small intestine
Pancreatic enzymes trypsin and chymotrypsin take over the process
Oligopeptides taken apart one amino acid at a time by three more enzymes
Proteins broken in three amino acid
Carboxypeptidase of the brush border continues to remove amino acids from the carboxyl (-COOH) end.
Aminopeptidase of the brush border removes one amino acid at a time from the amino (-NH) end
Dipeptidase splits dipeptides ( 09 into separate amino acids (9).
Lipids
Hydrophobic quality of lipids makes their digestion and absorption more complicated than carbohydrates and proteins. Lipase enzymes digesting fat
Lingual lipase secreted by the intrinsic salivary glands of the tongue
• Active in mouth, but more active in stomach along with gastric lipase
• 10% to 15% of lipids digested before reaching duodenum
Fat enters duodenum as large globules exposed to lipase only at their surface
Globules broken up into smaller droplets by lecithin and bile acids (components of bile)
How are lipids broken up
Agitation by segmentation breaks up the fats into droplets
The coating of lecithin and bile acids keeps it broken up, exposing far more of its surface to enzymatic action
Lipase acts on triglycerides by removing the first and third fatty acids from glycerol backbone
• Two free fatty acids (FFAs) and a monoglyceride
Absorption of lipids
Absorption of these products and other lipids depends on minute droplets in the bile known as micelles
• Made in liver and consists of 20-40 bile acid molecules with their own hydrophilic side group
Micelles pass down bile duct into duodenum
• Absorb fat-soluble vitamins, cholesterol and FFAs
• They transport lipids to the surface of the intestinal absorptive cells
Bile
Not an digestive enzyme
Steps for lipids
Emulsification
Fat globule is broken up and coated by lecithin and bile acids.
Fat hydrolysis
Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the first and third fatty acids from triglycerides, usually leaving the middle fatty acid.
Lipid uptake by micelles
Micelles in the bile pass to the small intestine and pick up several types of dietary and semidigested lipids.
Chylomicron formation
Intestinal cells absorb lipids from micelles, resynthesize triglycerides, and package triglycerides, cholesterol, and phospholipids into protein-coated chylomicrons
Chylomicron exocytosis and lymphatic uptake
Golgi complex packages chylomicrons into secretory vesicles; chylomicrons are released from basal cell membrane by exocytosis and enter the lacteal (lymphatic capillary) of the villus.
Nucleic acid
Nucleic acids (DNA & RNA) present in smaller amounts
Ribonuclease and deoxyribonuclease, hydrolyse these to nucleotides
Brush border enzymes decompose nucleotides = phosphate ions, nitrogenous bases, and simple sugars (ribose from RNA and deoxyribose from DNA)
Transported across the intestinal epithelium by membrane carriers & enter capillary blood of villus
What is the primary function of the large intestine?
Absorption of water and electrolytes
Which anatomical feature marks the junction between the small intestine and the large intestine?
B. Ileocecal valve
How are glucose and galactose absorbed in the small intestine?
A. Through sodium-glucose transport proteins (SGLT)
What role does the Na+/K+ pump play in the mechanism of SGLT
D. It maintains the sodium gradient across the membrane
What is the role of sodium-dependent amino acid co-transporters in the absorption process?
B. They facilitate the movement of amino acids into epithelial cells
Propulsion
- Swallowing (oropharynx)
• Peristalsis (esophagus, stomach small intestine. large intestine
Mechanical digestion
- Chewing (mouth)
• Churning (stomach)
• Segmentation (small intestine)
Absorption
• Nutrients and water to blood vessels and lymph vessels (small intestine)
• Water to blood vessels (large intestine)
Mechanical digestion
Mechanical Digestion
• Mastication (Chewing)
• Breaks down food
• Increases surface area for chemical digestion
Bolus Formation
• Saliva
• Mixes with and lubricates food particles
• Tongue
• Assists in mixing
• Shapes the bolys
Churning
• Vigorous mixing and grinding initiated by the presence of food in the stomach
• Facilitated by contractions of the Muscularis
Externa
• Bolus transformed into semi-liquid mass called chyme
Purpose
• Increases surface area for chemical digestion
• Ensures thorough mixing with gastric juices containing enzymes and hydrochloric acid
Muscular externa
Muscularis externa:
Longitudinal layer
Circular laver
Oblique layer
Proteins
Proteins = Large chains of amino acids (AAs)
Amino Acids (AAs)
•Types (20 in total)
Essential (9): Obtained through diet
Non-essential: Synthesized in the body
Protein Sources
•Complete (contain all 9 essential AAs)
• Examples: meat, fish, poultry, cheese, eggs
•Incomplete (lack one or more essential AAs)
• Examples: green vegetables, grains, legumes
Protein Digestion in the Mouth and Stomach
Mouth
• Limited enzyme activity on proteins in the mouth
• Primarily initiated in the stomach
Stomach
• Highly acidic (pH 1.5 to 3.5), optimal for protease activity
•Pepsin (most important protein digesting enzyme)
- Produced as pepsinogen, an inactive precursor, by chief cells in the stomach lining
-Breaks proteins down into smaller peptides
PRO Digestion in the Small Intestine
Trypsin (Pancreatic Juice)
Polypeptides -› smaller peptides
Chymotrypsin (Pancreatic Juice)
Further reduces peptides to smaller fragments
Carboxypeptidase (Pancreatic Juice)
Cleaves amino acids from the carboxyl end of peptides
Aminopeptidase (Brush border)
Cleaves amino acids from the from the N-terminus (amino end)
Dipeptidase (Brush border)
Dipeptides -› Amino acids
PRO Digestion in the Large Intestine
Not the primary site for protein digestion
Some proteolysis through microbial enzymes
Some AA synthesized by microbes
Protein
Protein digestion begins in the stomach by hydrochloric acid and the enzyme pepsin
. Protein-digesting enzymes are secreted from the pancreas into the small intestine
The small intestine is the major site of protein digestion;
final digestion occurs here
A small amount of dietary protein is lost in the feces
Protein-digesting enzymes are also secreted from the brush border
The liver regulates distribution of amino acids to the rest of the body
Which enzyme is responsible for initiating the breakdown of proteins in the stomach?
Pepsin
In the small intestine, what are the end products of protein digestion?
Amino acids
Which enzyme is responsible for breaking down dipeptides into amino acids
Dipentidase
During the final stages of protein digestion, what is the role of the brush border enzymes?
Final breakdown of peptides into amino acids
Protein absorption
Carboxypeptidase of the brush border continues to remove amino acids from the carboxyl (-COOH) end.
Aminopeptidase of the brush border removes one amino acid at a time from the amino (-NH) end.
Dipeptidase splits dipeptides into separate amino acids.
Where do the amino acids get absorbed after being transported across the epithelial cells?
Into the hepatic portal vein
Lipids
Triglycerides (95%)
Fats (solid) & Oils (liquid)
- Saturated (single bonds)
- Unsaturated (at least one double bond)
Chain Length
•Short: soft at room temp
• Long: hard at room temp
Other 5%
• Cholesterol
• Phospholipids
Lipid Digestion in the Mouth and Stomach
Mouth
• Lingual Lipase
• Breaks down triglycerides into diglycerides and free fatty acids
• Optimal pH: 6.7-7.0
• Incomplete due to short time in the mouth
Stomach
• Highly acidic (pH 1.5 to 3.5) due to hydrochloric acid (HCI)
• Inactivates lingual lipase
• Gastric lipase
• Secreted from chief cells in the stomach
• Triglycerides broken into diglycerides and fatty acids
