The Digestive SystemCh. 25

Digestive Functions

• Processes, extracts, eliminates

1. Ingestion- selective intake of food.

2. Digestion- mechanical and chemical breakdown of food into a usable form.

3. Absorption- uptake of nutrient molecules into the cells of the digestive tract and then into the blood.

4. Defecation- elimination of undigested residue.

Digestion

2 Stages:

1. Mechanical-

2. Chemical-

Some nutrients are already in usable form, vitamins, free AA, minerals, cholesterol, and water

Physical breakdown of food into smaller particles.

Purpose: To expose more food surface to the action of digestive enzymes.

Series of hydrolysis reactions that break dietary macromolecules into their monomers.

I.e.- protein into amino acids, nucleic acids into nucleotides.

General Anatomy

2 anatomical subdivisions:

1. Digestive tract

2. Accessory organs

Digestive Tract

Is a tube that extends from the mouth to anus, measuring 30 feet long.

Includes: oral cavitypharynxesophagusstomachsmall intestinelarge intestine

Accessory Organs

Consists of:

TeethTongueSalivary glandsLiverGallbladderPancreas

General Anatomy

Digestive tract follows a basic structural plan.

Walls composed of the following layers from the inner to the outer surface.

General Anatomy

Inner surface to outer surface:

Mucosa: lining of the lumen

1. Some type of epithelium layer (depends on region)

- most of the DT is simple columnar.- Oral cavity through esophagus, lower anal canal is

stratified squamous.

2. Lamina propria- loose connective tissue layer

3. Muscularis mucosae- tenses the mucosa, creating grooves and ridges. Purpose?

↑ surface area, improve efficiency of digestion and nutrient absorption

General Anatomy

Submucosa Layer:- thick layer of connective tissue containing:

- blood vessels- lymphatic vessels- nerve plexus- some places glands that secrete lubricating

mucus into the lumen

General Anatomy

Muscularis Externa:

Consists of two layers of smooth muscle.

Inner layer – encircle the tract.Outer layer – runs longitudinally.

Purpose- to help propel food/residue through DT.

Serosa Layer- outer most layer, thin areolar tissue, extends from lower esophagus to rectum.

Enteric Nervous System

This system regulates digestive tract motility, secretion, and blood flow.

Can function completely independent from the CNS, but there is some influence. (Opinion) part of ANS.

Composed of two nerve networks:1. Submucosal (Meissner) plexus - submucosa- controls movement of muscularis mucosa and glandular secretions of the mucosa

2. Myenteric (Auerbach) plexus – between the two layers of the muscularis externa.- controls peristalsis and other contractions

Relationship to the PeritoneumDuring food processing the stomach and intestine undergo

strenuous contractions and need freedom to move in the abdominal cavity.

They are loosely suspended from the abdominal wall by connective tissue sheets called mesenteries.

Functions: 1. hold abdominal viscera in proper relationship to each other.

2. prevent intestine from becoming twisted and tangled when changing positions

3. provide passage for blood vessels and nerves that supply DT and contain lymph nodes and lymph vessels

Lesser Omentum – extends from the stomach to liver.Greater Omentum – hangs from inferior margin of stomach

and covers the sm. intestine like an apron.Mesocolon – anchors colon to the posterior abdominal wall.Intraperitoneal – organ enclosed by mesentary on both

sides (within peritoneal cavity).Retroperitoneal – organ lies on the dorsal wall and covered

by peritoneum on the ventral side only, (outside peritoneal cavity).

Regulation of the Digestive TractMotility and secretion of the DT are controlled by neural,

hormonal, and paracrine mechanisms.

Neural controls include a short and long autonomic reflex.

1. Short (myenteric) reflexes- stretching or chemical stimulation of DT via myenteric nerve plexus, stimulating contractions in nearby muscularis externa, i.e. peristalic contractions of swallowing.

2. Long (vagovagal) reflexes – act through autonomic nerve fibers that carry sensory signals from the DT to the CNS, motor commands back to the DT.

Hormones- produced by DT are gastrin and secretin, in blood stream

Paracrine secretions – histamine and prostaglandins, stimulate digestive functions, local stimulation

Mouth through Esophagus

Mouth aka oral or buccal cavity.

Function:- ingestion (food intake)- sensory response to food- mastication- chemical digestion- deglutition- speech, respiration

Oral orifice to faucesLined with stratified squamous epithelium

Cheeks & LipsFunction of cheeks:

- retain food and push it between teeth for chewing,- speech,- sucking & blowing actions (suckling by infants).

Lips:- attached to gums by labial frenulum- vestibule – space between cheeks / lips and external teeth

Divided into two areas- Cutaneous area- mustache area- Vermilion (red) area- lipstick area, blood caps, nerves close to surface

Tongue

Very agile & sensitive organ.

- Manipulates food between teeth without getting bitten,- Extracts particles from teeth after a meal,- Sensitive enough to feel a stay hair in food.

Nonkeratinized stratified squamous epitheliumContains lingual papillae (taste buds).

Divide into body and root separated by Vallate papillae and terminal sulcus.

Attached to the floor of the mouth by lingual frenulum.

Tongue Muscles

Extrinsic muscles:Styloglossus m.Hyoglossus m.Genioglossus m.Palatoglossus m.

How does proprioception protect the tongue from being bitten?

Palate

Hard palate and soft palate. Separate oral & nasal cavity, makes it possible to breath while

chewing food.

Hard palate:- contains transverse friction ridges (palatal rugae) help to hold on to food.

Muscular arches called palatoglossal arch (anterior) and palatopharyngeal arch (posterior, marks beginning of pharynx)

Dentition (teeth)

Tooth & Gum Disease

Mouth contains 700 species of microorganisms, esp. bacteria.

Bacteria & sugars together form plaque, bacteria metabolize sugar producing lactic acid dissolve enamal = dental caries (cavities). If penetrates into dentin = extraction or root canal therapy.

Calculus (tartar) – when plaque calcifies. When in the gingival sulcus it wedges the tooth and gum apart allowing bacteria to invade = gingivitis

Bacteria spreading from the sulcus to the alveolar bone begin to dissolve it = periodontal disease.

Mastication or Chewing

• Breaks food into smaller pieces to be swallowed surface area exposed to digestive enzymes

• Requires little thought because contact of food with sensory receptors triggers chewing reflex– tongue, buccinator and orbicularis oris manipulate food– masseter and temporalis elevate the teeth to crush food– medial and lateral pterygoids swing teeth in side-to-side

grinding action of molars

First step in mechanical digestion

Saliva & Salivary Glands

Saliva

Functions:- Moistens mouth- digests a little starch and fat- cleanses teeth- inhibits bacterial growth- dissolves molecules to bind to taste buds- moistens and binds food together for

swallowing

Saliva

Hypotonic solution of 97% - 99.5% water and

• Salivary amylase – an enzyme that digests starch;• Lingual lipase- an enzyme that is activated by stomach

acid & digests fat after food is swallowed.• Mucus – binds and lubricates food mass and aids in

swallowing;• Lysozyme – enzyme that kills bacteria;• Immunoglobulin A (IgA) – antibody that inhibits

bacteria growth;• Electrolytes – Na, K, Cl, P, and bicarbonate salts.• pH 6.8-7.0

Salivary Glands

2 Types: Intrinsic and extrinsic

Intrinsic glands: lingual glandslabial glandsbuccal glands

Secrete small amounts at a constant rate, either eating or not (lingual lipases & lysozymes / moisten mouth & inhibit bacterial growth).

Extrinsic glands: are three discrete organs, that connect to oral cavity via a duct.

- Parotid, Submandibular, Sublingual glands

All are compound tubuloacinar glands.

Some acini are only mucous cells or serous cells or both.

Mucous cells = salivary mucusSerous cells = thinner fluid rich

in amylase and electrolytes.

Salivation

Cells filter water from blood and add other substances

Food stimulates receptors (pressure, taste, tactile) that signal salivatory nuclei in medulla and pons. Also signals from…

Higher brain centers stimulate salivatory nuclei so sight, smell and thought of food cause salivation.

Salivary nuclei send signals to the glands by way of CN VII & IXparasympathetic stimulation salivary glands produce thin

saliva, rich in enzymessympathetic stimulation produce less abundant, thicker

saliva, with more mucus

Pharynx

Skeletal Muscles Layers:- Deep – longitudinally orientated - Superficial – circular orientation

- divided into 3 Pharyngeal constrictors:1. Superior2. Middle 3. Inferior – considered the upper esophogeal sphincter (UES)

When not swallowing food, the inferior constrictor remains contracted to exclude air from the esophagus.

However, considered a “physiological sphincter” because it disappears at the time of death.

Esophagus

Straight muscular tube that penetrates the diaphragm at the esophageal hiatus, continues to meet the stomach at the cardiac orifice.

Food pauses at another physiological sphincter called the lower esophageal sphincter (LES). Believed to prevent regurgitation of food / acid.

Mucosa - nonkeratinized stratified squamous.Submucosa - esophageal glands = lubricating.

Muscularis externa – Skeletal m. upper 1/3rd, mix of skeletal and smooth m. middle 1/3rd , and smooth m. inferior 1/3rd .

Transition from voluntary to involuntary phases of passing the food bolus down the esophagus.

Swallowing

Aka deglutition, involves 22 muscles from the mouth, pharynx, and esophagus, coordinated by the swallowing center in the MO and Pons.

Swallowing center communicates with the muscles of the pharynx and esophagus by way of CN V, VII, IX, and XII.

Swallowing occurs in two phases:1. Buccal phase2. Pharyngeal-esophageal phase

Deglutition

Buccal Phase: - tongue collects food- presses it against the palate to form bolus- pushing it back into the oropharynx- bolus stimulates tactile receptors and activates next phase.

Pharyngeal-esophageal phase: (3 blocking steps)- root of the tongue blocks oral cavity- soft palate rises & blocks nasopharynx- infrahyoid muscle pulls larynx = blocking airway

Deglutition

The bolus slides off the epiglottis and enters the esophagus, triggering waves of muscle contraction called peristalsis.

Modulated by a short reflex through the myenteric nerve plexus.

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1

What are some functions of the tongue.

1

Manipulate food, extract particles, taste etc.Home

2

Name the tissues, in order, from the mandibular bone to the root canal of a tooth.

2

Peridontal ligament, cementum, dentin, pulpHome

3

What is the difference in function and location between intrinsic & extrinsic salivary glands?

3

Function: intrinsic: constant, lipase and lysozymes,glands

extrinsic: organs, mucous glands,serous glandsHome

4

Name the extrinsic salivary glands and describe their locations.

4

Parotid: beneath skin, ant. to earlobe, duct?

Submandibular:halfway along the body of mandible,medial

Sublingual: floor of the mouthHome

5

Describe the muscularis externa of the esophagus and its action in peristalsis.

5

Inner: circular m., outer: longitudinal m. bolus stim. stretch receptors, cm behind bolus contracts =push bolus, cm ahead

relaxes, Lm contracts=shortening & widening esophagusHome

6

Describe the mechanism that prevent food from entering the nasal cavity during

swallowing.

6

The soft palate rises and blocks the nasopharynx.

Home

7

Describe the mechanisms that prevent food from entering the larynx during swallowing.

7

The infahyoid pulls the larynx up, epiglottis covers it’s opening, vestibular folds are adducted to close

airway.Home

8

Name the nutrients that are absorbed without being digested.

8

Vitamins, free AA, minerals, cholesterol, and water.

Home

9

How does proprioception protect the tongue from being bitten?

9

— To keep from biting the tongue any more than we already do, it is important that the neuromuscular system coordinate tongue and chewing movements. To do this, it must be “aware” at all times of the contraction of

the lingual and jaw muscles, and it is the proprioceptors that provide the brain with this information.Home

Stomach

Divided into 4 regions- Cardiac region - Fundic region- Body region- Pyloric region

- Atrium- Pyloric canal- Pylorus

Pyloric sphincter (gastroduodenal sphincter)

Innervation & Circulation

Parasympathetic innervation from CN X.Sympathetic innervation from celiac ganglia

Supplied by blood from the celiac trunk.Blood from stomach and intestines enters hepatic

portal circulation, filters through liver, heart.

The Stomach Wall

Mucosa is lined with simple columnar glandular epith.

Muscularis externa consists of 3 layers:1. outer longitudinal layer2. middle circular layer3. inner oblique layer

Stomach Wall

Gastric Pit: epithelial layer = columnar epitheliumlamina propria (bottom of pit) = glands

3 tubular gland (located in the bottom of pit)1. cardiac glands (cardiac region)2. pyloric glands (pyloric region)3. gastric glands (rest of the stomach)

All contain mucous, stem, parietal, chief and enteroendocrine cells

Stomach Wall

Mucous cells = secrete mucus, predominate in cardiac and pyloric glands, also in gastric glands.

Regenerative (stem) cells = rapidly divide and produce new cells. Cells migrate up or down.

Parietal cells = found in upper half of gland, secrete hydrochloric acid and intrinsic factor. Mostly in gastric glands.

Chief cells = most numerous, secrete chymosin and lipase in infancy and pepsinogen throughout life. Located in gastric glands and absent from cardiac and pyloric glands.

Enteroendocrine cells = secrete hormone and paracrine messengers that regulate digestion.

Gastric Secretions

1. Hydrochloric acid = pH as low as 0.8.

Parietal cells contain carbonic anhydrase (CAH), which catalyzes:

CO2 + H2O H2CO3 HCO3- + H+

H+ is pumped into lumen by H+ - K+ ATPase.HCO3

- are exchanged for Cl- from the blood (Cl- shift)H+ joins with the Cl- to form HCl-

Alkaline tide – pH of blood leaving stomach is acidic due to the bicarbonate ions.

CAH

Hydrochloric acid

Functions:

1. activates enzymes pepsin and lingual lipase.

2. breaks up connective tissues and plant cell walls liquefies food and forms chyme.

3. converts ingested ferric ions (Fe3+) to absorbable iron ferrous ions (Fe2+).

4. contributes to nonspecific disease resistance by destroying ingested bacteria & other pathogens.

2. Pepsin

Some enzymes are secreted as inactive proteins called zymogens, become active after removal of some AA.

Chief cells produce a zymogen called pepsinogen. Which will be converted to pepsin by the removal of AA by HCl-.

Function is to digest dietary proteins into shorter peptide chains, which are then absorbed by the small intestine.

Autocatalytic effect as some pepsin is formed.

3. Other enzymes

Chief cells (infancy) also produce:

Gastric lipase – digest butterfat of milk.

Chymosin (rennin) – curdles milk by coagulating its proteins.

4. Intrinsic Factors

Produced by parietal cells, essential for the absorption of Vitamin B12 by the small intestine.

Intrinsic factors bind to B12 and the small intestine will absorb it via receptor mediated endocytosis.

B12 is important for hemoglobin synthesis and prevention of pernicious anemia.

Clinical aspects: Gastrectomy – removal of stomach, still able to digest food but person must take B12 injections or B12 and intrinsic factor orally.

5. Chemical Messengers

Gastric and pyloric glands produce about 20 different chemical messengers – most are hormones that can be systemic or paracine secretions.

Some of these peptides are produced in the CNS as well as digestive tract – called gut-brain peptides.

Substance P, vasoactive intestinal peptide (VIP), secretin, gastric inhibitory peptide (GIP), cholecystokinin, and neuropeptide Y (NPY).

Gastric Motility

Swallowing actives the swallowing center in the MO, causing the stomach to relax.

Food entering the stomach activates the receptive-relaxation response of smooth m. allowing relaxation of stomach to accommodate food.

Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer.

-mild ripples from fundus to pyloric region every 20 seconds- after 30 min strong contractions, churning food.

Antrium holds about 30ml of chyme, as peristalsis moves toward antrum it squirts 3ml of food into duodenum at a time.

When the wave reaches the pyloric sphincter it shuts, thus remaining chyme is churned some more.

Benefits of letting a little chyme into the duodenum:

- allows it to neutralize stomach acid a little at a time.

- more complete digestion of nutrients.

- prevention – overfilled duodenum results in inhibition of gastric motility = vomiting.

Vomiting

Multiple muscle actions integrated by emetic center in MO.

Induced by:- overstretching of stomach or duodenum- chemical irritants (alcohol, bacterial toxins)- visceral trauma (pelvic organs)- intense pain- psychological and sensory stimuli that activate emetic center. (repugnant sights, smells, thoughts)

Vomiting is usually preceded by nausea & retching.

Retching – thoracic expansion and abdominal contraction creating a pressure difference that dilates the esophagus.

Retching accompanied by:- tachycardia- profuse salivation- sweating

Vomiting occurs when the abdominal contraction and rising thoracic pressure force the upper esophagus sphincter open, reverse peristalsis.

Projectile vomiting- no nausea or retching. Cause: neurological lesion, common after feeding infants.

Digestion & Absorbtion

All nutrients are absorbed by the small intestine.

Stomach does not absorb much nutrients but will absorb aspirin and some lipid-soluble drugs.

Alcohol is absorbed by the small intestine, so its effects will depend upon how fast the stomach is emptied.

Protection of stomach

Protected by 3 ways from the acid and enzymes:

1. Mucous coat – thick, highly alkaline mucous

2. Epithelial cell replacement – cells live for 3 – 6 days, and are rapidly replaced by dividing cell from the gastic pits.

3. Tight Junctions – epithelial cells are tightly adhered, preventing juices from digesting outside stomach.

Result of a breakdown of these protection mechanisms = inflammation and peptic ulcers.

Regulation of Gastric Function

Nervous & Endocrine systems collaborate to increase or decrease gastric secretions and motility.

Gastric activities are divided into 3 stages (based on whether the stomach is controlled by the brain, itself, or by the small intestine). All can occur simultaneously.

1. Cephalic Phase2. Gastric Phase3. Intestinal Phase

Cephalic Phase

Response due to sight, smell, taste, or thought of food.

Pathway: - sensory/mental input to hypothalamus- which relays to MO- Vagus nerves from MO stimulate

enteric nervous system, thus- stimulating gastric activity

Gastric PhaseActivated by the presence of food and/or

semidigested proteins.

Ingested food stimulates gastric juice by 2 ways:1. stretching the stomach (2 reflexes)

- short reflex by myenteric nerve plexus- long reflex by vagus n. and brainstem

2. raising the pH of the stomach contents

Gastric secretions stim. by 3 chemicals:- ACh -Histamine- Gastrin

Acetylcholine (ACh): secreted by parasympathetic n.

- stimulates mucus secretion- stimulates pepsinogen secretion from Chief cells.

Histamine: paracrine secretion from enteroendocrine cells in the gastric glands.

Gastrin: hormone secreted from enteroendocrineG cells in the pyloric glands.- stimulate pepsinogen secretion from Chief cells

ALL three will stimulate parietal cells to produce HCl- and intrinsic factors.

Positive feedback loop:- as food is broken down into peptides and AA, these directly stimulate G-cells to secrete more gastrin = ↑ parietal cell activation = ↑ HCl-

Negative feedback loop:- as digestion continues the pH drops lower, ↓ pH 2, stomach acid inhibits parietal and G cell secretion = winding down of gastric phase

Intestinal PhaseDuodenum regulates gastric activity through hormones and

nervous reflexes.1st it will ↑ gastric secretions – chyme stretches the duodenum, activating

vagovagal reflexes which stimulates the stomach. Also, peptides and AA in chyme stimulate G cells of duodenum to secrete

gastrin = further stomach stim.

Acid and semidigested fats in duodenum trigger the enterogastric reflex.

Duodenum sends inhibitory signals to the stomach and MO which

1) inhibits vagal nuclei2) stim. sympathetic neurons = inhibitory signals3) chyme stim. secretin, cholecystokinin (CCK) &

gastric inhibitory peptide (GIP) release = ↓ gastric secretion, motility in the stomach.

Before We Go On

1. Name four types of epithelial cells of the gastric and pyloric glands and state what each one secretes.

2. Explain how the gastric glands produce hydrochloric acid and how this produces an alkaline tide.

3. What positive feedback cycle can you identify in the formation and action of pepsin?

4. How does food in the duodenum inhibit motility and secretion in the stomach?

Liver, Gallbladder, and Pancreas

Liver

Locted: right hypochondriac and epigastric regions.

Microscopic Anatomy

Hepatic lobule – consists of central vein, hepatocytes, hepatic sinusoids, hepatic triad, bile canaliculi.

Sinusoids lined with fenestrated epithelium that separates hepatocytes from the blood stream, allows blood plasma in between. Removing glucose, AA, iron, vitamins etc. for metabolism and/or storage.

Liver secretes bile into bile canaliculi.

Bile canaliculi → bile ducts of triad → R/L hepatic ducts

Hepatopancreatic ampulla

Gallbladder & BileInternally has simple columnar epithelium, functions to store

and concentrate bile.

Bile - yellowish – green fluid containing mineral, cholesterol, fats, phospholipids, bile pigment and bile acids.

Bile pigment is bilirubin (derived from decomp of hemoglobin), bilirubin is metabolized to urobilinogen = gives feces its brown color.

Absent of pigment = acholic feces- grayish white and marked with fat. Usually indicative of complete bile duct obstruction.

Bile acids – steroids syn. from cholesterol, aid in digestion and absorption of fat

Enterohepatic circulation

Gross Anatomy of PancreasBoth an exocrine and endocrine gland.Endocrine part is the pancreatic islets = insulin & glucagon.

Exocrine part secretes 1200-1500 mL of pancreatic juices/day.

Pancreas is made up of secretory acini (cells) that contain RER and zymogen granules filled with secretion.

Acini open to a system of larger and larger ducts eventually converging with the pancreatic duct.

Pancreatic duct runs midline and joins with the hepatopancreatic ampulla (opens at sphincter of Oddi)

Accessory pancreatic duct (independent) that opens to the duodenum at the minor duodenal papilla.

Pancreatic JuicesAlkaline mixture of water, enzymes, zymogens, sodium

bicarbonate, and other electrolytes.

Acini secrete enzymes and zymogens.

Ducts secrete sodium bicarbonate (buffers HCl from stomach).

Pancreatic zymogens:- trypsinogen → trypsin by enterokinase – digest protein- chymotrypsinogen → Chymotrypsin by trypsin- procarboxypeptidase → Carboxypeptidase by trypsin

Pancreatic Enzymes:- pancreatic amylase- pancreatic lipase-ribonuclease & deoxyribonuclease

Regulation of Secretion

Bile and pancreatic juices are secreted in response to:

- Parasympathetic (vagal) stimulation;- Sympathetic inhibit stimulation;- hormones:

- Cholecystokinin (CCK)- gastrin- secretin

Regulation of SecretionCCK released from duodenum in response to acid & fats

arriving from the stomach.

CCK triggers 3 responses:- contraction of the gallbladder = forces bile into bile duct;- secretion of pancreatic enzymes;- relaxation of the hepatopancreatic sphincter = allows bile and pancreatic juices to be released into duodenum.

Gastrin from stomach and duodenum weakly stimulates gallbladder contraction and pancreatic enzyme secretion

Secretin released from duodenum in response to acidic chyme- Stimulates bile & pancreatic ducts to secrete more bicarbonate

(helping to neutralize acid in the duodenum.

Before We Go On!

1. What does the liver contribute to digestion?

2. Trace the pathway taken by bile acids from the liver and back. What is the pathway called?

3. Name two hormones, four enzymes, and one buffer secreted by the pancreas, and state the function of each.

4. What stimulates CCK secretion, and how does CCK affect other parts of the digestive system?

Small Intestine

Nearly all chemical digestion and nutrient absorption occurs in small intestine

Small IntestineDivided into three regions:

1. Duodenum – pyloric valve → denojejunal flexure - receives stomach contents, pancreatic juice and bile- neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH

increase, pancreatic enzymes

2. Jejunum – next 8ft or first 40% of small intestine- begins upper left quadrant of abdomen, but lies mostly within umbilical

region (fig A.6);- walls are thick and muscular, rich with blood supply;- Most digestion and nutrient absorption occurs here.

3. ileum – last 12ft, ends at ilieocecal junction/ ilieocecal valve- occupies hypogastric region and part of pelvic cavity- thinner walls, less muscular & less vascular- contains lymphatic nodules called Peyer Patches

Microscopic Anatomy

Circular folds (plicae circularis) – from duodenum to middle ileum.

- Allows chyme to flow in a circular motion to increase contact with mucosa, promotes mixing and nutrient absorbtion.

Mucosa contain Villi (finger-like) projections- Increase surface area- Larger in duodenum, small in distal regions

Contain 2 cell types:1. Absorptive cells (enterocytes)2. Goblet cells – mucous secreting

In the middle of each villi contains:- an arteriole; - a capillary network - nutrient absorb; - a venule; - lymphatic capillary (Lacteal) – fat absorption.

Absorptive cells have a brush border of microvilli.- increases the surface area of the absorptive cells;- contain brush border enzymes, for final stages of enzymatic digestion;- enzymes release by process called contact digestion.

Intestinal crypts:- absorptive cells, goblet cells, stem - Paneth cells – lysozyme, phospholipase, defensins – protection against bacterial pathogens.

Intestinal MotilityContractions of the small intestine = 3 functions:

1. mix chyme with intestinal juice, bile and pancreatic juice, allowing fluids to neutralize acid and digest nutrients;

2. churn chyme, bringing it in contact with mucosa for contact digestion & nutrient absorption;

3. move residue toward large intestine.

Segmentation- most common movement of the sm. intestine.- ringlike constrictions along intestine.- purpose = knead or churn contents. - pacemaker cells set the rhythm of muscularis externa

When nutrients are absorbed, the residue is moved along via peristaltic waves.

Overlapping waves of contractions called a migrating motor complex.

Refilling the stomach suppresses peristalsis and reactivates segmentation.

Once at the ileocecal junction, residue passes through the ileocecal valve into cecum.

Food in the stomach triggers the gastroileal reflex, which helps relax the valve.

Before We Go On

1. What three structures increase the absorptive surface area of the small intestine?

2. Sketch a villus and label its epithelium, brush border, lamina propria, blood capillaries, and lacteal.

3. Distinguish between segmentation and the migrating motor cortex of the small intestine. How do these differ in function?

Chemical Digestion & Absorption

Carbohydrate DigestionProcess begin in the mouth, salivary amylase hydrolyzes starch

into oligosaccharides.

Once in the stomach amylase still works as long as it does not come into contact with acid, once it does it is digested.

50% of dietary starch is digested before reaching the sm. Intestine.

In sm. Intestine the chyme mixes with pancreatic amylase, breaks starch down to oligosaccharides and maltose. Through contact digestion, dextrinase, glucoamylase, and maltase enzymes hydrolyze the starch further.

Sucrace and lactase enzymes hydrolyze sucrose and lactose into monosaccharides.

Carbohydrate Absorption

80% of glucose is taken up by a sodium-glucose transport protein (SGLT).

SGLT also absorb galatose.Fructose is absorbed by facilitated diffusion.

Inside the cell fructose is converted to glucose.

Glucose, galatose, and some fructose pass out the base of the cell via facilitated diffusion and absorbed by blood capillaries of the villus.

Proteins Digestion & Absorption

Begins in the stomach, with enzymes called proteases (peptidase), which digest proteins.

Pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine = smaller polypeptides.

In small intestine, pancreatic enzymes trypsin & chymotrypsin take over hydrolyzing polypeptides into smaller oligopeptides.

Finally, brush border (contact digestion) enzymes take apart one AA at a time.

Carboxypeptidase = removes the –COOH group.Aminopeptidase = removes the –NH2 group.Dipeptidase = splits dipeptides in the middle releasing two AA.

Protein Digestion & Absorption

Once in the cell, AA are released into the blood stream via facilitated diffusion, enter the capillaries of the villus.

The absorptive cells of infants take up intact proteins via pinocytosis and release them into the blood via exocytosis.

This allow IgA from breast milk to pass into infant circulation resulting in passive immunity from mother to infant.

Lipid Digestion & Absorption

Fat is digested by enzymes called lipases.

Mouth = lingual lipases activated by stomach acid; Stomach = gastric lipases (infants);Small intestine = pancreatic lipases (most fat digestion).

1st step is Emulsification:- certain components of bile- lectihin and bile acids.

2nd step Fat Hydrolysis:

Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the 1st and 3rd fatty acids from the triglycerides leaving a monoglyceride.

3rd Step: Lipid uptake by micelles.

Micelles consist of 20-40 bile acid molecules that come together.

They pass into the small intestine and pick up several type of dietary and semi-digested lipids.

Transport the lipids to the surface of the intestinal absorptive cells, lipids leave the micelles and diffuse through the plasma membrane into cell.

Without micelles, the small intestine would only absorb 40 – 50% of dietary fat and almost no cholesterol.

4th Step: Chylomicron Formation

Intestinal cells absorb lipids from the micelles, resynthesis triglycerides (SER), the Golgi complex packages triglycerides, phospholipids and cholesterol into protein-coated chylomicrons.

5th Step: Chylomicron exocytosis and lymphatic uptake.

The chylomicrons are packaged into secretory vesicles, that migrate to the basal surface of the cells and released into the villus.

Some FFA enter capillaries but chylomicrons are too large and enter the lacteal into the lymph, thus eventually entering into the blood stream via left subclavian vein.

Nucleic Acids, Vitamins, Minerals

Nucleases hydrolyze DNA and RNA to nucleotides- nucleosidases and phosphatases of brush border split them into

phosphate ions, ribose or deoxyribose sugar and nitrogenous bases

Vitamins are absorbed unchanged- A, D, E and K with other lipids -- B complex and C by simple

diffusion and B12 if bound to intrinsic factorMinerals (electrolytes) are absorbed all along

small intestine regardless of need, up to the kidney to decide.- Na+ cotransported with sugars and amino acids- Cl- exchanged for bicarbonate - Iron and calcium absorbed as needed. Ca+ absorbed in

duodenum and jejunum.

Calcium absorption:

Parathyroid hormone induces kidney to release active Vit. D (calcitriol) → Vit. D stimulates Ca2+ absorption by small intestine → Ca2+ passes through calcium channels into epithelial cells and bind to calbindin → calcium is pumped out of the cell (basal side) via calcium-ATPase and Na+ - Ca2+ antiport.

Water

Digestive tract receives about 9 L of water/day-.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal

secretions (saliva, gastric, pancreactic juices, bile)-8 L is absorbed by small intestine and 0.8 L by large

intestine, 0.2 L voided in daily fecal output.

Water is absorbed by osmosis following the absorption of salts and organic nutrients that create an osmotic gradient from intestinal lumen to the ECF.

Diarrhea occurs when too little water is absorbed-intestine irritated or feces pass through too quickly -feces contains high concentrations of a solute (lactose).

Large Intestine

Gross AnatomyMuscularis externa –

circles the intestine like the sm. Intestine but its longitudinal fibers are [ ] in a thickened strip called Tenia coli.

When contracted forms pouches called haustra.

Anal canal

Passes through the levator ani muscle and terminates at the anus.

Contains two sphincters:1. Internal anal sphincter2. External anal sphincter

Composed of skeletal m.

Microscopic Anatomy

Mucosa- simple columnar epithelium,- except lower half of anal canal is

nonkeratinized stratified squamous epithelium.

No circular folds or villi in the large intestine.

Contain intestinal crypts – have a greater density of goblet cells, mucus is their only significant secretion.

Bacterial Flora

800 species of bacteria populating our lg. intestine.

Mutually beneficial relationship, they extract nutrients from food that we are not equipped to extract on our own.

Thus we get more nutrients from our food than we would get without them.

DefecationStretching of the rectum stimulates the defecation reflex.

Process involves two reflexes:

1. Intrinsic defecation reflex- (via myenteric nerve plexus).

- activates a weak peristaltic wave- relaxes internal anal sphincter- requires coordination of the next reflex

2. Parasympathetic defecation reflex- spinal reflex, sensory in to spinal cord, motor signals out via

pelvic nerves;- intensifies peristaltic waves in descending sigmoid colon

and rectum;- relaxes internal anal sphincter.

Reflexes are INVOLUNTARY and the sole means of defecation in infants and people with transected SCI.

External anal sphincter is under VOLUNTARY control, thus preventing untimely defecation.