Chapter 23 The digestive system

6. List and describe the function of the secretions produced in these digestive organs: salivary glands, esophagus, stomach, small intestines, liver, gall bladder, and pancreas.

...

Be able to draw and/or label the major layers of the digestive tract.

...

Describe all major cell types found in gastric glands and gastric pits, and their secretions (focus on parietal cells, chief cells, and enteroendocrine cells).

...

Describe the components and function of bile

...

Describe the function, site of production and site of action of the digestive hormones gastrin, secretin, and CCK.

...

Describe the histological organization of the digestive tract.

...

Describe the regulation of gastric activity (3 overlapping phases; intestinal phase has two parts to it).

...

For the mucosal layer, also know the sublayers. Understand the salient (important) features of each layer. Be able to recognize any specialized features found in different parts of the digestive tract (e.g., extra muscle layer in stomach; specialized features in epithelia of stomach and small intestine).

...

Know the basic categories of pancreatic enzymes, and their substrates.

...

Know two local neurotransmitters that stimulate and two local neurotransmitters that inhibit smooth muscle contraction in the gut.)

...

Liver: Microscopic Anatomy

...

Understand the basic anatomy of this organ (hepatocytes, lobules, and portal triads

...

esophagus

...

gall bladder

...

liver

...

pancreas

...

salivary glands

...

small intestines

...

stomach

...

the function of carbonic acid

...

Enteric Nervous System: stimulate

ACh and Substance P stimulate smooth muscle contraction

Accessory Digestive Organs

Accessory Digestive Organs Teeth Physically grind down foods Tongue Manipulates food, assists with swallowing Salivary glands (Sublingual, Submanibular, Parotid), Gallbladder, Liver, and Pancreas

Understand the role of pancreatic acinar cells?

Acinar cells, forming the bulk of the gland, produce an enzyme rich juice that is carried by ducts to the small intestine during digestion

pancreas: Exocrine function

Acini (clusters of secretory cells) secrete pancreatic juice Zymogen granules of secretory cells contain digestive enzymes Epithelial cells in walls of smaller ducts release bicarbonate in response to secretin

peristalsis

Adjacent segments of alimentary tract organs alternately contract and relax, which, along with gravity, moves food down the tract

First of 2 Groups of Organs

Alimentary Canal (Gastrointestinal or GI tract; also called "gut") One long muscular tube, open on both ends Consists of mouth, pharynx, esophagus, stomach, small intestine, and large intestine Digests and absorbs food

alkaline tide is

As H+ is pumped from the cell and HC03- is ejected through the basal cell membrane into the capillary blood. As a result, blood draining from the stomach is more alkaline than the blood serving it. This is called alkaline tide.

CCK and secretin

Both secretin and cholecystokinin prompt the pancreas to secrete its juice. Secretin, released in response to the presnce of HCl in the intestin,e, mainly targets the pancreatic duct cells, resulting in a watery bicarbonate rich pancreatic juice. CCK, released in response to proteins and fats in chyme, stimulates the acini to release enzyme rich pancreatic juice, and potentiatesthe effect of secretin.

Know the roles of the brush border and its enzymes, and why the location of the enzymes is advantageous?

Brush border enzymes take breakdown of products of digestion to production of monomers that can be absorbed into blood and lymphatic vessels.

What does CCK cause at the intestine/gall bladder/ pancreas

CCK is secreted in response to the entry of chyme rich in protein and fat into the duodenum. It causes the pancreatic acini to secrete digestive enzymes and stimulates the gallbladder to contract

Mastication

Chewing food is called mastication. Food mixes with saliva (including water, mucus, and digestive enzymes).

Intrinsic nerve supply of alimentary canal

Contains > 100 million neurons - more than are found in spinal cord!

Chemical Digestion of Carbohydrates

Digestive enzymes Salivary amylase Pancreatic amylase Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, and sucrase)

Defecation:

Elimination of undigestible substances from body via anus as feces

What is enteropeptidase?

Enteropeptidase (also called enterokinase) is an enzyme produced by cells of the duodenum and involved in human digestion. It is secreted from intestinal glands (the crypts of Lieberkühn) following the entry of ingested food passing from the stomach.

Chemical Digestion of Proteins

Enzymes: pepsin in the stomach Pancreatic proteases Trypsin, chymotrypsin, and carboxypeptidase Brush border enzymes Aminopeptidases, carboxypeptidases, and dipeptidases

Salivary Glands: extrinsic

Extrinsic salivary glands (parotid, submandibular, and sublingual)

Serous membrane

Familiar parietal and visceral layers with fluid-filled cavity in between Parietal layer lines walls of cavity Visceral layer is transparent and directly lines most organs Fluid lubricates mobile organs in the cavity

esophagus

Flat muscular tube from laryngopharynx to stomach Joins stomach at cardiac orifice Esophageal mucosa contains stratified squamous epithelium Changes to simple columnar at the stomach Esophageal glands in submucosa secrete mucus to aid in bolus movement Muscularis: skeletal superiorly; smooth inferiorly Adventitia instead of serosa

Villi of the Duodenum

For electrolyte and nutrient absorption and for chemical digestion Are fingerlike extensions (~1 mm high) of the mucosa Epithelium of villi: Simple columnar absorptive cells (enterocytes) Many goblet cells release mucus

Be familiar with the hepatic portal circulation

Hepatic portal circulation is nutrient-rich venous blood that drains from digestive viscera and is delivered to liver, where it is "treated" before being returned to the rest of the body

Liver lobules

Hexagonal structural and functional units -Filter and process nutrient-rich blood -Composed of plates of hepatocytes (liver cells) Longitudinal central vein

Segmentation

In segmentation, nonadjacent segments of alimentary tract organs alternately contract and relax, moving food forward and backward. Slows movement through tract Increases food mixing and contact with enzymes and wall of lumen

Submucosal nerve plexus

In submucosa Sensory and motor fibers Regulates glands and smooth muscle in mucosa

Salivary Glands: intrinsic

Intrinsic (buccal) salivary glands are scattered in the oral mucosa (mouth)

Deglutition (swallowing)

Involves the tongue, soft palate, pharynx, esophagus, and 22 muscle groups Buccal phase Voluntary contraction of the tongue Pharyngeal-esophageal phase Involuntary Control center in medulla and lower pons

swallowing

It is mechanically broken apart to form a bolus, which is then swallowed. Bolus passes through pharynx with no chemical or mechanical digestion; is called chyme after it enters the stomach

Enteric nervous system

Linked to CNS via afferent visceral fibers Long ANS fibers synapse with enteric plexuses Sympathetic impulses inhibit secretion and motility Parasympathetic impulses stimulate

long relexes

Long reflexes involve the central nervous system

Be able to recognize or describe a short reflex versus a long reflex.

Long reflexes involve the central nervous system Short reflexes are carried out by the enteric nervous system with no CNS involvement

Small Intestine: Gross Anatomy

Major organ of digestion, absorption 2-4 m long; from pyloric sphincter to ileocecal valve Subdivisions Duodenum Jejunum Ileum

Digestive System Functions

Makes it possible to engage in one of life's greatest pleasures - eating! Breaks down foods to forms of nutrients that can be absorbed and used by the body Site of vitamin synthesis Absorbs usable substances, discards the rest Protects against foreign invaders Plays a role in water and electrolyte balance

Know what the alkaline tide is

Meanwhile, HCO3- accumulates in the parietal cell HCO3- is ejected through basal (side away from lumen) cell membrane into blood/interstitial fluid in exchange for Cl-. This is the alkaline tide Consequence: blood returning from stomach region is more alkaline than usual Consequence: Cl- supply in parietal cells is replenished

Alimentary Canal Organs, In Order

Mouth Pharynx Esophagus Stomach Small Intestine (Duodenum Jejunum Ileum) Large Intestine (Cecum (w/Appendix) Ascending Colon Transverse Colon Descending Colon Sigmoid Colon Rectum Anal Canal)

Also know which organs/structures are involved in mastication and swallowing (deglutition)

Mouth to pharynx Chewing food is called mastication. Food mixes with saliva (including water, mucus, and digestive enzymes). It is mechanically broken apart to form a bolus, which is then swallowed. Bolus passes through pharynx with no chemical or mechanical digestion; is called chyme after it enters the stomach

Stomach: Microscopic Anatomy

Mucosa -Simple columnar epithelium composed entirely of mucous cells Layer of alkaline mucus traps bicarbonate-rich fluid beneath it -Otherwise smooth lining dotted with gastric pits that lead into tubular gastric glands that produce gastric juice, among other things

Enteric Nervous system: inhibit

NO and vasoactive intestinal peptide (VIP) inhibit smooth muscle contractions

gastric pits

Otherwise smooth lining dotted with gastric pits that lead into tubular gastric glands that produce gastric juice, among other things

Pancreatic enzymes

Pancreatic enzymes are natural chemicals that help break down fats, proteins and carbohydrates.

other features of duodenum

Paneth cells, deep in crypt, release antimicrobial/antibacterial substances and help to determine which bacteria colonize intestinal lumen Submucosa contains duodenal (Brunner's) glands that secrete alkaline mucus to counteract acidity of recently arrived chyme

parasympathetic

Parasympathetic nervous system activity stimulates salivation Parasympathetic impulses stimulate

How do the parasympathetic and sympathetic nervous systems influence digestive functions?

Parasympathetic nervous system activity stimulates salivation Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia) Sympathetic impulses inhibit secretion and motility Parasympathetic impulses stimulate

Absorption:

Passage of digested end products across digestive system lumens through mucoscal cells and into blood or lymph

Understand where the parietal and visceral peritoneum are found.

Peritoneum is mucus membrane that lines abdominopelvic cavity

Digestive Processes in the Stomach

Physical digestion via churning Denaturation of proteins in highly acidic environment Enzymatic digestion of proteins by pepsin Secretes intrinsic factor required for absorption of vitamin B12 Lack of intrinsic factor pernicious anemia Delivers chyme to small intestine with controlled rate of delivery

Chemical Digestion of Lipids

Pre-treatment—emulsification by bile salts Enzyme—pancreatic lipase

Gastric Gland Secretions

Primary functions of gastric juice: Provide HCl (creates acidic environment needed for protein denaturation and function of pepsin in hydrolysing proteins Provide pepsin to catalyse breakdown of proteins Provide chemical messengers that assist with HCl release and coordinate or bring about other digestive activity in stomach and intestines Assist with vitamin B12 absorption

What roles are played by the ENS?

Produce and coordinate local movements Orchestrate secretions Work independently or in association with the central nervous system (via short or long reflexes)

A Form of Propulsion: Peristalsis

Propulsion moves food through alimentary canal. One tool (besides swallowing) is peristalsis, in which Adjacent segments of alimentary tract organs alternately contract and relax, which, along with gravity, moves food down the tract

Enteroendocrine cells, including G cells

Secrete gastrin, histamine, serotonin, somatostatin, and other chemical messengers May exert local or hormonal effects depending on messenger

Parietal cells

Secrete hydrochloric acid (HCl) and intrinsic factor

Chief cells

Secrete pepsinogen

What role does secretin play?

Secretin, released by intstinal cells exposed to fatty chyme, also stimulates liver cell to secrete bile.

Salivary glands function

Secretion of lubricating fluid containing enzymes that break down carbodydrates

Understand the mechanics and function of peristalsis and how it is different from segmentation.

Segmentation mixes food with digestive juices and increases the efficiency of absorption by repeatedly moving different parts of the food mass over the intestine wall. Segmentation is part of mechanical digestion. Peristalsis is part of propulsion. Peristalsis the major means of propulsion, involves alternate waves of contraction and relaxation of muscles in the organ walls. Its main effect is to squeeze food along the tract, but some mixing occurs as well.

Chemical digestion

Series of steps in which complex food molecules are broken down into their chemical building blocks by enzymes (catabolic activity)

short reflexes

Short reflexes are carried out by the enteric nervous system with no CNS involvement

Cholecystokinin (CCK)

Site of production: Duodenal mucosa Stimulus for production: fatty chyme, in particular, but also partially digested proteins Target organ: liver/pancreas: protentiates secretin's action on these organs gallbladder: stimulates organ to contract and expel stored bile hepatopancreatic sphincter: relaxes sphincter to allow entry of bile and pancreatic juice into duodenum.

Gastrin

Site of production: stomach mucosa (g cells) Stimulus or production: food in stomach (chemical stimulation); acetylcholine released by nerve fibers target organ stomach (parietal cell): increases HCl secretion and stimulate gastric emptying Small intestine: stimulates contraction of intestinal muscle ileocecal valve: relaxes ileocecal valve large intestine: stimulates gastric glands and motility

Digestive Processes: six essential activitest

Six essential activities 1. Ingestion 2. Propulsion 3. Mechanical digestion 4. Chemical digestion 5. Absorption 6. Defecation

sympathetic

Strong sympathetic stimulation inhibits salivation and results in dry mouth (xerostomia) Sympathetic impulses inhibit secretion and motility

The stomach

Temporary "storage tank" Can expand tremendously From volume of ~ 50 ml to ~ 4,000 ml As it collapses inward, mucosa and submucosa form large longitudinal folds (rugae)

teeth incisors

The chisel shaped incisors are adapted for cutting or nipping off pieces of food

List and describe the major digestive functions of the liver that pertain to digestion.

The liver has multiple functions, but its main function within the digestive system is to process the nutrients absorbed from the small intestine. Bile from the liver secreted into the small intestine also plays an important role in digesting fat. In addition, the liver is the body's chemical "factory." It takes the raw materials absorbed by the intestine and makes all the various chemicals the body needs to function. The liver also detoxifies potentially harmful chemicals. It breaks down and secretes many drugs.

What protections help to prevent or minimize damage from digestion of our own organs by acidity and by enzymes that catalyze breakdown of proteins

The mucosal barrier, which protects the stomach from self digestion and HCl, reflects the fact that the mucosal cells are connected by tight junctions, secrete a thick mucus, and are quickly replaced when damaged

teeth molars

The premolars and molars have broad crowns with rounded cusps and are best suited for grinding or crushing.

Brush border

The small intestine tract: This is where absorption takes place.[2][3The brush borders of the intestinal lining are the site of terminal carbohydrate digestions. The microvilli that constitute the brush border have enzymes for this final part of digestion anchored into their apical plasma membrane as integral membrane proteins. These enzymes are found near to the transporters that will then allow absorption of the digested nutrients. The kidney: Here the brush border is useful in distinguishing the proximal tubule (which possesses the brush border) from the distal tubule (which does not).[5][6] The brush border morphology increases a cell's surface area, a trait which is especially useful in absorptive cells. Cells that absorb substances need a large surface area in contact with the substance to be efficient.[7]

tongue

The tongue occupies the floor of the mouth and fills most of the oral cavity when the mouth is closed. The tongue also mixes food with saliva forming it into a compact mass called a bolus, and then intiiates swallowing by pushing the bolus posteriorly into the pharynx.

What are two types of cells found in the Salivary glands

Two types of secretory cells: Serous cells produce watery secretion with enzymes, ions, a bit of mucin Mucous cells secrete mucus

Understand how cells of the stomach contribute to the formation of hydrochloric acid (HCl).

When protein foods are in the stomach, the pH of the gastric contents generally rises because proteins act as buffers to tie up H+. The rise in pH stimulates gastrin secretion and subsequently HCl release, which in turn provides the acidic conditions needed for protein digestion. The more protein in the meal, the greater the amount of gastrin and HCl released. As proteins are digested, the gastric contents gradually become more acidic, which again inhibits the gastrin secreting cell. This negative feedback mechanism helps maintain optimal pH and working conditions for the gastric enzymes

large intestine

ascending, transverse, descending, and sigmoid colon and rectum/anus

liver: portal triad

at each corner of lobule -Bile duct receives bile from bile canaliculi -Portal arteriole is branch of hepatic artery -Hepatic venule is branch of hepatic portal vein

CCK

besides cuasing the gallbladder to contract, CCK stimulates secretion of pancreatic juice and relaxes the hepatopancreatic sphincter so that bile and pancreatic juice can enter the duodenum.

stomach function

chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions

Define the enteric nervous system (ENS).

communicate widely with one another to regulate digestive system activity.

small intestine (duodenum, jejunum, and ileum),

duodenum, jejunum, and ileum

large intestine function

enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

splanchnic circulation

includes arteries that branch off abdominal aorta and serve digestive organs

Ingestion

is act of putting food in mouth

Mechanical digestion

is physical process of preparing food for chemical digestion; includes chewing, mixing, churning, segmentation*

What is the function of these structures, and of the mesenteries (omenta)?

it's a double layer of peritoneum Provides routes for blood vessels, lymphatics, and nerves Holds organs in place; stores fat

Propulsion

moves food through the system; includes swallowing and peristalsis*

Myenteric nerve plexus

n muscularis externa Provide major nerve supply to GI tract wall Controls GI tract motility

Intraperitoneal (peritoneal)

organs are surrounded by peritoneum

Retroperitoneal

organs lie posterior to peritoneum

Pancreas: endocrine function

pancreatic islets secrete insulin and glucagon

Mucous neck cells

secrete thin, acidic mucus

liver function

secretion of bile (important for lipid digestion) storage of nutrients, many other vital functions

secretin

site of production: duodenal mucosa stimulus or production: acidic chyme (also partially digested proteins, fats, hypertonic or hypotonic fluids, or irritants in chyme. stomach: inhibits gastric gland secretion and gastric motility during gastric phase of secretion

gallbladder

storage and concentration of bile

teeth canines

the conical or fanglike canines (cuspids or eye teeth) tear and pierce

Esophagus function

transport of materials to the stomach