Digestive System

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I. Introduction

Digestion refers to the mechanical and chemical breakdown of foods so that nutrients can be absorbed by cells.

The digestive system carries out the process of digestion.

The digestive system consists of the alimentary canal/ GI tract, leading from mouth to anus, w/c caries the whole menu of digestive functions and several accessory organs (teeth, tongue, several large digestive glands) which assist the process of digestive breakdown in various ways.

Functions of the Digestive System

1. Ingestion- take in food› occurs when materials enter digestive tract via

the mouth

2. Mechanical processing- break down food– crushing and shearing makes materials

easier to propel along digestive tract

3. Digestion-absorb digested molecules– is the chemical breakdown of food– into small organic fragments– for absorption by digestive epithelium

4. Secretion:› is the release of water, acids, enzymes, buffers,

and salts › by epithelium of digestive tract› by glandular organs

5. Absorption: – movement of organic substrates, electrolytes,

vitamins, and water– across digestive epithelium– into interstitial fluid of digestive tract

6. Excretion: – removal of waste products from body

fluids

Alimentary Canal/GI tract/Digestive Tract- tube extending from the mouth to the anus, plus the associated organs, which secrete fluids into the digestive tract.- consists of oral cavity, pharynx, esophagus, stomach, small intestine, large intestine and the anus, accessory organs such as salivary glands( empty into the oral cavity), liver and the pancreas w/c are connected to the small intestine.

Layers of Digestive Tract

1.Mucosa layer2. Submucosal layer3. Muscularis layer4. Serosa layer

Mucosa

Epithelium› stratified squamous (in mouth, esophagus & anus) = tough › simple columnar in the rest

secretes enzymes and absorbs nutrients specialized cells (goblet) secrete mucous onto cell surfaces enteroendocrine cells---secrete hormones controlling organ

function Lamina propria

› thin layer of loose connective tissue› contains BV and lymphatic tissue

Muscularis mucosae---thin layer of smooth muscle

› causes folds to form in mucosal layer › increases local movements - increasing Absorption with exposure to “new” nutrients

-Innermost layer

Submucosa

Loose connective tissue› containing BV, glands and lymphatic tissue

Meissner’s plexus› part of the enteric nervous system› “brain of the gut”› parasympathetic divisions› innervation by sensory and motor neurons

vasoconstriction local movement by

muscularis mucosa smooth muscle

supply the secretory cells of the mucosal epithelium connected to the myenteric plexus (in the muscularis layer) by a

series of interneurons

Muscularis - consist of an inner layer of

circular smooth muscle and an outer layer of longitudinal smooth muscle.

-lies between the two muscle layer is the is the enteric plexus (relating to the intestine).

*enteric plexus- important in the control of movement and secretion within the tract.

Serosa

The outermost layer of the digestive tract Consists of a single layer of flat serous fluid

producing cells, the VISCERAL PERITONIUM.

PARIETAL PERITONIUM-serous membrane that lines the wall of the abdominal cavity

In the regions of digestive tract w/c are not covered by peritonium, the DT is covered by a connective tissue layer called adventitia (foreign, coming from outside).

PeritoneumPeritoneum

-visceral layer covers organs-parietal layer lines the walls of body cavity

Peritoneal cavity-potential space containing a bit of serous fluid

• Mesentery – middle intestines• Mesocolon – large intestine

• Lesser omentum- messentary connecting the lesser curvature of stomach to the liver & diaphragm

• Greater omentum- connecting the greater curvature of stomach to the transverse colon and posterior body wall

• Peritonitis = inflammation of peritoneal membranes– trauma – rupture of GI tract– appendicitis– perforated ulcer

Greater Omentum, Mesentery & Mesocolon

Lesser Omentum

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The path of food:oral cavity/teeth/salivary glands

oropharynx/epiglottis

esophagus

stomach

small intestine: duodenum

small intestine: ileum

small intestine: jejunum

large intestine: ascending colon

large intestine: transverse colon

large intestine: descending colon

sigmoid colon rectum anus

PeristalsisConsists of waves of muscular contractions

Moves a bolus along the length of the digestive tract

Bolus is a small, oval mass of digestive contents

1. Circular muscles contract behind bolus:

while circular muscles ahead of bolus relax

2. Longitudinal muscles ahead of bolus contract:

shortening adjacent segments

3. Wave of contraction in circular muscles:

forces bolus forward

Mechanical Digestion in the Small Intestine 1. Weak peristalsis in

comparison to the stomach---chyme remains for 3 to 5 hours› occurs after most the meal has

been absorbed› also called the migrating

motility complex (MMC)› starts at the lower portion of

the stomach and pushes the chyme forward

› reaches the end of the ileum after 90 – 120 minutes

› then another wave starts in the stomach

2. Segmentation---local mixing of chyme with digestive juices in the SI› do NOT push the food through

the tract› move chyme back and forth› done in specific segments that

are defined by the smooth muscle layers

› most rapid in the duodenum and slows at it reaches the ileum

The Oral Cavity

4 Functions of the Oral Cavity

Lubrication:› mixing with mucus and salivary gland

secretions Limited digestion:

› of carbohydrates and lipids• Sensory analysis:

– of material before swallowing• Mechanical processing:

– through actions of teeth, tongue, and palatal surfaces

Labia• Also called lips• Muscular structures formed mostly by

orbicularis oris muscle

Gingivae (Gums) • Ridges of oral mucosa• Surround base of each tooth on alveolar

processes of maxillary bones and mandible

D. Palate – forms the roof of the oral cavity*hard palate – (anterior) formed by the palatine processes of the maxillary bones*soft palate – (posterior) forms a muscular arch which extends downward to form the uvula*uvula – during swallowing, muscles draw the soft palate & the uvula upward; this action closes the opening between the nasal cavity & the pharynx preventing food from entering the nasal cavity*palatine tonsils – masses of lymphatic tissues on either side of the tongue & help protect the body against infection; they sometimes get infected & have to be surgically removed*pharyngeal tonsils (adenoids) – lymphatic tissue on the posterior wall of the pharynx; if they enlarge they can block the passage between the pharynx & the nasal cavity; can be surgically removed

Tonsil Lies between palatoglossal and

palatopharyngeal arches, on each side

The Tongue Manipulates materials inside mouth May bring foods into oral cavity4 Functions of the Tongue

1. Mechanical processing:– compression, abrasion, and distortion

2. Manipulation:– assists in chewing

3. prepares material for swallowing Sensory analysis:– touch, temperature, and taste receptors

4. Secretion:– mucins– enzyme lingual lipase

Frenulum- thin fold of tissue which serves as the anterior attachment to the floor of the mouth-Prevents extreme movements of the tongue -Connects body of tongue to floor of oral cavity

Primary and Secondary Dentition -primary: 20 teeth starting at 6 months -secondary/adult: between 6 and 12 years = 32 teeth 8 incisors - biting 4 canines (cuspids) - tearing 8 premolars (bicuspids) - grinding 12 molars (tricupids) - grinding ** third pair of molars (wisdom teeth) may not erupt

-impacted

Teeth grinding, tearing and shearing of food

-two main divisions: crown and root-crown: above gumline-innermost layer - pulp (nerves/blood vessels)

-outer covering of specialized calcified connective

tissue - dentin-covered with a layer of enamel

-root: entry of nerves and blood vessels-secures the tooth into the jaw (cementum)-covered by a periodontal membrane - unites with gums

Incisors Blade-shaped teeth Located at front of mouth Used for clipping or cutting Have a single root

Cuspids (Canines) Conical Sharp ridgeline Pointed tip Used for tearing or slashing Have a single root

Bicuspids (Premolars)

Flattened crowns Prominent ridges Used to crush, mash, and grind Have 1 or 2 roots

Molars Very large, flat crowns With prominent ridges Used for crushing and grinding Have 3 or more roots

Salivary Glands

4 Functions of Saliva

Lubricating the mouth

Moistening and lubricating materials in the mouth

Dissolving chemicals that:› stimulate taste buds› provide sensory information Initiate digestion of: › complex carbohydrates by enzyme salivary

amylase (ptyalin or alpha-amylase)› lipids by enzyme lingual lipase

Secretion of Oral Cavity

Salivary Glands Produce 1.0–1.5 liters of saliva each day:

› 70% by submandibular glands› 25% by parotids› 5% by sublingual glands

Saliva• 99.4% water• 0.6% includes:

– electrolytes (Na+, Cl—, and HCO3—)

– buffers– glycoproteins (mucins)– antibodies– enzymes– waste products

*3 pairs of major salivary glands:-parotid, submandibular, & sublingual

glands

Salivary Secretions*salivary glands-include 2 types of secretory cells = serous cells that secrete digestive enzymes & mucous cells that secrete mucus

*amylase – digestive enzyme produced by serous cells (1st step in chemical digestion of carbohydrates)

*mucus – thick liquid produced by mucous cells that providesmoisture for swallowing food

Major Salivary Glands

*3 pairs of major salivary glands:-parotid, submandibular, &

sublingual glands1) parotid glands – largest, secrete saliva rich in amylase; located near the ears2) submandibular glands – produce viscous (thick & hard to flow) saliva; located on floor of the mouth3) sublingual glands – smallest, secrete mucus (thick), located in floor of the mouth

Digestion in the Mouth

Mechanical digestion (mastication or chewing) breaks into pieces mixes with saliva so it forms a bolus

Chemical digestion› salivary amylase

begins starch digestion at pH of 6.5 or 7.0 found in mouth

when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases

› lingual lipase secreted by glands in tongue begins breakdown of triglycerides into fatty acids and

glycerol

Pharynx

Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly)

Skeletal muscle lined by mucous membrane

Deglutition or swallowing is facilitated by saliva and mucus› starts when bolus is pushed into the

oropharynx› sensory nerves send signals to

deglutition center in brainstem

Esophagus•Collapsed muscular tube•In front of vertebrae•Posterior to trachea•Posterior to the heart •Pierces the diaphragm at hiatus

hiatal hernia or diaphragmatic hernia

•A hollow muscular tube•About 25 cm long and 2 cm wide

• Conveys solid food and liquids to the stomach

• Begins posterior to cricoid cartilage

• Is innervated by fibers from the esophageal plexus

Physiology of the Esophagus - SwallowingAlso called deglutitionCan be initiated voluntarilyProceeds automatically

1. Voluntary phase---tongue pushes food to back of oral cavity-stimulates receptors in the oropharynxmessage travels to the deglutition center

2. Involuntary phase----pharyngeal stage

breathing stops & airways are closedsoft palate & uvula are lifted to close off nasopharynxvocal cords closeepiglottis becomes bent over airway (glottis) as larynx is liftedcontrolled by autonomic nervous system

• 3. Esophageal phase (involuntary) - Peristalsis pushes food down– circular fibers behind

bolus contract– longitudinal fibers in

front of bolus also contract to shorten the distance of travel and widens the espophagus

Stomach4 Functions of the Stomach1. Storage of ingested food2. Mechanical breakdown

of ingested food3. Disruption of chemical

bonds in food material:– by acids and enzymes

4. Production of intrinsic factor:– glycoprotein required

for absorption of vitamin B12 in small intestine

The Stomach

Anatomy of the Stomach

• The stomach is shaped like an expanded J:• short lesser curvature forms medial surface• long greater curvature forms lateral surface

• Anterior and posterior surfaces are smoothly rounded

• Shape and size vary:• from individual to individual• from 1 meal to the next• Stomach typically extends between levels of

vertebrae T7 and L3

4 Regions of the Stomach

Cardia Fundus Body Pylorus

Secretion in the Stomach

As food enters in the stomach, it is mixed with stomach secretions to become a semifluid mixture called chyme.

Stomach secretions from gastric glands are:› Mucus-lubricates and protects epithelial cells› HCl-produces a pH of 2.0 in the stomach› Pepsinogen-converted by HCl to active enzyme

pepsin.› Intrinsic factor-binds w/ vit.B12( impt. In RBC

production)› Gastrin- or belly/ stomach; hormone that helps

regulate stomach secretions

Movement in the Stomach Mixing and Emptying Actions

*food entering the stomach stretches the muscles in its wall & internal pressure is unchanged

*the muscular wall of the pyloric region regulates chyme movement into the small intestine

*the rate of emptying depends of the fluidity of the chyme & the type of food present

*the upper part of the small intestine fills & an enterogastric reflexinhibits peristalsis in the stomach

* vomiting results from a reflex that has many stimuli

Segments of the Intestine

Small Intestine The lengthy small intestine extends from the pyloric

sphincter to the large intestine. It receives secretionsfrom the pancreas and liver, completes digestion of thenutrients in chyme, absorbs the products of digestion, and transports the remaining residues to the large intestine. (about 18-20 feet long)

B. Parts of the Small Intestine (3)*duodenum – C-shaped path, shortest & most fixed part*jejunum – mobile & lies free in the peritoneal cavity*ileum – most distal portion of the small intestine*mesentery – peritoneal tissue that suspends the jejunum & ileum to the abdominal wall; has blood vessels, nerves,& lymphatic vessels that supply the intestinal wall*greater omentum – double fold (drape) of peritoneum that helps protect the peritoneal cavity

The Duodenum • The segment of small intestine

closest to stomach• 25 cm (10 in.) long• “Mixing bowl” that receives:

– chyme from stomach (Mixture of secretions and food in the stomach)

– digestive secretions from pancreas and liver

The Jejunum Is the middle segment of small

intestine 2.5 meters (8.2 ft) long Is the location of most:

› chemical digestion› nutrient absorption

The Ileum• The final segment of small intestine• 3.5 meters (11.48 ft) long

Intestinal Villi A series of fingerlike

projections:› in mucosa of small

intestine Covered by simple

columnar epithelium:› covered with

microvilli

The mucosa of small intestine is simple columnar epithelium with 4 major cell types:

Absorptive cells- have microvilli w/c produce digestive enzymes and absorb digested food

Goblet cells- produces protective mucus Glandular cells ( paneth’s cell)- protects

intestinal epithelium from bacteria Endocrene cells- produce regulatory

hormones

The Duodenum Has few plicae Small villi

Functions of the Duodenum • To receive chyme from stomach• To neutralize acids before they can damage the

absorptive surfaces of the small intestineIntestinal Secretions

Watery intestinal juice 1.8 liters per day enter intestinal lumen Moistens chyme Assists in buffering acids Keeps digestive enzymes and products of

digestion in solution

Regulation of Small Intestinal Secretions

*secretion is stimulated by gastric juice, chyme, & reflexesstimulated by distension of the small intestinal wall

Absorption in the Small Intestine* occurs in duodenum and jejunum, others in ileum

*villi absorb monosaccharides, amino acids, fatty acids, & glycerol

*villi also absorb water & electrolytes

*fat molecules with longer chains of carbon atoms enter the lacteals of the villi; fatty acids with relatively short carbon chains enter the blood capillaries of the villi

Movements of the Small Intestine*mixing by segmentation & peristalsis

*over distension or irritation may stimulate a peristaltic rush & bring on diarrhea

*the ileocecal sphincter controls movement of the intestinalcontents from the small intestine into the large intestine

Pancreas

Pancreas

Lies posterior to stomach:› from duodenum toward spleen

Is bound to posterior wall of abdominal cavity Is wrapped in thin, connective-tissue capsule

Regions of Pancreas Head:

› broad› in loop of duodenum

Body: › slender› extends toward spleen

Tail:› short and rounded

Common Bile Duct •From the liver and

gallbladder•Meets pancreatic duct near duodenum

Duodenal Papilla• Chamber that

receives secretions from:– common bile duct– pancreatic duct

• Located halfway along length of duodenum

Acini- dark clusters › 99% of gland› produce pancreatic juice

Islets of Langerhans› 1% of gland› pale staining cells› produce hormones

Functions of the Pancreas1. Endocrine cells:

› of pancreatic islets› secrete insulin and glucagon into bloodstream

2. Exocrine cells:› acinar cells › epithelial cells of duct system

Regulation of Pancreatic Secretion*nervous & endocrine systems regulate the release of pancreatic juice*secretin (a peptide hormone) – from the duodenum stimulates the release of pancreatic juice in response to the acid in chyme*the high bicarbonate concentration in pancreatic juice helps neutralize chyme & causes the intestinal contents to be alkaline*cholescystokinin from the intestinal wall stimulates the release of pancreatic juice that has a high concentration of digestive enzymes & can break down fats & proteins

*trypsinogen, chymotrypsin, & carboxypeptidase – proteolytic enzymesthat digest proteins; they are inactive until they are activated by other enzymes; trypsinogen is activated to trypsin;chymotrypsin & carboxypeptidase are activated by pepsin. -This mechanism prevents enzymatic digestion of proteins within the secreting cells and the pancreatic ducts.

The Liver Is the largest visceral organ (1.5 kg) Lies in right hypochondriac and

epigastric regions Extends to left hypochondriac and

umbilical regions Performs essential metabolic and

synthetic functions

Anatomy of the Liver Is wrapped in tough fibrous capsule Is covered by visceral peritoneum Is divided into lobes

The Falciform Ligament

On anterior surface Divides left lobe and right lobe

Two smaller lobes, the CAUDATE(having a tail) and QUADRATE(square)- can be seen from inferior view.

PORTA(gate)- can be seen from the inferior view through w/c the blood vessels, ducts and nerves enter or exit the liver.

3 Functions of the Liver1. Metabolic regulation2. Hematological regulation3. Bile production

Metabolic RegulationThe liver regulates:

1. composition of circulating blood2. nutrient metabolism3. waste product removal4. nutrient storage 5. drug inactivation

Metabolic Activities of the Liver

Carbohydrate metabolism Lipid metabolism Amino acid metabolism Waste product removal Vitamin storage Mineral storage Drug inactivation

Bile Production

• One quart of bile/day is secreted by the liver– yellow-green in color & pH 7.6 to 8.6

• Components– water & cholesterol– bile salts = Na & K salts of bile acids – bile pigments (bilirubin) from hemoglobin

molecule

BILE- for emulsification (break down of fats)

The Gallbladder Is a pear-shaped, muscular sac Stores and concentrates bile prior to excretion

into small intestine Is located in the fossa on the posterior surface of

the liver’s right lobe

3 regions of Gallbladder Fundus Body Neck

Functions: Stores bile (40-70ml) Releases bile into duodenum:

› only under stimulation of hormone cholecystokinin (CCK)

Intestinal Hormones

Intestinal tract secretes peptide hormones with multiple effects:› in several regions of digestive tract› in accessory glandular organs

Secretin Is released when chyme arrives in

duodenum Increases secretion of bile and buffers by

liver and pancreasCholecystokinin (CCK) • Is secreted in duodenum:

– when chyme contains lipids and partially digested proteins

• Accelerates pancreatic production and secretion of digestive enzymes

Cholecystokinin (CCK) Relaxes hepatopancreatic sphincter and

gallbladder:› ejecting bile and pancreatic juice into duodenum

Gastric Inhibitory Peptide (GIP) • Is secreted when fats and carbohydrates

enter small intestine

Vasoactive Intestinal Peptide (VIP)• Stimulates secretion of intestinal glands• Dilates regional capillaries• Inhibits acid production in stomach

Gastrin Is secreted by G cells in duodenum:

› when exposed to incompletely digested proteins Promotes increased stomach motility Stimulates acids and enzyme production

The Large Intestine

Figure 24–23

Is horseshoe-shaped Extends from end of

ileum to anus Lies inferior to

stomach and liver Frames the small

intestine Also called large

bowel Is about 1.5 meters

long and 7.5 cm wide

Functions of the Large Intestine

Reabsorption of water Compaction of intestinal contents

into feces Absorption of important vitamins

produced by bacteria Storage of fecal material prior to

defecation

3 Parts of the Large Intestine

1. Cecum: › the pouchlike first

portion

2. Colon: › the largest portion

3. Rectum: › the last 15 cm of

digestive tract

The Ileum Attaches to the

medial surface of cecum

Opens into the cecum at the ileocecal valve

The Cecum

Is an expanded pouch

Receives material arriving from the ileum

Stores materials and begins compaction

3 Vitamins Produced in the Large

Intestine 1. Vitamin K:

› a fat-soluble vitamin› required by liver for synthesizing 4 clotting

factors, including prothrombin

2. Biotin:– a water-soluble vitamin – important in glucose metabolism

3. Pantothenic acid:– a water-soluble vitamin – required in manufacture of steroid

hormones and some neurotransmitters

The Appendix

Also called vermiform appendix Is a slender, hollow appendage (about

9 cm long) Is dominated by lymphoid nodules (a

lymphoid organ) Is attached to posteromedial surface of

cecum:› mesoappendix connects appendix to ileum

and cecum

The Rectum Forms last 15 cm of digestive tract Is an expandable organ for temporary

storage of feces Movement of fecal material into rectum

triggers urge to defecate

The Anal Canal• Is the last portion of the rectum• Contains small longitudinal folds called

anal columns The Anus

• Also called anal orifice• Is exit of the anal canal

Metabolism Is the total chemical reaction that occur in

the body. ANABOLISM- energy-requiring process by

which small molecules are joined to form larger ones. Occurs in all cells of the body.

CATABOLISM- energy-releasing process by w/c large molecules are broken down into smaller ones. begins during the process of digestion and is concluded w/in individual cells.- Energy derived from catabolism used to drive

anabolic reactions.

Anaerobic respiration- breakdown of glucose in the absence of oxygen to produce two molecules of Lactic acid and 2 ATP mol.

Aerobic respiration- presence of O2 to produce CO2, H2O, 38 ATP mol.

Cellular Metabolism- chemical rxns that occur w/in the cells

Digestive products: carbohydrates, proteins, lipids

ATP- energy currency of the cell Cellular Respiration- oxygen-using

events. Includes glycolysis, Kreb cycle, electron transport chain

Carbohydrate, fat and protein Metabolism in cells

Monosaccharides are the products Glucose is the most important

Used as source of energy Also known as blood sugarGlycogen- excess of glucose in the blood w/c

is partially broken down into fats.Glycolysis- Series of Chemical rxns that occurs in the fluid part of cytoplasm. The results are two pyruvic acid

Krebs cycle- produces virtually all the CO2 and water that results during cell respiration.› Like glycolysis it yields a small amount of ATP

by transferring high energy phosphate groups directly from phosphorylated subs. to ADP.

Electron transport chain- is where the action is for ATP production. It occurs in mitochondria.

HYPERGLYCEMIA- glucose is stored as glycogen or converted to fat.

HYPOGLYCEMIA- glycogenolysis, gluconeogenesis, and fat breakdown occur to restore normal blood glucose level.

Fat metabolism› Fats insulate the body, protect organs, build

some cell structures (membranes and myelin sheaths), and provide reserve enegy.

› When carbs are in limited supply, more fats are oxidized to produced ATP.

› Excessive fat breakdown causes blood to become acidic (acidosis or ketoacidosis)

oProtein MetabolismoCarefully conserved by body cells. Amino acids

are oxidized to form ATP mainly when other fuel sources are not available.

oAmmonia, released as amino acids are catabolized, is detoxified by liver cells that combine it w/ CO2 to form UREA.

3 major types of foods are oxidized for energy. They yield diff. amounts of energy.

Carbs and proteins: 4kcal/gram Fats: 9kcal/gram.

Basal metabolic rate- total amt. of energy used by the body when one is in basal (resting) state. Age, sex, body surface are, amt of thyroxine produced influence BMR.

Total metabolic rate-number of calories used by the body to accomplish all ongoing daily activities. It increases when muscle activity increases.-when TMR equals total caloric intake, weight remains constant.

Liver- body’s key metabolic organ. Its cells remove nutrients from hepatic portal blood. It performs glycogenesis, glycogenolysis and gluconeogenesis to maintain homeostasis of blood glucose levels.

Hypothalamus- initiates heat-loss processes (radiation of heat from skin and evaporation of sweat) or heat-promoting process (vasoconstriction of skin blood vessels and shivering) as necessary to maintain body temperature w/in normal limits.

Fever( hyperthemia) represents body temp. regulated at higher-than-normal levels.