Chapter 16

The Digestive SystemMechanical and chemical breakdown of food

to molecules small enough to be transportable and fit into cells

Mechanical – breaks large chunks into small chunks – mouth and stomach

Chemical – breaks little chunks into small molecules – stomach and small intestine

Includes organs through which food passes and accessory organs (salivary glands, liver, gall bladder, pancreas

digestive animation

Layers of Wall of Digestive System

Beginning to end it is between 20 and 30 feet long

Wall/lining is continuous and unbroken

Four layers – from deep to superficialMucosa – surface epithelium, goblet cells,

connective tissue, little amount of smooth muscle; may be folded into villi and microvilli, may secrete enzymes function – absorption and secretion

Submucosa – areolar (loose) connective tissue, blood vessels, glands, lymphatic vessels and nerves

function – delivery of blood to carry away nutrients

Muscularis – usually two layers of smooth muscle, two layers of muscle fibers – circular and longitudinal fibers

function - responsible for movement of

food through tube,

Serosa – outermost layer, epithelial tissue outside above connective tissue

function – secretes serous fluid to reduce friction when organs in abdominal cavity rub against each other

Four layers are continuous along entire lengthCavity in the interior of the layers is the

lumenAlimentary canal wall contains two important

nerve plexuses – submucosal and mycentirc

Networks are part of the autonomic nervous system and help regulate digestive system activity subconsciously

Movements are mixing and propelling

The MouthOrgan of mastication where food is mixed

with saliva to form a bolusInterior of mouth is oral cavity while space

between teeth and cheeks is vestibule

Lining is stratified squamous epitheliumUnder lining is muscle, fat and outer layer of

skin

The lingual frenulum secures the tongue to the floor of the mouth

The LipsThin stratified squamous epithelium, no fat, Impressive muscularity and highly

concentrated neurons and blood vessels close to the surface

Lip-skin border is where the mucous membranes of the alimentary canal start

Necessary for:Airtight seal while suckling as babyFor making some of the letter sounds

The TongueThick, mostly muscle

Has several body attachments – to the hyoid bone and the styloid process of the skull

Muscle fibers run in a number of directions which allows for flexibility

Lingual tonsils lie atop the posterior portions of the tongue

The PalateRoof of the oral cavity – hard is anterior and

soft is posteriorHard palate – palatine processes of maxillary

boneSoft palate – muscular arch that ends in uvula

Palatine tonsils –back of the mouth on either side of soft palate – lymphatic tissue

Pharyngeal tonsils/adenoids are third pair of tonsil, above and beyond the border of the soft palate

The TeethHarder than bone and made of enamelTwo sets form during development – deciduous (20) teeth and

permanent teeth (32)Deciduous erupt through gingiva at about 6 months until 2-4

years Permanent teeth have deeper roots and erupt from age 6

through adolescence with the exception of wisdom teeth/third molars

Deciduous TeethUsually lower front incisor erupts first @ 6 months20 in totalFall out in the same order in which they appearedRoots are dissolved by odontoblasts, and teeth pushed

out of socket by pressure from permanent teeth

Permanent TeethAppear around age 6 with third molars last around 17 –

25Incisors – chisel shaped used to biteCanines – pointed fangs for ripping and tearingPremolars (bicuspids) and Molars – for grinding

The ToothEach tooth has a crown (above the gum) and a root

(below the gum) that meet at the neck (gumline)Covered by white shiny enamel that is principally

calcium and once lost cannot be replaced

Beneath enamel is dentin – still harder than boneDentin surrounds the pulp cavity which contains

blood vessels, nerves, softer connective tissue

Pulp cavity becomes root canalRoots are anchored to mandible and maxillary

bones by cementum and tough periodontal ligament that contains bundles of collagenous fibers

Salivary GlandsProduce salivaFunctions of saliva

Moisten food so easier to swallowDissolve food and help separate tastants to

allow taste buds to detect tasteBegins chemical digestion of carbohydrates

using amylaseContains bicarbonate ions to help neutralize

bacterial acids that cause tooth decayMaintains pH of mouth at 6.5 – 7.5

Produced in 3 main salivary glands that are scattered throughout walls of mouth and tongue

Major Salivary GlandsParotid Glands – largest, underneath cheek

and over masseter, secretes clear watery form of saliva rich in amylase – lymphoid gland

Submandibular (Submaxillary) – on floor of the mouth on inside surface of jaw, most cells are serous few are mucosal, saliva more viscous than parotid, ducts open under the tongue

Sublingual – smallest of three, located under the tongue, cells are mucosal and secretions are thick and stringy

The PharynxConnects the mouth to the esophagusMuscular wall to assist in swallowingNo digestive action since food is not present

there long enough

Pharyngeal muscles are skeletal arranged in two layers – inner circular and outer longitudinal

Divided into three regions

When swallowing breathing is interrupted. Swallowing raises the pharynx, pushing the epiglottis up against the trachea, sealing it so food won’t go down the wrong tube – when it does we cough to force food out

3 Regions of the PharynxNasopharynx – located above soft palate,

communicates with nasal cavity, passageway for breathing – eustachian and auditory tubes open into here

Oropharynx – behind the mouth, where postnasal drip occurs

Laryngopharynx – below oropharynx, passageway for food only, connects to esophagus

The Esophagus9 – 14 inches longWalls are collapsible so the stomach contents are

not “detected”

Descends behind trachea and passes through the mediastinum down to opening in diaphragm called esophageal hiatus and then joins with stomach

Mucous glands found in mucosa and submucosaAt base is lower esophageal sphincter (cardiac

sphincter) that is usually closed and opens to allow food entry

Peristalsis - action of contractile smooth muscle waves that pass through the esophagus and rest of alimentary canal

The StomachShaped like the letter J, under hangs the

diaphragm on the left sideAverage capacity is 1 liter

Inner surface lined with rugae that disappear when the stomach holds a large amount of food

Receives food through the gastroesophageal sphincter, mixes it with gastric juice, kills bacteria in food, achieves a small amount of absorption, mixes all components of your meal making them into a pasty substance called chyme and passes them to the duodenum in the small intestine

Parts of the StomachFrom the exterior the stomach is divided into

four regionsCardiac region – at the top of the stomach just

past the gastroesophageal sphincter

Fundus – balloon like portion that overlies the cardiac region, usually filled with air

Body region- part of the stomach that holds chyme

Pyloric region – tapers down to pyloric sphincterAt end of stomach pyloric sphincter prevents

food that passes into the small intestine from backing up into the stomach

Interior of the StomachStomach has a super thick mucosaInner surface of the stomach is dotted with

gastric pits that are openings to the deeper gastric glands

Gastric glands contain three types of secretory cellsGoblet cells – for mucus secretion

Chief cells – secrete the protein-digesting enzymes of the stomach

Parietal cells – secrete hydrochloric acid

Digestive Enzymes of StomachPepsin – most important of the stomach’s

digestive enzymes, begins protein digestion, pepsin will not digest mucinSecreted as inactive precursor enzyme

pepsinogen and denatured into pepsin when comes in contact with hydrochloric acid

Gastric lipase – found in minute quantities, acts on fats like butterfat

Intrinsic factor – not an enzyme, but a gastric juice component that enables vitamin B12 absorption by the small intestine

Control of Gastric JuiceHormone gastrin stimulates the gastric glands

to secrete gastric juicesGastrin is produced under parasympathetic

control when food enters the stomach or when one’s thoughts turns to food

Parasympathetic impulses to the stomach are carried by the vagus nerve

Absorption by the Stomach

Stomach wall is poorly adapted for food absorption, it has too much mucus

The materials that WILL pass through the stomach wall include water, glucose, certain salts, alcohol and lipid based drugs of several types

Contraction of Stomach WallThe stomach has three layers, peristalsis is

non-directional and results in the mixing of food rather than the forward movement of food

Chyme (food mixed with gastric juice) enters the small intestine one little squirt at a time so that it may be neutralized by duodenal secretions of bicarbonate

Solid fat stays in the stomach the longest (3-6 hours), followed by proteins, carbs and liquids which remain in the stomach for only a short period of time

http://www.youtube.com/watch?v=08VyJOEcDos

Reverse Peristalsis2nd way stomach may enter – vomitingVomiting center is located in the medulla oblongataImminent vomiting is signaled by taking deep

breaths and excessive salivation accompanied by pallor and feelings of nausea

Both the diaphragm and abdominal muscles contract during the act to increase the force of repulsion

The vomiting center is stimulated by overstretching of receptors in the stomach wall, some drugs (emetics), bacterial toxins in food, and sometimes rapid changes in body’s direction

Small IntestineChyme is squirted in small quantities into the

duodenum (1st foot or so) of the small intestine

Chyme must be liquefied to pass through the pyloric sphincter

As food enters the duodenum, it is mixed with fluids from the pancreas, liver and gall bladder

Pancreatic duct connects with the duodenum at the same place where the common bile duct from the liver and gall bladder enters the duodenum; A ring of smooth muscle – sphincter of Oddi - surrounds the opening

Structure of Small IntestineSmall intestine fills most of the abdominal

cavity At least14 feet long in a living personWhere chemical digestion is completed, most

food materials are absorbed and indigestible residues are passed on to the large intestine

Has three regions – Duodenum, Jejunum, Ilieum

Jejunum and ileum are suspended by a double layered but think membrane called mesentary

Mesentary contains blood vessels, nerves, lymphatic vessels and lymph nodes

Membrane called greater omentum covers the stomach and small intestine like an apron, this is where fat is deposited

Inside the Small IntestineInner wall covered with uncountable tiny

projections called intestinal villi that project inward toward the intestinal lumen

Each villus has a lining of simple columnar epithelium interspersed with goblet cells

Within the center of each villus are blood capillaries, a single lymph capillary called lacteal and nerve fibers

Also contains mucus secreting glands called Brunner’s glands that are buried in the submucosa

Gland SecretionsIntestinal glands at the base of each villus

secrete large amounts of watery fluid – neutral pH with no enzymes

Enzyme secretion occurs from epithelial cells on the surface of the microvilliPeptidases – split polypeptides into 20 different

amino acidsSucrase, maltase and lactase – split

disaccharides into monosaccharides

Intestinal lipase – splits fats into fatty acids and glycerol

Nuclease – splits DNA and RNA into nucleotides

AbsorptionEnd products of digestion are small enough to

fit through the mucosa of the small intestine and the walls of the blood capillaries and lacteals

End products include – monosaccharides, amino acids, fatty acids and nucleotides

Small intestine has an internal absorptive surface area the size of a tennis court

Occurs by active or passive transport and also involves water and electrolytes

Products AbsorbedWater – absorbed by osmosis, more water absorbed by

small intestine than large (2L a day vs 1L per day)Electrolytes – active transportMonosaccharide – transport by facilitated diffusion or active

transport

Amino acids are absorbed by active transport through the intestinal lining. Once across the lining, the monomers move passively into the bloodstream.

The end products of fat digestion are typically fatty acids, glycerol, phospholipids, and cholesterol molecules. These molecules readily diffuse into the epithelial walls of the small intestine BECAUSE THEY ARE FAT SOLUBLE. Here, they are resynthesized into fat droplets (called chlyomicrons) that diffuse into the lacteals. Lymph in the lacteals carries these droplets to the bloodstream.

The major mixing movement in the small intestine is termed segmentation. Thechyme is cut into segments by sphincter-like contractions in the walls, and the food in each segment is "swished" back and forth so that digestion and absorption may occur.

Peristalsis in the small intestine is weaker and slower than in the esophagus or stomach.

A sphincter muscle called the ileocecal valve separates chyme in the small intestine from the large intestine.

http://www.youtube.com/watch?v=P1sDOJM65Bc

The large intestine as a diameter (not a length) greater than the small intestine. And thus, its name.

Its length can reach 1 1/2 meters (5 feet).

It begins on the lower right side of the abdominal cavity- here, the ileum and the cecum unite. Then, the L.I. rises, transverses the abdominal cavity over the small intestine, and plunges rapidly on the body's left side.Cecum

The cecum is a dilated, pouch-like structure that hangs beneath the ileocecal junction. Hanging underneath it is the appendix.

The appendix contains lymphatic tissue. If it becomes inflamed or infected, it might require removal. If it enlarges and breaks open, the peritoneum becomes infected (peritonitis), a potentially fatal condition.

The four sections of the L.I. are the ascending colon, the transverse colon, the descending colon, and the S-shaped sigmoid colon.

At the end of the sigmoid colon is the rectum, which lies against the sacrum, to which it is attached.

The last 1 1/2-2 inches of the L.I. is the anal canal. Two sphincter muscles, the internal and external anal sphincters, regulate the canal.

Structure of the Wall of the Large IntestineThe wall of the large intestine includes the 4 tissue layers

that typify the GI tract.There are no villi in the large intestine.The wall is arranged in a series of pouches called haustra.

Here, fat is deposited in the serosa of the wall (this fat is not present in the small intestine serosa).

Functions of the Large Intestine

Very limited or no chemical digestive function.There is an overabundance of mucous-producing

goblet cells line the mucosa. Mucus helps hold the fecal mass together and helps maintain a neutral pH (to combat bacterial acids).

Water and electrolytes are reabsorbed in the first half of the L.I. 90% of the water that enters the L.I. is reabsorbed.

Intestinal Bacteria Intestinal flora - the bacteria that occupy the L.I. - chiefly

E. coli.These bacteria synthesize vitamin K, B12, thiamine, and

riboflavin. These vitamins are absorbed by the intestinal mucosa.

Vitamin K – helps modify proteins after they are synthesized on ribosomes.

Vitamin B12 – aids the nervous system and helps in blood formation.

Thiamine – necessary for neural function and carbohydrate metabolism.

Riboflavin – plays a key role in energy metabolism.

Food materials that humans are incapable of digesting end up in the fecal mass:

•Cellulose - because we lack the enzyme necessary to

digest it! •Seed coats and covers •Bone, cartilage, and tendon •Certain lipids •Mucin •Capsaisins

The color of the fecal mass relates to the bile pigments from dead red blood cells. The bile pigments (shades of red, brown, and green) enter the food tube in the duodenum and are altered by intestinal bacteria. The color of the fecal mass does NOT relate to the items upon the dinner plate.

The fecal mass is about 75% water.

The PancreasSynthesizing digestive enzymes is the

digestive system function of the pancreas.Enzyme synthesis is an exocrine function (to

contrast it with the endocrine function of the pancreas )

The "head" of the pancreas is tucked in the duodenal loop and the tail bumps up against the spleen.

The cells that synthesize the digestive pancreatic fluids are called acinar cells.

Acinar cells secrete their enzymes into small tubes, which unite to form larger tubes and ultimately, form the pancreatic duct.

The pancreatic duct unites with the common bile duct to form a short tube called the hepatopancreatic ampulla.

It is here that digestive enzymes from the pancreas, mixed with bile from the liver, enters the small intestine to aid in food digestion.

Pancreatic JuicePancreatic juice contains several digestive

enzymes that digest all 4 organic molecule groups (carbohydrates, lipids, proteins, & nucleic acids.)

Here are the most important Pancreatic Enzymes:

Pancreatic amylase Splits starch or glycogen into disaccharides.

Pancreatic lipase Splits triglycerides into fatty acids and glycerol.

Trypsin, Chymotrypsin, and Carboxypeptidase Reduce proteins to amino acids.

Pancreatic nucleases Split nucleic acids (DNA, RNA, ATP) into nucleotides.

Pancreatic juice also contains bicarbonate to neutralize the harsh acids arriving in the duodenum from the stomach.

The hormone secretin is released into the bloodstream from the intestinal mucosa upon the arrival of chyme in the duodenum. Secretin stimulates the pancreas, which secretes pancreatic juice.

The LiverThe liver is on the body's right side,

underneath the diaphragm, protected by the lower ribs. It is reddish brown in color, highly vascular.

Liver FunctionsThe liver has more functions than any other

organ in the human body!The liver is the chief chemical converter

in the human body.

It also serves as a site of destruction, astorage facility, and a protein

metabolizer.It is a good organ to eat because it is where

iron is recycled.

Functions of the Liver It helps maintain levels of blood sugar by storing excess

carbohydrates as glycogen.Lipids are metabolized in the liver and lipids are converted

to phospholipids, lipoproteins, and cholesterol.Carbohydrates and proteins are converted to fat in the liver.

Amino acid deamination occurs in the liver. This means that amino acids are stripped of their NH3 group so that they are structurally similar to carbohydrates. In this form, they may be used for energy. The leftover NH3 molecule is converted to urea and becomes a component of urine.

Blood proteins (clotting factors) are produced in the liver

The liver stores glycogen, iron, vitamins A, D, and B12.

Liver cells help destroy damaged red blood cells.

Liver cells phagocytize foreign antigens.

The liver removes toxic substances (such as alcohol) from the blood.

The liver stores 200-400 mL of blood.The liver secretes bile for fat emulsification during

digestion.

The Structure of the LiverThe liver is surrounded by a fibrous capsule.The human liver has 2 major lobes and two

minor lobes (your cat will have 5!)

Major lobes: Left lobe and Right lobe•Minor lobes: Quadrate lobe (near the gall

bladder) and Caudate lobe (near the vena cava)•The falciform ligament separates the two

lobes.

Each lobe is divided into many hepatic lobules.

•Within the lobule there are many hepatic cells that radiate outward from a central hepatic vein. Cells are separated by hepatic sinuses.

Also within the lobules are bile canals, which receive secretions from the liver cells. The bile canals unite to form bile ducts.

Blood is carried to the liver through the hepatic portal vein (from the stomach and small intestine) and the hepatic artery (which carries oxygenated blood from the aorta).

The Composition of BileBile is the yellowish, green fluid produced by hepatic

(liver) cells.Bile is the only secretion of the liver. It is a greenish-

yellow fluid that exits the liver through the hepatic duct and goes to either the small intestine (for use) or to the gall bladder (for storage). In either instance, its fate is to end up in the small intestine.

The Composition of Bile: water, bile salts (most common bile solid), bile pigments, cholesterol, electrolytes.

The bile salts perform the digestive function of emulsification - reducing larger droplets of fat into tiny droplets that digestive enzymes (lipases) can act upon.

The bile pigments (bilirubin and biliverdin) are the breakdown products of hemoglobin. Bilirubin is reddish, biliverdin is green. You see traces of these colors in a bruise, as RBCs are broken down by phagocytes in the same way as they are in the spleen and liver.

The yellowish skin of jaundice results from the deposition of bile pigments.

•The liver secretes NO DIGESTIVE ENZYMES.

The Gall BladderThe gall bladder is a pear-shaped sac located

in a depression on the inferior surface of the liver.

The liver sends bile to the gall bladder through the cystic duct, which unites with the hepatic duct to send bile to

the duodenum through the common bile duct.

It is lined with columnar epithelial cells.Its wall is lined with smooth muscle.The gall bladder stores and concentrates bile

between meals. When a meal arrives at the small intestine from the stomach, the gall bladder is filled with bile!

The gall bladder releases bile when stimulated by cholecystokinin from the small intestine.

Gallstones may form in the duct system or within the gall bladder if bile is too concentrated!