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P & A Notes Digestive System

February 07, 2011

Chapter 15
Digestive System

- Alimentary Canal: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.

- Accessory organs: salivary glands, liver, gallbladder, and pancreas.
- Walls of the alimentary canal are made of 4 layers:
Innermost layer:
1.) Mucous membrane - protects tissue beneath it and carries an absorption of nutrients and secretion of mucus and digestive enzymes.

2.) Submucosa - contains much connective tissue, also contains blood vessels, lymphatic vessels, and nerves. Blood nourishes surrounding tissue and carries away absorbed material.

3.) Muscular layer - 2 coats of smooth muscle. Inner layer encircle the tube (decrease diameter) Outer layer runs length wise (shortens tube) This layer is responsible for peristalsis.

4.) Serous layer - Outer covering of the tube. It composes the visceral peritoneum. It also releases serous fluid, keeping the outer surface moist and lubricated.

Movements of tube:

1.) Mixing - In stomach
Waves of contractions from one end to other mix contents.

2.) Peristalsis -


Mouth: Consists of oral cavity and vestibule. Lips and cheeks used to position food in mouth for chewing.
Tongue is hooked to the floor of the mouth by a fold of skin called the frenulum. Used to mix food with saliva and position it for chewing. Papillae are rough projections that help in handling the food. Are also responsible for taste. (Taste buds) Back of tongue is held down by the hyoid bone. It is covered by lymphatic tissue called lingual tonsils. Hard palate and soft palate form roof of mouth.

In the center of the soft palate hangs the uvula. When we swallow the uvula and S.P. swings up and blocks off the nasal cavity so food doesn’t squirt out your nose. In the back of the mouth, just back of the soft palate is the palatine tonsils. They help in removing germs and bacteria from our system. They themselves can become inflamed nd infected ---- tonsillitis.
Pharyngeal tonsils or adenoids - another mass of lymphatic tissue, located at the back of the nasal cavity.

Teeth - Through development we have two sets. 1st set we get between 6 months and 2 1/2 years. Primary set or deciduous teeth. (20 teeth)’
At about age 6 the roots of the deciduous teeth begin to resorb. These teeth will be pushed out by the developing secondary or permanent set. Permanent set consists of 32 teeth, 16/jaw. This isn’t complete until the 3rd molars appear between 17-25 years of age.

Function of teeth - The more pieces food is broken into the more surface area the digestive enzymes can act on and the better the digestion.
Chewing is physical breakdown of food.

Tooth: composed of an exposed crown and a root that anchor the tooth to the jaw bone.

*Crown - covered with a white enamel ----> hardest
substance in body.

* Dentine is under the enamel. Is similar to bone, but
a little harder. The dentine surrounds the pulp cavity
which contains blood vessels, nerves and
connective tissue.
* Root canal - where nerves and B.V. enter tooth at
* Cementum - on outside of tooth's root - A bonelike
* Periodontal ligament - Firmly attaches the
cementum to the bone of the jaw.

Salivary glands:
* secrete saliva. saliva is made of 2 types of secretions.
1) serous cells in the glands produce a watery fluid containing the enzyme amylase ----> splits starch into a disaccharide. (chemical digestion)

2) Mucous cells secrete a thick stringy fluid --- mucus: helps lubricate and hold the food together when swallowing. there are three large salivary glands.

1) Parotid glands
* largest of the three
* in front and some what below the ear. Lies just over the top of the masseter.

* Is the gland that becomes infected with the mumps virus.

2.) sublingual
* smallest of three
* On the floor of mouth under tongue.
* Cells are mainly mucous cells, where other two types are primarily serous cells.

3.) Submandibular glands
* located on floor of mouth just inside of the jaw bone

Pharynx - cavity that connects the oral cavity, nasal cavity, larynx and esophagus.

Esophagus - tube that goes from pharynx to stomach.

Act of Swallowing:
* The tongue presses against the hard palate,
forcing food to the back of the oral cavity.
* Uvula and soft palate are raised preventing food
from entering the nasal cavity.
* The hyoid bone and the larynx are elevated,
causing the epiglottis to cover the glottis. This
prevents food from entering the trachea.
* Muscles in the lower part of the pharynx relax and
the esophagus will open.
* Peristalsis begins in the pharynx and pushes the
food down the length of the esophagus.
* When food reaches the lower end of the
esophagus the cardiac sphincter will open allowing
the food to enter the stomach.
* The cardiac sphincter will close preventing food
from going back into the esophagus.

Esophagus - collapsible tube, about 10 in. long. Lies just behind the trachea. It passes through the diaphragm and opens into stomach.

Cardiac Sphincter - heavy band of muscle at end of esophagus.

Stomach - located in upper left portion of abdominal cavity. Functions mainly in digestion, very little in absorption.
Divided into 4 portions
* Cardiac region (below esophagus)
* Fundic region (arches above cardiac)
* Body region (main portion)
* Pyloric region (lower portion)
Pyloric sphincter is located at the very bottom of this region. It holds food in stomach until it is ready to be released - then only a small amount at a time. Stomach has a thick mucous membrane. This membrane has gastric glands that contain three types of cells: mucous cells, chief cells and parietal cells.

* Mucous cells - Produces mucus and a thick alkaline secretion. Mucus forms a protective lining and the alkaline secretion neutralizes the acid next to the wall, stopping the digestive action of pepsin. (prevents ulcers)
* Chief cells - produce digestive enzymes, with pepsin being the most important.
* Parietal cells - produce hydrochloric acid.

The products of all three cells combine to produce gastric juice.
* Pepsinogen - quickly changed into pepsin when mixed with HCI. Pepsin break proteins down.
The semi fluid gob of food and gastric juice in the stomach is called chyme.
The pyloric sphincter allows just a small amount of chyme to enter the small intestine at one time.
Speed the stomach empties depends on several things.
- liquids move through rapidly
- fatty foods remain in the stomach for 3-6 hours
- foods high in proteins 2-3 hours
- foods high in carbohydrates 1-1 1/2 hours

When chyme enters the duodenum the small intestine and its accessory organs add their secretions. (pancreas, liver, gallbladder)
Pancreas: is attached to the duodenum by the
pancreatic duct.

Pancreatic juice contains several enzymes.
- Amylase: splits starch ---> disaccharide
- Lipase: converts emulsified fats into fatty acids and glycerol.
- The Proteolytic enzymes:
1.) trypsin
2.) chymotrypsin
3.) carboxypeptidase
Breaks proteins, and partially digested proteins down to peptides
* As chyme enters the duodenum a hormone is released by the S.I. into the blood. It stimulates the goblet cells of the duodenum to produce mucus. It also stimulates the pancreas to release its digestive enzymes. Pancreatic juice has high numbers of carbonate ions which neutralize the acid in chyme. If the chyme has protein and fat in it the duodenum will release the hormone cholecystokinin, causing the pancreas to release high concentrations of digestive enzymes.

Liver: upper right of abdominal cavity.
* Largest gland in the body.
* Liver has many functions
* Function of bile secretion is the one that is important to digestion.
* Liver is divided into a large right lobe and a smaller left lobe. Each lobe is separated into many tiny hepatic lobules (functional units)
* In the hepatic lobules are many bile canaliculi which receives secretions from the hepatic cells. These canals unite and form larger ducts -----> unite and become the hepatic ducts.
* Gallbladder is on the underside of the liver, it stores bile that the liver produces between meals. When stimulated by cholecystokinin from the S.I. it releases its bile. It leaves the gallbladder by way of the cystic duct. Cystic duct joins the hepatic ducts and forms the common bile duct -----> dumps into the duodenum.
* the CBD has a sphincter muscle at the point where it dumps into the S.I. (hepatopancreatic sphincter)
* The sphincter usually remains contracted, so when the liver produces bile it will back up the ducts and fill the gallbladder.

Bile: yellowish-green
contains water, bile salts, bile pigments, cholesterol,
and various electrolytes.

Bile Salts: makeup largest percent of bile and is the only substance with digestive function. Doesn’t act as a enzyme, more like “dish soap” by emulsification of fat. It breaks the fat globules up in smaller droplets -----> more surface area for fat-splitting enzymes to act on.
Bile salts aid in absorption of fatty acids, cholesterol and the fat soluble vitamins - A, D, E, and K.

Small Intestine: Made up of the duodenum, jejunum, and ileum.
* Secretions - mucus and digestive enzymes that
split sugar, proteins, fat and nucleic acids into basic
building blocks.
* Wall is lined with villi which increase surface area
-----> absorption.
* Duodenum and jejunum are the major sites of
nutrient absorption.
* Ileocecal valve contracts movement of material
from S.I. -----> L.I.

Large Intestine:
Function - absorption of water and electrolytes. Form and store feces until defecation. L.I. consists of: cecum, colon, rectum, and anal canal.

- Cecum - beginning of L.I. at lower end has a close ended tube - vermiform appendix
(no digestive function, contains lymphatic tissue)
- Colon: divided into four portions.
1.) Ascending colon
2.) Transverse colon
3.) Descending colon
4.) Sigmoid colon
- Rectum -----> and anal canal -----> anus
Anus is guarded by 2 sphincter muscles - internal and external anal sphincters.

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