Chapter 15

The Gastrointestinal System

Gastrointestinal System (GI)

• Takes in (ingests) raw materials• Breaks them down (digests) both physically

and chemically to usable elements• Absorbs those elements• Eliminates what isn’t usable

GI System

• The processes of the GI incorporate several main and accessory organs and substances

Something to think about

• The food that enters your mouth, travels through you digestive system, and is eventually eliminated is never once inside your body

• It remains in a tube like “highway” with certain materials exiting the ramp at different locations in the body.

Tube Like Highway

• The tube begins at the mouth and ends at the anus

• In between the mouth and anus are:» The Pharynx» Esophagus» Stomach» Small and large intestine

• In addition, there are accessory organs

Accessory Organs

• Accessory Organs include:» Teeth» Salivary glands» Liver» Pancreas» Fallbladder

• ALL ARE NECESSARY FOR PROCESSING MATERIALS INTO USABLE SUBSTANCES.

GI overview from 4/27/10

• The digestive tract, often called the alimentary tract or canal, is a muscular tube that contains the organs of digestion.

• The tube begins with the mouth and ends at the anus.

• In between these two points are the pharynx, esophagus, stomach, small intestines, and large intestines.

• Accessory organs of digestion include the teeth, salivary glands, liver, pancreas, and gallbladder.

Functions of the Gastrointestinal Tract

• Ingestion: food enters the mouth• Mastication (chewing): mechanically grinding

food with the teeth and tongue – beginning the process of physically breaking it down

• Digestion: the chemical act of breaking down food into small molecules

Mouth and Oral Cavity

• Your mouth leads to the buccal (oral) cavity.• Your lips act as the door to this cavity.• The hard and soft palate create the roof.• The tongue acts as the floor.• The cheeks form the walls.

Mouth and Oral Cavity (cont’d)

• The tongue’s base (area of attachment) and the uvula are the barrier to the next part of the system, the pharynx.

• The uvula aids in swallowing, directing food toward the pharynx and blocking food from entering your nose.

• The mouth receives, tastes, mechanically breaks down, and begins the process of chemical breakdown of food, adding saliva.

Tongue

• Your tongue is a muscle that provides taste stimuli to your brain, determines temperature, manipulates food, and aids in swallowing.

• As the tongue moves food around in the oral cavity, saliva is added to moisten and soften it, while teeth crush the food.

Tongue

• The tongue pushes the food into a ball-like mass, called a bolus, so it may be swallowed - passed to the pharynx.

• The lingual frenulum, a membrane under the tongue, keeps you from swallowing your tongue and aids in speaking.

Clinical Application: Sublingual Medication

• The area under the tongue has many blood vessels. This sublingual blood vessel network readily absorbs substances and is a rapid means of administering medication.

• One medication given by this route is nitroglycerine, used to treat angina. Angina develops as a result of poor oxygen supply to the myocardium because of diminished blood flow.

Clinical Application: Sublingual Medication (cont’d)

• Nitroglycerine dilates arteries, improving blood supply and oxygenation – hopefully relieving angina symptoms.

Salivary Glands

• There are 2 pairs of salivary glands controlled by the autonomic nervous system.

• A large parotid salivary gland is found slightly inferior and anterior to each ear. These are the ones that swell when you get mumps.

• The ducts from these glands empty into the upper portion of the oral cavity.

Salivary Glands

• The sublingual salivary glands are found under the tongue.

• The submandibular salivary glands are located along both sides of the inner surface of the mandible, or lower jaw.

Saliva

• The salivary glands produce 1–1.5 liters of saliva daily.

• Small amounts of saliva keep the mouth moist, but the idea or presence of food increase production significantly.

• Saliva is 99.4% water, and contains antibodies, buffers, ions, waste products, and enzymes.

Saliva (cont’d)

• Enzymes act as organic catalysts to speed up chemical reactions.

• One enzyme, salivary amylase, speeds the chemical activity of breaking down carbohydrates.

• After eating, saliva cleans the oral surfaces, reducing the amount of bacteria that grows in your mouth.

Teeth

• The first set of teeth you grow as a baby are the deciduous teeth, falling out in time.

• The 1st tooth appears around 6 months of age. The lower central incisors appear first, with all 20 teeth in place by age 2½.

• Between 6 and 12 years these teeth fall out and are replaced by 32 permanent teeth.

• Wisdom teeth appear by the time we turn 21.

Types of Teeth

• Incisors are located at the front of the mouth, are blade shaped, and are used to cut food.

• Canine teeth are for holding, tearing, or slashing food. They are also known as eyeteeth or cuspids, and are located next to incisors.

• Bicuspids, or premolars, are transitional teeth.• Molars have flattened tops. Both bicuspid and

molars are responsible for crushing and grinding food.

Tooth Structure

• Teeth have a crown, neck, and root.• The crown is the part you normally see and is

covered by the hardest biologically manufactured substance, enamel.

• The neck is the transitional section that leads to the root.

• Most teeth are made up of dentin, a mineralized bone-like substance.

Tooth Structure (cont’d)

• The next layer is connective tissue, pulp, located in the pulp cavity.

• The pulp cavity contains blood vessels and nerves providing nutrients and sensation. The nerves and blood vessels get to the pulp cavity via the root canal.

The Root

• The root is nestled in a bony socket and is held in place by fibers of the periodontal ligament.

• In addition, cementum covers the dentin of the root, aiding in securing the periodontal ligament.

• Cementum is a soft version of bone.• Healthy gums, or gingiva, help hold the teeth in

place.• Epithelial cells form a tight seal around the tooth

to prevent bacteria from coming into contact with the tooth’s cementum.

Pharynx

• There are 3 parts to the pharynx:– The nasopharynx is primarily part of the respiratory

system, blocked by the soft palate.– The oropharynx and the laryngopharynx act as a

passageway for food, water, and air. The epiglottis covers the trachea to prevent food from entering the lungs, forcing food into the opening for the esophagus.

Esophagus

• The esophagus is approximately 10 inches long and is connected to the stomach.

• It extends from the pharynx, through the thoracic cavity, through the diaphragm, connecting to the stomach in the peritoneal cavity.

• The esophagus is normally a collapsed tube until a bolus of food is swallowed.

Esophagus (cont’d)

• Rhythmic contractions, called peristalsis, pushes food down the esophagus.

• The esophageal walls are lined with stratified squamous epithelium that secrete mucus to make the walls slippery. These cells make the lining resistant to abrasion, temperature extremes, and irritation.

Esophageal Sphincters

• A muscular ring at the top of the esophagus, called the pharyngoesophageal sphincter, relaxes to open the esophagus so food can enter.

• At the entrance to the stomach is the lower esophageal sphincter, or cardiac sphincter, opening the door to the stomach and closing to prevent acidic gastric juices from splashing into the esophagus – causing heartburn.

• The whole process of swallowing food takes about 9 seconds. Fluid takes less time.

GERD

• See Video

Walls of the Alimentary Canal

• Four basic types of tissue line the entire alimentary canal from the esophagus onward.

• The innermost layer, the mucosa, lines the lumen of the canal.

• This layer is composed mostly of surface epithelium with some connective tissue and has a thin smooth muscle layer surrounding it.

• The mucosa also possesses cells that secrete digestive enzymes to break down foodstuffs and goblet cells that secrete mucus for lubrication.

Walls of the Alimentary Canal (cont’d)

• The submucosa is the next layer, and is composed of soft connective tissue.

• This layer contains:– Blood vessels– Lymph vessels– Lymph nodes (called Peyer patches; are similar to your

tonsils)– Nerve endings

Walls of the Alimentary Canal (cont’d)

• The next layer is called the muscularis externa, and is composed of two layers of smooth muscle.

• The innermost layer of muscle encircles the canal, while the outer layer of muscle is longitudinal in nature, so it lies in the direction of the canal.

• The stomach is surrounded by a third layer of oblique smooth muscle.

Walls of the Alimentary Canal (cont’d)

• The outermost layer, the serosa, is composed of a single, thin layer of flat, serous fluid producing cells supported by connective tissue.

• The serosa is called the visceral peritoneum in most of the canal.

Walls of the Alimentary Canal (cont’d)

• The fluid secreted is important to keep the outer surface of the intestine moist and allow friction free movement of the intestine against the abdominopelvic cavity.

• The esophagus differs in that it only possesses a loose layer of connective tissue called the adventitia.

Stomach

• The stomach is located in the left side of the abdominal cavity, under the diaphragm, and is covered completely by the liver.

• It is approximately 10 inches long with a diameter that depends on how much you just ate.

• It can hold up to 4 liters when filled.• Rugae, or folds, help the stomach expand and

contract.

Stomach Functions

• The stomach has 4 functions:– Temporary holding area for received food– Secretes gastric acids and enzymes that mix with

food, performing chemical digestion– Regulates the rate the new, partially digested food

(a thick, heavy, cream-like liquid called chyme) enters the intestine

– Absorbs small amounts of water and substances on a very limited basis (the stomach does absorb alcohol)

Movement of Foodin the Stomach

• It takes about 4 hours for the stomach to empty following a meal.

• Liquids pass through fairly quickly.• Carbohydras move through quickly.• Proteins take more time to pass through.• Fats take the longest, usually between 4–6

hours.

Regions of the Stomach

• The stomach is divided into 4 regions.• Near the heart is the cardiac region,

surrounding the lower esophageal sphincter.• The fundus, laterally and slightly superior to

the cardiac region, temporarily holds the food as it enters the stomach.

Regions of the Stomach (cont’d)

• The body is the mid-portion of the stomach.• The funnel shaped, terminal end of the

stomach is called the pylorus. Most of the work of the stomach is performed here. This is where food passes through the pyloric sphincter into the small intestine.

• The 2 curves are the lesser curvature (concave) and the greater curvature (larger convex curve).

Muscle Layersof the Stomach

• The muscular action of the stomach works like a cement mixer, achieved by 3 layers of muscles.

• The three layers are the longitudinal layer, circular layer, and oblique layer.

• The stomach churns food as it mixes with gastric juices excreted by gastric glands in the gastric pits of the columnar epithelial lining of the stomach.

Muscle Layersof the Stomach (cont’d)

• The stomach works the food toward the pyloric sphincter through peristaltic activity of the muscles.

• Both chemical and physical digestion occurs here.

Gastric Juice

• Gastric juice is comprised of hydrochloric acid (HCl), pepsinogen, and mucus. About 1500 mls is produced daily by gastric glands.

• Pepsinogen is secreted by the chief cells while HCl is secreted by parietal cells combining to produce pepsin, the chief digestive enzyme.

• Pepsin breaks down protein. HCl breaks down the connective tissue.

Gastric Juice (cont’d)

• HCl has a pH of 1.5–2, and is effective at killing pathogens.

• Mucous cells generate a thick layer of mucus shielding the stomach from the effects of the stomach acids.

• The stomach also secretes intrinsic factor, allowing vitamin B12 to be absorbed.