Anatomy and Physiology

The Digestive System

Overview Of Digestive System

Alimentary System (Canal)

• Also Gastrointestinal (GI) Tract

• Continuous muscular digestive tube

• Digestion and absorption

• Mouth, pharynx, esophagus, stomach, intestines

Accessory Digestive System

• Contribute to the digestion of food

• Teeth, tongue, gallbladder, liver, pancreas

Digestive Processes

1. Ingestion: taking in food

2. Mechanical processing: squashing, tearing, crushing (in mouth)- includes peristalsis and segmentation

3. Digestion: chemical breakdown of food by digestive enzymes

4. Secretion: release of enzymes, etc.

5. Absorption: movement of organic molecules across epithelium of digestive tract so that they can be used for nourishment of cells

6. Defecation: elimination of solid waste products

Peristalsis: the forward movement

of materials in the digestive tract

Segmentation: churning and mixing of

food in the digestive tract: no

directional movement

Layers of the Alimentary Canal

(Digestive Tract)

• Lumen: central canal where food is

transported

• Mucosa (Mucous Membrane):

– Innermost layer; moist epithelial membrane

– Secretion, absorption, protection

• Submucosa: just external to mucosa

– Contains blood vessels, nerve fibers

– Gives walls its elastic characteristic

Layers of the Alimentary Canal

(Cont.)

• Muscularis Externa: external to

submucosa

– Contains smooth muscle

• Inner circular layer, Outer Longitudinal Layer

– Responsible for peristalsis & segmentation

• Serosa: protective outermost layer

– Composed of connective tissue & epithelium

Mesenteries• Peritoneum: serous membrane that lines and

lubricates the peritoneal cavity.

• The serosa is part of the peritoneum.

• Mesenteries: are double sheets of peritoneal membrane.

• The ventral mesenteries on the ventral surface of the stomach are called the lesser omentum.

• Dorsal mesenteries hang from the lateral and inferior borders of the stomach and are called the greater omentum.

Mesenteries

• Functions:– Suspend portions of

the digestive tract

within the peritoneal

cavity

– provide an access

route for the passage

of blood vessels and

nerve

– Stabilize the position

of intestines and

prevent them from

becoming entangled.

The Mouth

• Also called oral cavity or buccal cavity

• Oral Orifice: anterior opening

• Stratified Squamous Epithelium

– Helps to withstand friction forces

• Only organ involved with Ingestion

The Lips and Cheeks

• Lips (Labia)

– Orbicularis Oris Muscle

– Red Margin, Labial Frenulum

• Cheeks

– Buccinators Muscles

• The two combined help keep foodbetween teeth, help with speech

• Gums

– Called gingivae

The Palate

• Forms the roof of the mouth

• Hard Palate

– Food forced against it by the tongue

• Soft Palate

– Uvula: prevents food from traveling to the

nasal cavity

The Tongue

• Occupies the floor of the mouth

• Helps to reposition food between teeth

• Occupies the taste buds (lingual papillae)

• Intrinsic Muscles

– Changes shape of tongue, not position

• Extrinsic Muscles

– Changes position of tongue, not shape

The Salivary Glands

• Produces Saliva

Functions of Saliva

1. Cleanses the mouth

2. Dissolves food chemicals for taste

3. Moistens food, aids in bolus formation

4. Begins chemical digestion of food

Sets of Salivary Glands

1. Parotid glands: produce amylase:

enzyme that breaks down starch

2. Sublingual salivary glands: produce a

watery, mucous secretion that acts as a

buffer (neutralizes pH) and a lubricant

3. Submandibular salivary glands: secrete

buffers, mucins (lubricating action), and

amylase

The Teeth

• Adults contain 32 teeth

– 8 incisors, 4 canines, 8 premolars, 12 molars

• Crown: portion exposed above gums

– Covered by enamel (hardest substance in

body)

• Root: portion embedded in jaws

• Teeth initiate the breakdown of food

– Chewing (Mastication)

The Pharynx

• Oropharynx, Laryngopharynx

– Common passageway for air & food

– Contain two layers of skeletal muscle

• Inner: Longitudinal Muscle

• Outer: Pharyngeal Constrictor Muscles

– Muscles initiate the process of swallowing

(called deglutition). This delivers the material

(bolus) along the esophagus and into the

stomach.

The Esophagus• Muscular tube; collapsed when no food

• Lies posterior to the trachea

– Epiglottis directs food into esophagus

• Joins the stomach at the Cardiac Orifice.

• Enters the abdominal cavity through an

opening in the diaphragm called the

esophageal hiatus.

• A tear of this opening results in a hiatal

hernia.

The Esophagus

Gastroesophageal Sphincter: prevents backflow of stomach materials into the esophagus

• A weakened or permanently relaxed esophageal sphincter muscle can cause esophagitis (inflammation of esophagus) as gatric enzymes and acids backflow into the esophagus and erode its tissues. Causes occasional occurences of heartburn and Acid Reflux Disease.

Mouth, Pharynx, Esophagus

• Mastication (Chewing)

– Lips, cheeks, tongue keep food between teeth

• Deglutition (Swallowing)

– Tongue blocks off the mouth

– Uvula rises to block of nasopharynx

– Larynx rises so allow epiglottis to block off the

trachea and the lower airways

The Stomach• Temporary storage tank of food, J-shaped pouch

• Coverts food into thick, acidic, soupy mixture (Chyme)

Anatomical Regions of Stomach

1. Cardiac Region: smallest portion of stomach

2. Glands in this regoin secrete large amounts of mucous that protect connection with espohagus from gastric acids and enzymes

• Gastroesophageal Sphincter

3. Fundus: comes into contact with diaphragm

4. Body: largest region of stomach; mixes ingested food and gastric juices; glands in it secrete enzymes and acids

5. Pyloric Region: curve of the J• Pyloric Sphincter: muscle that controls release of chyme into

duodenum (upper portion of small intestine)

Rugae: folds in stomach that expand to increase the volume of the stomach for meals.

No absorption of food takes place

in the stomach!

Gastric Glands

• Gastric pits: secrete mucous to protect

stomach from enzymes and acids

• Parietal cells: secrete H+ and Cl-

separately into lumen of stomach, which

then combine to produce hydrochloric

acid. This keeps the pH of the stomach

around 1.5 – 2. This sterilizes ingested

food and helps to breakdown proteins.

Gastric Glands

• Chief cells: secrete enzyme pepsin that

digests proteins

• Pyloric glands: secrete mucous and

various gastric hormones that stimulate

the secretion of other glands and cells,

including parietal and chief cells.

The Small Intestine

• Body’s Major Digestive Organ, about 20 feet in length

– Digestion completed, absorption occurs

Subdivisions of Small Intestine

1. Duodenum: “mixing bowl”, where chyme from stomach is mixed with enzymes from pancreas and liver; also contains numerous glands that secrete mucous to neutralize the pH of the acidic chyme entering from the stomach.

2. Jejunum: most digestion and absorption takes place here; contains lots of plicaes circulares (folds) and intestinal villi (finger-like projections on wall of jejunum) that help to increase the surface area available for digestion and absorption.

The Small Intestine

3. Ileum: longest segment of small intestine ( about 12 feet long). Ends in a sphincter muscle called the ileococcal valve, which controls the flow of small intestinal contents into cecum of large intestine.

Also contains 20-30 masses of lymphoid tissue called Peyer’s patches; these contain lymphocytes that protect the small intestine from bacteria living in the large intestine

It takes about 5 hours for chyme to pass from the duodenum to the end of the ileum.

Accessory Organs: The Liver• Largest visceral organ; large, firm, reddish-

brown

• Three major functions of liver:

1. Bile production: produces and transports bile to duodenum or gall bladder

2. Metabolic regulation: regulates composition of circulating blood; excess nutrients and toxins absorbed (and detoxified or excreted), nutrient deficiencies corrected by mobilizing reserves or synthesizing nutrients (storage of iron, vitamins)

3. Hematological regulation: liver is the largest blood reservoir in the body; phagocytuc cells (Kupffer cells) in the liver cleanse the blood of old/damaged red blood cells, debris, and pathogens

Accessory Organs: The Liver

4 Lobes of the Liver

1. Right Lobe

2. Left Lobe

3. Caudate Lobe

4. Quadrate Lobe

• Hepatitis: inflammation of liver; viral

– Six types: A-F

Accessory Organs: The Liver

Accessory Organs: The

Gallbladder

• Hollow, pear-shaped, muscular organ connected to liver

• Acts as storage site for bile– Reason for its green color

• Bile duct delivers bile from gall bladder and liver to duodenum of small intestine during meals

• Gallstones: crystallization of bile– Causes tremendous shooting pain in abdomen

– Rx: drugs, lasers, surgery

Bile

• Pancreatic enzymes are water soluble only.

• Lipids (fats) in stomach exist in large droplets

that are in soluble.

• Water soluble enzymes can attack the surface of

the fat droplets only.

• Bile breaks the large droplets into smaller

droplets (a process called emulsification).

• This increases the surface area available for

enzyme attack.

Accessory Organs: The Pancreas

• Found in the greater curvature of stomach

(behind stomach)

• Long, pinkish-gray organ; lumpy, nodular

texture

• Is mainly an exocrine organ: secretes

digestive enzymes and buffers, which are

delivered by the pancreatic duct to the

duodenum of the small intestine

Accessory Organs: The Pancreas

• Endocrine function of pancreas: secretes

the hormones insulin and glucagon, which

regulate blood sugar levels

• Too much sugar in blood- insulin released to

use sugar for growth, energy, etc. and to

build energy reserves

• Too little sugar- glucagon released to

reduce use of glucose and stimulate release

of additional sugar into blood stream

Accessory Organs: The Pancreas

Video Clip

• Watch digestive video clip

The Large Intestine

• Begins at ileum of small intestine and ends at anus, horseshoe-shaped

• Larger diameter than small intestine– Almost half as long as small intestine

• Functions:

1. Reabsorption of water and bile salts

2. Compaction of chyme into feces

3. Absorption of vitamins (vitamin K, biotin, B vitamins) released by bacteria in large intestine

4. Storage of feces until defecation

The Large Intestine

Large Intestine Subdivisions

1. Cecum: first portion of large intestine, collects

and stores chyme, begins compaction

2. Appendix: attached to cecum, is an immune

organ that contains lymphocytes

3. Colon: ascending, transverse, descending,

sigmoid, contains pouches called haustra that

permit expansion and elongation of colon

The Large Intestine

4. Rectum: expandable, temporarily stores feces, movement of feces into rectum triggers the urge to defecate

5. Anal Canal (anus): external anal sphincter guards the exit of the anal canal and is under voluntary control, powerful peristaltic contractions, called mass movements, move feces into rectum and cause the concious urge to defecate.

• A high fiber diet promotes peristaltic contractions leading to defecation